1. Describe the structure of DNA in detail.

DNA is an extraordinarily long, thin molecule made of sub-units called nucleotides that are linked together like a chain. Each nucleotide is constructed of three parts: a phosphate group, a five-carbon sugar molecules, and a nitrogen base. These parts arrange to form a nucleotide. The five-carbon sugar is called deoxyribose from which DNA gets its full name, deoxyribonucleic acid.

While the sugar molecule and the phosphate group are the same for each nucleotide in a molecule of DNA, the nitrogen base may be any one of four different kinds. There are four different nitrogen bases: adenine, guanine, thymine, and cytosine. Adenine and guanine belong to a class of organic molecules called purines. Purines are large molecules, each with a double ring of carbon and nitrogen atoms. Thymine and cytosine are pyrimidines. Pyrimidines have a single ring of carbon and nitrogen atoms.

In 1949, Erwin Chargaff, an American biochemists working at Columbia University in New York city, made an interesting observation about DNA. Chargaff’s research data showed that for the DNA in each organism, the amount of adenine always equals the amount of thymine. Likewise, the amount of guanine always equals the amount of cytosine. However, the amount of adenine and thymine and of guanine and cytosine varied between different organisms. These findings, known as Chargaff’s rules, or more commonly as the base-pairing rules, suggested that the precise arrangement of nucleotides within a DNA molecule specifies genes.

The significance of Chargaff’s rules became clear in the 1950s when scientists began using X-ray diffraction to study the structures of molecules. In X-ray diffraction, a beam of X rays focused at an object. The X rays bounce off the object and are scattered in a pattern onto a piece of film. By analyzing the complex patterns on the film, scientists can determine the structure of the molecule. In the winter of 1952, Maurice Wilkins and Rosalind Franklin, two scientists working at King’s College in London, developed some high-quality X-ray diffraction photographs of the DNA molecule. These photographs, suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides.

The problem now was to discover the three-dimensional structure of the DNA molecule. The model had to take into account Chargaff’s rules and the X-ray diffraction data. In 1953, James Watson and Francis Crick, two scientists at Cambridge University, used this information along with their knowledge of chemical bonding to came up with a solution. Using tin-and-wire models of molecules, they built a model of DNA with the configuration of a double helix, a "spiral staircase" of two strands of nucleotides twisting around a central axis.

The double helix looks something like a twisted ladder. The sides of the ladder are constructed of alternating sugar and phosphate units, and each rung is a purine and a pyrimide held together by hydrogen bonds. Why is a purine always paired with a pyrimidine? These base pairings are the only possible arrangement because adenine (A) can for hydrogen bonds only with thymine (T), and cytosine (C) can form hydrogen bonds only with guanine (G). Notice that this arrangement of the nitrogen bases explains Chargaff’s observations. The strictness of base pairing results in two strands that are complementary to each other; that is, the sequence of bases on one strand determines the sequence of bases on the other strand. For example, if the sequence of one strand of a DNA molecule is TCGAACT, the sequence on the other strand must be AGCTTGA.

2. Explain how DNA is copied for reproduction.

When the double helix was first discovered, scientists were very excited about the complementary relationship between the sequences of bases. They predicted that the complementary structure was used as a basis to make new DNA. Watson and Crick proposed that one strand could serve as a template, or surface, upon which the other strand is built. Within five years of the discovery of DNA’s structure, scientists had firm evidence, from a number of different kinds of experiments, that the complementary strands of the double helix did indeed separate and serve as templates for building new DNA.

The process of synthesizing a new strand of DNA is called replication. Before replication can begin, the double helix must be unwound. This is accomplished by enzymes called helicases, which open up the double helix by breaking the hydrogen bonds that link the complementary bases. Once the two strands are separated, additional enzymes and proteins attach to the individual strands and hold them apart, preventing them from twisting. The point at which the double helix separates is called the replication fork because of its Y shape. At the replication fork, enzymes known as DNA polymerase move along each of the DNA strands, adding nucleotides to the exposed bases according to the base-pairing rules. As the DNA polymerase’s move along, two new double helixes are formed. Once a DNA polymerase has begun adding nucleotides to a growing double helix, the enzyme remains attached to the strand until it reaches a signal that tells it to detach.

In the course of DNA synthesis, errors are sometimes made and the wrong nucleotide is added to the new strand. An important feature of DNA replication is that DNA polymerase has a "proofreading" role; it can add nucleotides to a growing strand only if the previous nucleotide is correctly paired to its complementary base. In the event of a mismatched nucleotide, DNA polymerase is capable of backtracking , removing the incorrect nucleotide, and replacing it with the correct one. This proofreading prevents errors in DNA replication. After proofreading, an error in the DNA may occur once per 1 billion nucleotides.

Replication does not begin at one end of the DNA molecules and end at the other. Circular DNA found in bacteria usually have two replication forks that begin at a single origin of replication and move away from each other until they meet on the opposite side of the DNA circle. Linear DNA molecules found in eukaryotic organisms usually have many replication forks that begin in the middle and move in both directions, creating replicating "bubbles" along the molecule. If replication did not occur this way, it would take 16 days to copy just one DNA molecule of a fruit fly. But because approximately 6,000 replication forks exist simultaneously, replication of the fruit fly DNA takes only three minutes. Human DNA is also copied in segments, with a replication fork approximately every 100,000 nucleotides.

3. Explain the many concepts of genes and how this relates to your traits.

The gene is the unit of heredity. Along with many other such units, it is transmitted from parents to offspring. Each gene, acting either alone or with genes, determines one or more characteristics of the resulting organism. The totality of genes that make up the heredity constitution of an organism is called a genome. Genes occur in strands of genetic material called chromosomes. In most cells each gene occupies a particular position within a specific chromosome. Chromosomes can break, however, and some of their genes may be transferred either to places on the same chromosome or to other chromosomes. When this happens, new combinations (recombinants) of the gene are formed. Genes can also change in chemical composition. In their altered recombinant or chemically varied form, they produce different elements from the unaltered genes. Depending on the characteristics transmitted by the gene, the environment may also play an important role in determining the extent to which the gene's potential effect is realized.

The Nature Of The Gene

The subdivision of genetics concerned with the structure and functioning of genes at the molecular level is called molecular genetics. Since the term gene was first proposed by the Danish geneticist Wilhelm Johannsen in 1909, concepts of the nature of the gene have undergone modification. Current understanding of gene structure and function at the molecular level had its origin in 1944, with the work of Canadian bacteriologist Oswald T. AVERY and American scientist Colin M. MACLEOD and Maclyn McCARTY. They showed that the genes of bacteria are composed of the chemical compound called deoxyribonucleic acid, or DNA. This was later found to be true of the genes of most other organisms.

A further advance was made in 1953, when American biochemist James D. WATSON and English scientist Francis CRICK jointly presented their model of the structure of the DNA molecule. The molecule was shown to consist of two chains of chemical compounds called polynucleotides, the chains between twisted into the form of a coil, or double helix. Subsequently, in 1961, U. S. biochemist M. W. NIRENBERG and others figured out the relationship between the composition of DNA and the composition of the proteins produced by genes. This relationship is known as the GENETIC CODE. It later became evident that another nucleic acid, called ribonucleic acid, or RNA, also functions to carry out protein synthesis.

At first it was thought that all genes functioned in an identical manner to produce the various characteristics of an organism. Three different classes of genes, however, are now recognized. One class consists of the structural genes, whose genetic codes determine the sequences of AMINO ACIDS that go to make up proteins or the smaller molecules known as polypeptides, including many hormones. Another class of genes has genetic codes that specify molecules that function in the physical and chemical processes involved in PROTEIN SYNTHESIS. The third gene class consists of regulatory genes, which are non-coding. They act solely as "recognition" sites for enzymes and other proteins involved in controlling protein synthesis.

Early studies seemed to indicate that a gene, wherever it happened to be located within a chromosome, consisted of a single continuous unit. Later it was found some genes have a region called the leader that precedes the coding segment, and a region called the trailer that follows it. In addition, the coding segment itself may actually be broken up into sections, with intervening coding portions called exons.

A far-reaching advance in gene study was made in 1973, when American geneticists Stanley Cohen and Herbert Boyer demonstrated that certain enzymes, called restriction endonucleases, could be used to make cuts in a DNA molecule at certain specific sites. This produced a series of segments with identical free ends, which could join with other free ends having the appropriate complementary configuration. The result was the reestablishment of a fully functional DNA double helix. Using this procedure, called gene splicing ), it became possible to take a gene from a human cell and transfer it to a bacterium, mouse, rat, or pig, where the human gene would functions as it would in a human being. It even became possible to transfer animal genes to plants. One projected use of this procedure would be to transfer appropriate normal human genes to cells of individuals suffering from hemophilia, cystic fibrosis, or other GENETIC DISEASES. Should such transfers provide successful, they could provide a means to cure such genetic diseases through direct gene therapy. The actual functioning of genes is complex. To understand it, the nature and structure of the nucleus acids DNA and RNA must be examined in greater detail.

DNA

DNA occurs as the genetic material in most viruses and in all cellular organisms. Some viruses, however, have no DNA. Instead, their genetic material is in the form of RNA. Depending on the particular DNA-containing organisms, most DNA is found either in a single chromosome, as in bacteria, blue-green algae, and DNA viruses, or in several chromosomes, as in all other living things. In addition to its presence in chromosomes, DNA is also found in many CELL organelles, such as plasmids in bacteria, chloroplasts in plants, and mitochondria in both plants and animals.

Structure

All DNA molecules consist of a linked series of units that are called nucleotides. Each DNA nucleotide is composed of three sub-units: a 5-carbon sugar called deoxyribose, a phosphate group that is joined to one end of the sugar molecule, and one of several different nitrogen-containing bases linked to the opposite end of the sugar molecule. The four bases that predominate in DNA are called adenine and guanine (double-ringed PURINE compounds), and thymine and cytosine (single-ringed PYRIMIDINE compounds). Four different types of DNA nucleotides can be formed, depending on the base involved.

The phosphate group of each nucleotide bonds to one of the carbon atoms of the sugar molecule in the adjacent nucleotide. This forms a so-called polynucleotide chain. The DNA of most organisms consists of two polynucleotide chains that are coiled to form a double helix. The backbone, or outside margin, of each chain consists of the sugar-phosphate sequence. The bases project inward from this backbone, into the helix. The bases of one chain are attracted to bases on the other chain by means of hydrogen bonds. This holds the double helix together. Exceptions to this type of structural organization are found in some viruses with genetic material consisting of a single DNA chain.

In a DNA double helix the pairing between bases of the two chains is highly specific. That is, adenine is always linked to thymine by two hydrogen bonds, and guanine is always linked to crystosine by three hydrogen bonds. This arrangement--a purine linked to a pyrimidine--results in a molecule of uniform diameter. Because of this specific way in which DNA nucleotides are paired through certain pairs of bases, the base sequence of the two strands in the helix is said to be complementary. This means that the base sequence of either strand may be converted to that of its partner by replacing adenine by thymine or thymine by adenine, and replacing guanine by cytosine or cytosine by guanine.

Functions

The genetic material DNA has two specific functions. It provides for protein synthesis and hence for the growth development of an organism. It also furnishes all descendants of the organism with protein-synthesizing information by replicating itself and passing a copy to each offspring. This information, known as the genetic code, lies in the sequence of bases of DNA, which specifies the sequence of amino acids in a protein. DNA does not act directly in the process of protein synthesis. Instead it acts through the formation of a particular type of RNA called the messenger RNA (mRNA) during the process of transcription.

DNA replication depends on the principle of complementarily mentioned above. During the process of replication, the two strands of DNA double helix separate from one another. As separation occurs, each base on each strand attracts its complementary base-containing nucleotide, to which it becomes attached by hydrogen bonds. For example, the base adenine attracts and bonds to the base thymine. As the complementary nucleotides are fitted into place, an enzyme called DNA polymerase performs its function. It binds the phosphate of one nucleotide to the sugar molecule of the adjacent nucleotide, forming a new polynucleotide chain. The new strand of DNA remains hydrogen-bonded to the old one, and together they form a new double-helix molecule. This type of replication is called semiconservative, because each newly formed double-stranded molecule consists of one previously existing DNA strand.

Viruses, which contain single-stranded DNA, replicate by a slightly more complicated process. When a virus enters a cell, it makes a complementary copy of itself, to which it remains attached. A virus in this condition is said to be in its replicative form (RF), temporarily becoming a double-stranded DNA virus. The two chains separate during replication, but only the one recently formed strand attracts complementary nucleotides. These newly attracted nucleotides are joined together by the enzyme DNA polymerase, their base sequence being exactly the same as that of the original DNA virus. The newly formed polynucleotide chain is released from the RF of the original virus and functions alone.

Mutations

Many environmental factors can alter the structure of a DNA molecule. Some factors may be physical, and others are chemical. A mutation occurs when such alterations lead to permanent change in the base sequence of a DNA molecule. Mutations in turn result in an inherited change in protein synthesis. Most mutations tend to be harmful in their effects, and a number of self-repair mechanisms exist to deal with the damage done to DNA by environmental factors. Processes of mutation and self-repair have been studied, for example, in the case of damage caused to DNA by exposure to ultraviolet (UV) light. The energy absorbed by DNA during UV exposure results in the formation of chemical bonds between adjacent bases of the same polynucleotide strand. This condition interferes with base pairing during replication and leads to mutations.

RNA

Ribonucleic acid, or RNA, is needed in all organisms in order for protein synthesis to occur. It is also the genetic material of some viruses, which are referred to as RNA viruses . Like DNA, all RNA molecules have a similar chemical organization consisting of nucleotides. Each RNA nucleotide, like those in DNA, consists of three sub-units. One is a 5-carbon sugar called ribose, the second is a phosphate group that is attached to one end of the sugar molecule, and the third is one of several different nitrogen-containing bases linked to the opposite end of the sugar molecule. Four bases predominate in RNA: adenine and guanine (double-ringed purine compounds), and uracil and cytosine (single-ringed pyrimidine compounds).

Structure

RNA differs from DNA in two aspects of its chemical organization. First, the sugar in RNA is of the ribose type, indicating that the second carbon molecule in the ring has a hydroxyl (OH) group attached to it. (In DNA the second carbon in the ring has only a hydrogen (H) atom--hence the prefix deoxy, meaning "lacking oxygen," in the DNA sugar deoxyribose.) Second, the base uracil is present only in RNA. (Thymine, the base comparable to uracil, is present only in DNA.) Both bases are single-ringed Pyrimidines, and their nucleotides substitute for one another, depending on whether the strand is RNA or DNA.

The nucleotides of RNA are joined in a polynucleotide chain by means of bonding the phosphate of each nucleotide to a carbon atom of the adjacent nucleotide's sugar sub-unit. In RNA viruses the RNA is in the form of either a double or a single polynucleotide chain. In double-stranded RNA versus, the geometric arrangement of the two polynucleotide chains is similar to that of double-stranded DNA, and the pairing between bases of the two RNA chains is highly specific. Adenine is always linked to cytosine by three hydrogen bonds. Again as in DNA, the specific pairing of RNA nucleotides according to the base concerned indicates that the base sequence of the two RNA strands is complementary. Thus if the base sequence of one strand is known, then the base sequence of the other strand can be specified.

Functions

Replication of double-stranded RNA follows the pattern described for DNA. The RNA chains separate, and each base attracts an RNA nucleotide carrying the complementary base, to which it is attached by hydrogen bonds. As the complementary nucleotide’s are fitted into place, an enzyme called RNA replicase binds the nucleotides together, forming a new polynucleotide chain. The new strand of RNA remains hydrogen-bonded to the old strand--another example of semiconservative replication.

Single-strand RNA viruses fall into two classes. The first group includes the polio virus that attacks the nerve cells of humans and other primates. When this type of virus enters a cell, the virus makes a complementary copy of itself, to which it remains attached. In this stage the virus is again said to be in its RF form, temporarily becoming a double-stranded RNA virus. During replication, although the two chains separate, only the recently formed strand attracts nucleotides with complementary bases. The newly attracted nucleotides are joined together by the enzyme RNA replicase. In their base sequence they are exactly the same as the original RNA virus. The newly formed chain is released from the RF of the original virus to function independently.

The second group of single stranded RNA viruses contains some that causes tumors in animals, such as mouse leukemia virus and mouse mammary tumor virus. Upon entering a cell, this type of virus makes a complementary strand of itself. This newly formed chain, however, is composed of DNA nucleotides. The single strand of DNA in turn makes a complementary DNA strand of itself, forming a DNA double helix. The newly formed DNA double helix becomes incorporated into one of the chromosomes of the host cell, where it is replicated along with the host DNA. While in the host cell, the RNA-derived viral DNA produces single-strand RNA viruses that leave the host cell and enter other cells. The enzyme involved in making a DNA complement of RNDA is called RNA-directed DNA polymerase, or reverse transcriptase--a name based on the action of reversing the transcription process. Such viruses are also referred to as RETROVIRUSES. One of them, called HIV (for human immunodeficiency virus), invades and kills the T-helper lymphocytes of a person's immune system, resulting in the disease called acquired immune deficiency syndrome, or AIDS.

Types of RNA

RNA that is involved in protein synthesis is single-stranded. It belongs to one of three distinct types, called ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). A cell's ribosomal RNA is associated with protein, forming bodies called RIBOSOMES. Ribosome’s are sites of protein synthesis. Ribosomal RNA varies in size and constitutes 85 to 90 percent of all the RNA in a cell. Transfer RNA, also called soluble RNA or adapter RNA, is a group of small molecules, each of which has a specific attraction for one of the amino acids. The function of each type of tRNA is to bring its specific amino acid to a ribosome for possible inclusion in the particular protein being synthesized. The tRNA molecules, about 80 nucleotides in a cloverleaf pattern, constitute about 5 percent of a cell's RNA.

The third type of cellular RNA, messenger RNA, constitutes 5 to 10 percent of a cell's total RNA. It acts as an intermediary between the genes located in the chromosomes, and the ribosome’s located in the cytoplasm. As its name implies, mRNA carries the genetic code contained in the sequence of bases in the cell's DNA. Because the DNA’s from various organisms differ only in the sequence of their bases, mRNA from different organisms must reflect this difference in base sequence. The synthesis of mRNA, called transcription, involves the formation of an RNA chain that is complementary to one of the two strands of a DNA double helix. In the transcription process, only nucleotides that contain ribose are used. In this process, uracil acts as the complement of adenine. The enzyme involved in transcription is known as RNA polymerase.

RNA as an Information Molecule

The earliest information molecule to have evolved must have been both relatively simple in structure and capable of enzymatic activity. RNA, which acts as the carrier of genetic messages in all organisms, is the simplest molecule known that has the capacity to store and transmit information.

Initially, evidence that RNA can also act as a catalyst of reactions centered on the enzyme called ribonuclease P, which consists of protein and RNA. The enzyme has been found on virtually all organisms. It is involved in the process that transforms the precursor molecules of tRNA's into their fully functional forms. Geneticists have since discovered that the RNA component of this enzyme, acting alone, can perform the catalytic activity of the enzyme, whereas the protein alone cannot. More recently, investigations have concentrated on the ribosomal-RNA specifying gene of the protozoan Tetrahymena thermophila. This gene consists of a non-coding sequence, or intron, between two coding portions, or exons. After transcription, the precursor RNA molecule has to have the intron-transcribed segment removed before the ribosomal-RNA molecule can become functional. The intron specified segment snips itself out of the precursor molecule and splices the loose ends together to form the functional molecule.

These findings, that RNA is capable of catalytic activity, lend support to the concept that RNA was indeed the earliest information molecule, and that DNA must have evolved from RNA. Support for this hypothesis is found in the life cycles of RNA tumor viruses, which, upon entering cells, make DNA copies of themselves.

Examination of the DNA of eukaryotic cells revealed four major surprises. First with some few exceptions, the amount of DNA per cell is the same for every diploid cell of any given species, but the variations among different species are enormous. Drosophila has about 1.4 * 10⁸ base pairs per haploid genome, only about 70 times more than E. coli. Humans (with approximately 3.5 * 10⁹ base pairs) have 25 times as much as Drosophila, somewhat more than a mouse, but about the same amount as a toad (3.32 * 10⁹ base pairs). The largest amount of DNA found so far has been located in a salamander with 8 * 10¹⁰ base pairs per haploid genome.

Second, in every eukaryotic cell, there is what appears to be a great excess of DENA, or at least of DNA whose functions are unknown. It is estimated that in eukaryotic cells less than 10% of all the DNA codes for proteins; in humans, it may even be as little as 1%. By contrast, prokaryotes, as we have seen, use their DNA very thriftily; viruses even more so. Except for regulatory or signal sequences, virtually all of their DNA is expressed.

Third, almost half of the DNA of the eukaryotic cell consists of nucleotide sequences that are repeated hundreds, even millions, of times. This was a particularly startling discovery. In E. coli, long the model for molecular genetics, each chromosomal DNA molecule typically contains only one copy of any given gene. (The principal exceptions are the genes coding for the ribosomal RNA’s.) Moreover, according to Mendelian genetics, a gene should be present only twice per diploid eukaryotic cell, not in a multitude of copies.

Introns

The fourth surprise – and perhaps the most unexpected of all – was that the protein-coding sequences of eukaryotic genes are usually not continuos but are instead interrupted by non-coding sequences. These non-coding interruptions within the gene are known as intervening sequences, or Introns, and the coding sequences, the sequences that are expressed, are called exons.

Introns were discovered in the course of hybridization experiments, when investigators found that there was not a perfect match between eukaryotic messenger RNA molecules and the genes from which they were transcribed. The nucleotide sequences of the genes were much longer than their complementary mRNA molecules that were found in the cytoplasm. Subsequently, Introns and exons were actually visualized in electron micrographs.

It is now known that most, but not all, structural genes of multi-cellular eukaryotes called Introns. The Introns are transcribed onto RNA molecules and excised before translation. The number of Introns per gene varies widely. For example, the gene for ovalbumin, a protein found in large quantities vertebrate egg cells, has seven Introns. By contrast, the mammalian gene for beta-globin, one of the polypeptides of the hemoglobin molecule, has only two Introns, one large and one small. In chickens, the gene for collagen, a very common protein, has 50 Introns. Introns have also been found in genes coding for transfer RNA’s and ribosomal RNA’s, and even in some viruses.

In general, the more complex an organism and the more recently it has evolved, the larger and more abundant are its Introns. It is not known which came first – continuos genes lacking Introns or interrupted genes containing Introns. It has been suggested that perhaps the latter came first, but that in bacteria and other unicellular organisms that are highly selected for rapid growth, any unneeded DNA has been eliminated in the course of their evolution.

Are Introns accidents, or do they have a function? One suggestion for their continued existence in multi-cellular eukaryotes is that they promote recombination; crossing over during meiosis is more likely in genes containing Introns than in genes lacking Introns, just because of the distances involved. There are also indications that, in some cases, different exons code for different structural and functional segments, or domains, of the finished protein. For example, the central exon of the beta-globin gene codes for the domain of the polypeptide that holds the heme group, and the other two exons code for domains of the molecule that fold around this central portion. It is hypothesized that new combinations of such domains, brought about by the reshuffling of exons, might foster the rapid evolution of new proteins.

Introns were not the only surprise that scientists found in eukaryotic genes. At first they assumed chromosomes carried one copy of each kind of gene – for example, one to make the enzyme that breaks down the sugar lactose, and another one to encode the protein hemoglobin that carries oxygen in your blood. They soon found out that this is not always true. Some genes in your cells exist in multiple copies, clusters of almost identical sequences called multigene families. Multigene families may contain as few as three or as many as several hundred versions of a gene. For example, your cells each contain 12 different hemoglobin genes in two families. Three genes are "silent", meaning they do not make proteins; five others are active only during embryonic or fetal development. Only four of the genes encode the two chains of human hemoglobin’s in adults. The nucleotide sequences of each of the 12 hemoglobin genes are clearly related to each other. Scientists theorize that the 12 different sequences arose from one ancestral gene, which duplicated and evolved into two gene families.

Some Genes Can Jump to New Locations

In both bacteria and eukaryotes, individual genes are scattered randomly about on chromosomes and may be repeated several times. These genes, called transposons, have the ability to move from one chromosomal location to another. Once every few thousand cell divisions, a transposons jumps to a new location on a chromosome. When a transposons jumps to a new location, it often inactivates a gene or causes mutations. Because transposons can cause mutations and bring together different combinations of genes, the transfer of these mobile genes has had an enormous impact on evolution. In the 1950s Barbara Mcclintock, a geneticist at Cold Spring Harbor Laboratory, discovered transposons in the course of her studies of maize. In 1983, Mcclintock, received a Nobel Prize for her work.

Some Genes Protect Cells From Mutation

Recently, scientists have discovered that some genes are able to prevent the ill effects caused by mutation. One such gene, which was the focus of intense research in 1993, is called p53( short for protein 53). In normal cells, p53 acts as a tumor suppressor – its protein product helps coordinate a complex system of responses to the presence of damaged DNA that might otherwise lead to cancer. The p53 gene works by shutting off cell growth and division in damaged cells or, in some cases, by causing cells to self-destruct. However, when the p53 gene loses its activity by mutation, the cells of the body lose the protection it provides and, as a result, can begin to grow unchecked, signaling the development of cancer. Since 1989, researchers have found mutated forms of the p53 gene in more than 51 kinds of human tumors. As scientists learn more about the molecules that interact with p53, they will uncover new possibilities for drug development.

Operon

It was the detection of the mutants that led to our current understanding of the regulation of transcription in prokaryotes. This understanding rests upon a model, known as the operon model, provided some years ago by the French scientists Francois Jacob and Jacques Monod, who shared the Nobel Prize in 1965 with their colleague Andre Lwoff. According to the model formulated by Jacob and Monod, groups of genes coding for proteins with related functions are arranged in units known as operons. An operon comprises the promoter, the structural genes, and another DNA sequence known as the operator. The operator is a sequence of nucleotides located between the promoter and the structural gene or genes; the operator may overlap the promoter, the adjacent structural gene, or both.

Transcription of the structural genes often depends on the activity of still another gene, the regulator, which may be located anywhere on the bacterial chromosome. The gene codes for a protein called the repressor, which binds to the operator. When a repressor is bound to the operator, it obstructs the promoter. As a consequence, RNA polymerase either cannot bind to the DNA molecule or, if bound, cannot begin its movement along the molecule. The result in either case is the same: no mRNA transcription occurs. However, when the repressor is removed, transcription may begin. Evidence for the existence of the regulator gene was derived from studies of E. coli cells that could not stop making beta-galactosidase. In these cells, a mutation in the regulator gene for the lactose (lac) operon provided the essential clue that such a gene existed in normal cells.

The capacity of the repressor to bind to the operator and thus to block protein synthesis depends, in turn, on another molecule that functions as an effector. Depending on the operon, an effector can either activate or inactivate the repressor for that particular operon. For example, when lactose is present in the growth medium, the first step in its metabolism produces a closely related sugar, allolactose, that binds to and inactivates the repressor, removing it from the operator of the lac operon. As a consequence, RNA polymerase can begin its movement along the DNA molecule, transcribing the structural genes of the operon into mRNA. In the case of the trptophan (trp) operon, the presence of the amino acid activates the repressor, which then binds to the operator and blocks the synthesis of the unneeded enzymes. Both allolactose and tryptophan – as well as the molecules that interact with the repressors of other operons – are allosteric effectors, exerting their effects by causing a change in the configuration of the repressor molecule.

Some 75 different operons have now been identified in E. coli, comprising 260 structural genes. Some are, like the lac operon, inducible, while others are, like the trp operon, responsible. Note, however, that both inducible and repressible systems are examples of negative control, since both involve repressors that turn off transcription.

When lactose is not present in the cell, one repressor attaches to the operator region of the lac operon while other repressor molecules float freely in the cell. A repressor is a molecule that inhibits a gene from being expressed. A regulator gene which lies close to the lac operon codes for the repressor. The presence of a repressor molecule prevents the action of an enzyme RNA polymerase. Since RNA polymerase is needed to make the mRNA that will produce the enzymes that break down lactose, no enzymes will be produced while a repressor is attached to the operon.

An inducer is a molecule that initiates gene expression. In this example lactose is the inducer. It binds to the repressor on the operator portion and removes it. Lactose also binds to all other repressor molecules in the cell, so that none can interfere with the promoter. RNA polymerase can than begin to act. It move from the promoter section of the operon, past the now-free operator region, and into the structural genes of the operon. Here RNA polymerase transcribes the DNA code into mRNA, which then migrates to the ribosome’s and forms three enzymes : betagalactosidase, permease, and transacetylase. These enzymes cleave lactose into glucose and galactose and regulate cell permeability to lactose. These actions decrease the amount of lactose in the cellular environment. When the level of lactose in the cellular environment is low, the repressor rebinds to the operator, and the lac operon is shut off.

Different Systems Used To Control The Lac Operon In Prokaryotic Cells

Transcription in the E. coli and other prokaryotes takes place, as we saw in the last chapter, by the synthesis of a molecule of mRNA along a template strand of DNA. The process begins when the enzyme RNA polymerase attaches to the DNA at a specific site known as the promoter. The RNA polymerase molecule binds tightly to the promoter and causes the DNA double helix to open, initiating transcription. The growing RNA strand remains hydrogen-bonded to the DNA template briefly - only about 10 or 12 ribonucleotides are bonded to the DNA at any one time - and then it peels off in a single strand.

A segment of DNA that codes for a polypeptide ( a protein ) is known as a structural gene. Often structural genes coding for polypeptides with related functions occur together in sequence on the bacterial chromosome. Such functional groups might include, for instance, two polypeptide chains that together constitute a particular enzyme or three enzymes that work in a single enzymatic pathway. Groups of genes coding for such molecules are typically transcribed into a single mRNA strand. Thus, a group of polypeptides that are needed by the cell at the same time can be synthesized simultaneously, a simple and efficient inventory-control system.

The newly synthesized mRNA molecule has a short "leader" sequence at its 5' end, part of which may assist in binding mRNA to the ribosome. The coding region of the molecule is a linear sequence of nucleotides that precisely dictates the linear sequence of amino acids in particular polypeptide chains. There may be several stop and start codons within the mRNA molecule, marking the end of one structural gene and the beginning of the next. An additional nucleotide sequence at the 3' end is known as a "trailer." As we have seen previously, ribosome’s attach to the mRNA molecule even before transcription is complete.

In the course of their long evolutionary history, E. coli and other prokaryotes have evolved ways to maximize their utilization of nutrients for cellular growth. If a bacterial cell could be said to have purpose or function, it would be to grow, and multiply as rapidly as possible. And, bacteria are excellent at achieving this; a culture of E. coli cells, for example, can double in number every 20 minutes.

One reason for E. coli's effectiveness in using nutrients is its versatility; it can make at least 1,700 enzymes and other proteins, enabling it to utilize a wide range of potential nutrients. A second reason is that the cell is highly efficient in its synthetic activities. It does not make all of its possible proteins all of the time, but only when they are needed and only in the amounts needed. For example, cells of E. coli supplied with the disaccharide lactose as a carbon and energy source require the enzyme beta-galactosidase to split the disaccharide. Cells growing on lactose have approximately 3,000 molecules of beta-galactosidase per cell. In the absence of lactose, there is an average of one molecule of the enzyme per cell. In short, the presence of lactose induces the production of the enzyme molecules needed to break it down. Such enzymes are said to be inducible.

Conversely, the presence of a particular nutrient may inhibit the transcription of a group of structural genes. E. coli, like other bacteria, can synthesize each of its amino acids from ammonia and a carbon source. The structural genes for the enzymes needed for the biosynthesis of the amino acid tryptophan, for instance, are grouped together and are transcribed into a single mRNA molecule. This mRNA is produced continuously in growing cells - unless tryptophan is present. In the presence of tryptophan, production of the enzyme ceases. Such enzymes, the synthesis of which is reduced by the presence of the products of the reactions they catalyze, are said to be repressible.

Mutants of E. coli sometimes occur that are unable to regulate enzyme production. These cells produce beta-galactosidase even in the absence of lactose, for example, or the enzymes that synthesize tryptophan even when tryptophan is present. These and similar mutants are generally at a disadvantage because they are squandering their energies and resources. Normal E. coli cells rapidly outmultiply them.

Although regulation of protein synthesis could theoretically take place at many points in the biosynthetic process, in prokaryotes it occurs mostly at the level of transcription. Regulation involves interactions between the chemical environment of the cell and special regulatory proteins, coded by regulatory genes. These proteins can work either as negative controls, repressing mRNA transcription, or as positive controls, enhancing transcription. The fact that mRNA is translated into protein so immediately ( before transcription is even completed) and broken down so rapidly further increases the efficiency of this strategy of regulation.

It was the detection of the mutants described above that led to our current understanding of the regulation of transcription in prokaryotes. This understanding rests upon a model, known as the operon model, proposed some years ago by the French scientists Francois Jacob and Jacques Monod, who shared the Nobel price in 1965 with their colleague Andre Lwoff. According to the model formulated by Jacob and Monod, groups of genes coding for proteins with related functions are arranged in units known as operons. An operon consists of a promoter, an operator, and structural genes ( that is that code for proteins, often enzymes that work sequentially in a particular reaction pathway.) The promoter, which precedes the operator, is the binding site for RNA polymerase. The operator is the site at which a repressor protein can bind; it may overlap the promoter, the first structural gene, or both. Another gene involved in operon function is the regulator, which codes for the repressor. Although the regulator may be adjacent to the operon, in most cases it is located elsewhere on the bacterial chromosome. An operon comprises the promoter, the structural genes, and another DNA sequence known as the operator. The operator is a sequence of nucleotides located between the promoter and the structural gene or genes; the operator may overlap the promoter, the adjacent structural gene, or both.

Transcription of the structural genes often depends on the activity of still another gene, the regulator, which may be located anywhere on the bacterial chromosome. This gene codes for a protein called the repressor, which binds to the operator. When a repressor is bound to the operator, it obstructs the promoter. As a consequence, RNA polymerase either cannot bind to the DNA molecule or, if bound, cannot begin its movement along the molecule. The result in either case is the same: no mRNA transcription occurs. However, when the repressor is removed, transcription may begin. Evidence for the existence of the regulator gene was derived from the studies of E. coli cells that could not stop making beta-galactosidase. In these cells, a mutation in the regulator gene for the lactose operon provided the essential clue that such a gene existed in normal cells.

The capacity of the repressor to bind to the operator and thus to block protein synthesis depends, in turn, on another molecule that functions as an effector. Depending on the operon, an effector can either activate or inactivate the repressor for that particular operon. For example, when lactose is present in the growth medium, the first step in its metabolism produces a closely related sugar, allolactose, that binds to and inactivates the repressor, removing it from the operator of the lac operon. As a consequence, RNA polymerase can begin its movement along the DNA molecule, transcribing the structural genes of the operon into mRNA. Inducible and repressible operons are both turned off by repressor proteins that are coded by regulator genes. The repressor binds to DNA at the operator and so prevents RNA polymerase from initiating transcription. In inducible operons, the inducer counteracts the effect of the repressor by binding with it and maintaining it in an inactive form. Thus, when the inducer is present, the repressor can no longer attach to the operator, permitting transcription and translation to proceed. In the case of the tryptophan operon, the presence of the amino acid activates the repressor, which then binds to the operator and blocks the synthesis of the unneeded enzymes. In repressible operons, the repressor can bind to the operator only when it is combined with a co-repressor. Thus transcription and translation proceed until a co-repressor is produced. Both allolactose and tryptophan - as well as the molecules that interact with the repressors of other operons - are allosteric effectors, exerting their effects by causing a change in the configuration of the repressor molecule.

Some 75 different operons have now been identified in E. coli, comprising 260 structural genes. Some are, like the lac operon, inducible, while others are, like the trp operon, repressible. Note, however, that both inducible and repressible systems are examples of negative control, since both involve repressors that turn off transcription. Catabolic activator protein, or CAP, is a regulatory protein that exerts positive control on the operon. Like the operon itself, the CAP system was initially investigated in relation to lactose metabolism and is now known to be of much wider significance. CAP combines with a molecule known as cyclic AMP (cAMP), and this combination binds to the promoter region of the operon. Only then, when the CAP-cAMP complex is bound to the promoter, does maximum transcription take place. Negative and positive regulation of the lac operon - In the lac operon ( and other operons regulated by the CAP-cAMP system), the promoter includes two distinct regions: a binding site for the CAP-cAMP complex and an entry site for RNA polymerase molecules. In order for RNA polymerase molecules to bind efficiently to the promoter, the CAP-cAMP complex must be in place on its binding site. In the absence of the inducer ( allolactose), the repressor binds to the operator, which in the lac operon, overlaps the first structural gene. Although RNA polymerase can bind to the promoter, it cannot move past the repressor to begin transcription. In the presence of the inducer, the repressor is inactivated and can no longer attach to the operator. If, under these circumstances, the CAP-CAMP complex is in place at its binding site, RNA polymerase molecules immediately begin transcription of mRNA molecules that direct the synthesis of three proteins: the enzyme beta-galactosidase, a transport protein that brings lactose from the external medium into the cell, and the enzyme transacetylase, which transfers an acetyl group from acetyl CoA to galactose. As we have seen, the operon is under the negative control of the repressor, no transcription takes place unless the repressor is removed. It is also under the positive control of the CAP-cAMP complex, which enhances transcription when it is bound to the operon.

Discovery of this control system came about because of the observation that E. coli will not use lactose as an energy source if glucose is present. In other words, in the presence of glucose, the lac operon remains repressed, even thought lactose is present in the cell. The intermediary in this regulatory process is cAMP. When the supply of glucose in the cell decreases, the level of cAMP increases, more CAP-cAMP complexes form and become available to bind to the lac operon, more of the proteins coded by the lac operon are produced, and more lactose is broken down. The process by which a decrease in the concentration of glucose leads to an increase in the concentration of cAMP remains a mystery.

These mechanisms are, in themselves, further examples of the precision with which the living cell regulates its biochemical activities. Their manipulation is, an essential component in the scientific trick of inducing bacterial cells to synthesize mammalian proteins of medical importance, such as human insulin.

Regulation Of Gene Expression In Eukaryotes

As seen in previous assignments, regulation of gene expression in prokaryotes largely involves the fine tuning of the metabolic machinery of the cell in response to changes in available nutrients in the environment. In eukaryotes, especially multi-cellular eukaryotes, the problems of regulation are very different. A multi-cellular organism usually starts life as a fertilized egg, the zygote. The zygote divides repeatedly by mitosis and cytokinesis, producing many cells. At some stage these cells begin to differentiate, becoming, for example, muscle cells, nerve cells, blood cells, intestinal cells, and so forth. Each cell type, as it differentiates, begins to produce characteristically different proteins that distinguish it from other types of cells. This is nicely illustrated by mammalian red blood cells. In the early stages of fetal life, developing red blood cells synthesize one type of fetal hemoglobin; then, sometime after the birth of the organism, the developing red blood cells begin to produce the alpha and beta chains characteristic of adult hemoglobin. Thus the genes are expressed in a carefully controlled sequence, one after the other. The DNA segments that code for these hemoglobin molecules are expressed only in the developing red blood cells. There is evidence, however, that all of the genetic information originally present in the zygote is also present in every diploid cell of the organism. In other words, the DNA segments that code for hemoglobin are present in skin cells and hearth cells and liver cells and nerve cells and, indeed, in every one of the nearly 200 different types of cells in the body. Similarly, the DNA sequence that codes for the hormone insulin is present not only in the specialized cells of the pancreas that manufacture insulin but also in all the other cells. Since each type of cell produces only its characteristic proteins - and not the proteins characteristic of other cell types - it becomes apparent that differentiation of the cells of a multi-cellular organism depends on the inactivation of certain groups of genes and the activation of others.

Many lines of evidence indicate that the degree of condensation of the DNA of the chromosome, as shown by chromatin staining, plays a major role in the regulation of gene expression in eukaryotic cells. Staining reveals two types of chromatin: euchromatin, the more open chromatin, which stains weakly, and heterochromatin, the more condensed chromatin, which stains strongly. During interphase, heterochromatin remains condensed, but euchromatin becomes dispersed. Transcription of DNA to RNA takes place only during interphase, when the euchromatin is dispersed.

Some regions of heterochromatin are constant from cell to cell and are never expressed. An example is the highly condensed chromatin located in the centromere region of the chromosome. This region, which does not code for protein, is believed to play a structural role in the movement of the chromosomes during mitosis and meiosis. Similarly, little or no transcription takes place from Barr bodies, which are X chromosome that are tightly condensed and irreversibly inactivated. Other regions of condensed chromatin, by contrast, vary from one type of cell to another within the same organisms, reflecting, it is believed, the biosynthesis of different proteins by different types of cells. Also, as a cell differentiates during embryonic development, the proportion of heterochromatin to euchromatin increases as the cell becomes more specialized.

Additional evidence linking the degree of chromosome condensation to gene expression comes from studies of the giant chromosomes of insects. At various stages of larval growth in insects, it is possible to observe diffuse thickenings, or "puffs", in various regions of these chromosomes. The puffs are open loops of DNA and studies with radioactive isotopes indicate that these loops are sites of rapid RNA synthesis. When ecdysone, a hormone that produces molting in insects, is injected, the puffs occur in a definite sequence that can be related to the developmental stage of the animal. For example, in one species of Drosophila, ecdysone initiates three new puffs and causes increases in 18 other puffs within 20 minutes after it is injected; during this same period, 12 other puffs decrease in size. After 4 to 6 hours, five additional puffs can be seen. The looping out of the DNA occurs before RNA synthesis is initiated. The mechanism of this spinning out of the chromosomal DNA is still poorly understood.

Once the DNA helix is formed, specific enzymes add methyl groups to nucleotides of cytosine. Methylation in eukaryotes is hypothesized to inhibit gene expression and so to provide a form of gene regulation. Methylcytosine is found almost exclusively at the complementary sequences (5')-C-G-(3') and (3')-G-C-(5'). In birds and mammals, 50 to 70 percent of cytosine in such sequences are methylated. Z-DNA contains a large proportion of methylated cytosine, and Z-DNA is believed to be inactive. On the other hand, some eukaryotes - insects, for example - have no methylated bases in their DNA and yet they regulate the expression of their genes with no apparent loss of efficiency.

In eukaryotes, as in prokaryotes, transcription is also regulated by proteins that bind t specific sites that have been identified. However, it is increasingly clear that this level of transcriptional control is far more complex in eukaryotes, particularly multi-cellular eukaryotes, than in prokaryotes. A gene in a multi-cellular organism appears to respond to the sum of many different regulatory proteins, some tending to turn the gene on and others to turn it off. The sites at which these regulatory proteins bind may be hundreds or even thousands of base pairs away from the promoter sequence at which RNA polymerase binds and transcription begins. This adds to the difficulty of identifying the regulatory molecules and also of understanding exactly how they exert their effects. Recent research suggests that changes in the activities of some of these regulators are liked with the development of cancers.

Enhancer Control

One of the best-understood mechanisms in regulating gene expression in eukaryotes occurs before transcription and involved a region on the eukaryotic chromosome called an enhancer. An enhancer is a sequence of nucleotides that, when activated by specific signal proteins, aids in exposing the RNA polymerase binding site of a specific gene. An enhancer is usually located thousand of nucleotides away from the beginning of a gene.

To understand how an enhancer stimulates transcription, look at the action of steroid hormones in the cells of vertebrates. A steroid hormone is a molecule made of lipids that acts as a chemical signal. Estrogen is a steroid hormone responsible for secondary sex characteristics in females. When estrogen passes through the cell membrane of specific cells it binds to a receptor protein in the nuclear envelope, forming a hormone-receptor complex. The hormone-receptor complex has the right shape to bind to a specific protein called an acceptor protein, which in turn binds to enhancer regions in the DNA. Binding of the acceptor protein to the enhancer region stimulates RNA polymerase to begin transcription.

Regulation of Gene Expression in Prokaryotic Cells

In order to survive, bacteria must be able to adjust to changes in their environment, such as fluctuations in available nutrients. For example, when the amino acid tryptophan is not present in the environment, the bacterium Escherichia coli must manufacture it from another compound. Later, when tryptophan is present in the environment, the bacterium stops making the amino acid. By being able to adjust its metabolism to changes in its environment, a bacterium is saved from wasting its energy and resources on producing a substance that is readily available.

Scientists first studied gene expression in bacteria. An example of gene regulation that is well understood at the molecular level is found in E. coli. When you consume a milk product, the disaccharide lactose (milk sugar) is soon present in your intestinal tract and available to the E. coli living there. Before a bacterium can absorb the lactose, it must first make beta-galactosidase, an enzyme that cleaves lactose into glucose and galactose.

As with typtophan, it is in a bacterium’s best interest to focus its energy on using available nutrients. Therefore, E. coli should make beta-galactosidase only when lactose is present. To understand how this happens, you must first understand the basic mechanism that controls gene regulation in prokaryotes – the operon. An operon is a cluster of genes that codes for proteins with related functions. The gene for beta-galactosidase is a part of a group of genes called the lac operon. The lac operon is divided into several different regions. One region is the promoter, the RNA polymerase binding site that signals the beginning of a gene. Another region is made of structural genes, the genes that code for polypeptides. Between the promoter and the structural genes is a region of DNA called the operator. Because of its position in the operon, the operator is able to control RNA polymerase’s access to the structural genes; it acts like a switch, turning the operon on or off.

What determines whether the lac operon in the "on" or "off" mode? When no molecules are bound to the operator, the lac operon is switched on; RNA polymerase can bind to the promoter, move across the operator, and transcribe the structural genes. The lac operon is switched off when a protein called a repressor is bound to the operator. A bound repressor creates a barrier, preventing RNA polymerase from transcribing the structural genes (although RNA polymerase can still bind to the promoter).

Transcription can resume when the repressor is removed by a molecule called an inducer. In the case of the lac operon, the inducer is allolactose, a molecule that is made from lactose when it enter the cell. When allolactose is present in the cell, it binds to the repressor and changes the shape of this protein. As a result, the repressor falls off the operator. Now, RNA polymerase has access to the structural genes. The bacterial cell can begin building the proteins it needs to metabolize lactose.

When lactose is not present in the intestinal tract, allolactose is not produced. Therefore, there is nothing to change the shape of the repressor, which remains bound to the operator. As a result, transcription of the lac operon is blocked and the structural genes remain unexpressed. By producing the enzymes only when the nutrient is available, the bacterium avoids wasting its energy making proteins it does not need.

Relationship Of Bacteriophages And Their Genetic Elements To Genetic Diversity In Prokaryotic Cells.

Viruses consists essentially of a molecule of nucleic acid enclosed in a protein coat, or capsid. They contain no cytoplasm or ribosome’s or other cellular machinery. However, they can move from cell to cell and, within a host cell, utilize its enzyme systems and organelles to replicate their nucleic acid and synthesize new coat proteins. The coat may consist of one protein molecule repeated over and over, as in tobacco mosaic virus, or a number of different kinds of proteins, as in the T-even Bacteriophages with their complex tail assemblies. The composition of the protein coat determines the attachment of the virus to the membrane of the host cell and the subsequent entry of the viral nucleic acid into the cell.

Once within the host cell, the viral nucleic acid directs the production of new viruses. This is accomplished using the raw materials of the cell- such as nucleotides, amino acids, and the cell's ATP and other energy sources - and also the cells metabolic machinery. Thus viruses are obligate parasites; they cannot multiply outside the host cell.

The nucleic acid of a virus - the viral chromosome - may be either DNA or RNA, single-stranded or double-stranded, circular or linear. Viral chromosomes vary greatly in size from some 5,400 nucleotides for a small, single-stranded DNA bacteriophage, to 180000 for the T-even bacteriophages. The viral chromosome always codes for the coat protein or proteins, and also for one or more enzymes involved in replication of the viral chromosome. These enzymes insure the rapid replication of viral nucleic acid in preference to the nucleic acid of the host cell. The viral chromosome also codes for an enzyme or enzymes that, once the virus particles are assembled, enable them to lyse the host cell and escape. The infection cycle is complete when the viral nucleic acid molecules are packaged into the newly synthesized protein coats and the virus particles break out of the host cell.

The genetic makeup of the DNA of bacterial cells can be altered, as we have seen, by the introduction of DNA from other bacterial cells. Both transformation and conjugation can result in recombination between donor and host DNA. Similarly, the transfer of plasmids and their insertion into the chromosome of a bacterial cell can change the DNA composition of the recipient cell. Viruses can also play role as vectors ( carriers) that move pieces of DNA from one bacterium to another.

Early in the study of bacteriophages, it was noted that a virus infection could suddenly erupt in a colony of apparently uninfected cells. Such cells were termed lysogenic, because of their capacity to generate a cycle of cell lysis that spread through neighboring cells. The cause of lysogeny, it was discovered, was the capacity of certain viruses to set up a long-term relationship with their host cell, remaining latent for many cellular generations before initiating an infection cycle. Such viruses became known as temperate bacteriophages. The DNA of temperate phages, like that of the F Plasmid, may become integrated at specific sites in the host chromosome, replicating along with the chromosome. Such integrated bacteriophages are known as prophages. Prophages break loose from the host chromosome spontaneously about once in every 10000 cell divisions, triggering a lytic cycle. In the laboratory, lysis may be triggered by ultraviolet light, x-rays, or other agents that damage nucleic acids. Temperate phages resemble plasmids in that (1) they are autonomously replicating molecules of DNA, and (2) they may become integrated into the bacterial cell chromosome. They differ from plasmids both in their capacity to manufacture a protein coat and thus to exist ( though not to replicate ) outside the cell and in their capacity to lyse the host cell.

The process known as transduction is the transfer of cellular DNA from one host cell to another by means of viruses. In the course of a lytic cycle, as we have noted, viruses exploit the resources of the host cell. During the lytic cycle of many viruses, the host DNA becomes fragmented; when these viruses leave the cell some of them may contain DNA fragments from the host chromosome. Since the amount of DNA that can be packaged within the protein coat is limited, such viruses lack some or all of their own necessary genetic information. Although they may be able to infect a new host cell, they are not able to complete a lytic cycle. However,, the genes they carry from their previous host may become incorporated into the chromosome of the new host. Depending on the genes that are transferred, these recombination’s may be detected. This process is called general transduction because virtually any gene can be transferred by this mechanism. General transduction occurs when a non-temperate bacteriophage infects a bacterial cell. The viral DNA enters the bacterial cell and undergoes a lytic cycle, in the course of this cycle the DNA of the host cell is broken apart, and some of the fragments are accidentally incorporated into newly formed virus particles. When released, a virus particle containing bacterial DNA may infect another bacterial cell. Although such a virus is defective and unable to set up a lytic cycle, the bacterial DNA it has introduced may recombine with the DNA of the new host cell.

When prophages break loose from a host chromosome to initiate a lytic cycle, they may, similarly, take a fragment of the host chromosome with them. The chromosome of each newly formed phage then consists of both host DNA and viral DNA. In this situation, the host DNA is not picked up at random, as is the case with non-temperate phages, but is, quiet specifically, restricted to the portions of the host chromosome adjoining the insertion site of the prophage. Hence this process is known as specialized, or restricted, transduction.

Specialized, or restricted, transduction occurs when a temperate bacteriophage infects a bacterial cell and enters a lysogenic cycle. The viral DNA is incorporated into the host chromosome, where it may remain as a prophage for many generations. When the prophage leaves the bacterial chromosome, it often takes with it a piece of bacterial DNA. In this case, only DNA adjacent to the insertion site of the prophage is picked up with the viral DNA. The linked bacterial and viral DNA is replicated and incorporated into new virus particles that are released from the cell when it is lysed. These particles infect other bacterial cells, and the genes of the first host cell may recombine with those of the new host cell. Viral DNA may also become integrated into the DNA of the new host cell.

Transduction resembles conjugation in that it involves the transfer of bacterial genes from one bacterial cell to another. It differs from conjugation in that in transduction the genes are carried by viruses.

Lamba, the best studied of the temperate bacteriophages, has several interesting and instructive features. When the double-stranded DNA of the virus is packaged in its protein coat, it is linear - that is, it has two free ends. Once the viral chromosome is released into the host cell, however, it forms a circle. This closing of the circle takes place because a single strand of 12 nucleotides protrudes on the 5' end of each strand of the DNA molecule. These strands are exactly complementary to one another and are said to be sticky. Hydrogen bonds form between the complementary base pairs, joining the ends of the molecule together. Such a cohesive nucleotide sequence, which has since been found in other DNA’s, is known in short hand as a COS region.

When the DNA of lambda's circular chromosome is replicated it is rolled out in numerous copies, joined in a single long molecule. A special enzyme then cleaves the molecule over and over at the COS sites and packages the individual lambda chromosomes into the waiting protein coats.

Lambda's integration into the E. coli chromosome takes place because the E. coli chromosome contains a short nucleotide sequence identical to a sequence on the lambda chromosome. Attachment of the bacteriophage chromosome to the bacterial chromosome is brought about by a special enzyme that recognizes both sequences, brings the two circular DNA helixes together, and initiates the cutting and sealing reactions. This enzyme, lambda intergrase, is coded by the lambda chromosome. When lambda leaves the bacterial chromosome to begin a lytic cycle, another enzyme frees it from the bacterial chromosome by making a staggered cut that leaves "sticky" ends protruding. These ends rapidly rejoin one another, forming a circular chromosome once more.

More recently, another type of movable genetic element has been found; it is known as a transpoon. Like episomes and prophages, transpoons are segments of DNA that are integrated into the chromosomal DNA. However, they differ from episomes and prophages in that they contain a gene that codes an enzyme, transposes, that catalyzes their insertion into a new site. Also at each end they have a repeated nucleotide sequence. This sequence may consist of direct repeats such as ATTCAG and ATTCAG or of inverted pairs such as ATTCAG and GACTTA. The repeated sequences are typically 20 to 40 nucleotides in length. At the time of insertion, the target site on the host chromosome - the site at which the transpoon becomes inserted - is duplicated. The target sequence, which is 5 to 10 base pairs in length, then flanks the transpoon. In some cases, the transpoons do not actually move - that is, they do not disappear from their initial site when they appear at a new location. Instead the original, parental transpoon gives rise to a new copy that becomes inserted elsewhere.

Two kinds of transpoons are known, simple and complex, Simple transpoons, also called insertion sequences are only about 600 to 1500 base pairs in length and do not carry any genes beyond those essential for the process of transposition. At least six different simple transpoons have been found in E. coli. They are detectable because they cause mutations. If one of these transpoons becomes inserted into a gene, it inactivates it. Simple transpoons also contain promoter sequences, which may lead to the inappropriate initiation of transcription of previously inactive genes of the host chromosome. Simple transpoons appear to have no function but to duplicate themselves; thus they are among a group of molecules known as selfish DNA. Complex transpoons are much larger and carry genes that code for additional proteins. As in the case with simple transpoons, complex transpoons may cause mutations, but they are also detectable because of their gene products. Genes that are part of a complex transpoon can move from place to place on a chromosome or from chromosome to chromosome, and are therefore known as jumping genes. Drug-resistance genes are often part of transpoons, and so can be transferred readily from Plasmid to Plasmid and from Plasmid to bacterial chromosome to Plasmid again. Complex transpoons are often found to have simple transpoons flanking them - one at each end - which suggests that complex transpoons might have come about by two simple transpoons jumping at the same time, taking with them everything in between.

We have now described four different ways that new, information-carrying DNA can be introduced into a bacterial cell : transformation, which is the uptake of fragments of DNA; conjugation, which is the direct transfer of DNA from one cell to another; viral infection, with the injection of viral nucleic acid; and transduction, the transfer by viruses of non-viral genetic material from one cell to another.

We have also, described two different ways in which genetic recombination can take place. One involves exchange between homologous segments of DNA. When two such segments of double stranded DNA are aligned with one another, exchanges occur between the molecules in such a way that genes may be transferred from one molecule to the other. This phenomenon occurs in eukaryotic cells during meiosis as a result of crossing over, it also takes place during conjugation, transformation, and transduction in bacterial cells. Several models of how homologous recombination takes place have been proposed.

A second kind of recombination involves the insertion of movable genetic elements. These elements can enter or leave the DNA of a chromosome without the occurrence of homologous recombination. The F Plasmid, lambda phage, and transpoons are all examples of this phenomenon.

The many concepts of genes relate to myself and everybody else in many ways. They give me my different traits and if it weren’t for all these concepts I probably wouldn’t even be living. I wouldn’t be able to learn about them in biology and have tremendous fun doing Punnett squares, but then again how can you learn something if you’re not even alive without the many concept’s of genes. Because of genes, a genetic code exists which codes for almost everything in our bodies, at least all the traits. Also, because of this I have to worry about not getting any genetic diseases, but also because of this they are able to cure many diseases.

4. Explain protein synthesis in detail.

Proteins are not built directly from genes. Your cells preserve hereditary information by transferring the information in genes into sets of working instructions for use in building proteins. The working instructions of the genes are made of molecules of ribonucleic acid, or RNA. RNA, like DNA, is a nucleic acid. However, it differs from DNA in three ways. First, RNA consists of a single strand of nucleotides instead of the two strands that form the DNA double helix. Second, RNA contains the five carbon sugar ribose rather than the sugar deoxyribose. And third, RNA has a nitrogen-containing base called uracil – abbreviated as U – instead of the base thymanine found in DNA. Like thymine, uracil is complementary to adenine.

RNA is present in cells in three different forms, each of which has a different function: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). All three types of RNA are essential for processing the information from DNA into proteins, a process called gene expression. Gene expression occurs in two stages – transcription and translation. In transcription, the information in DNA is transferred to mRNA. In translation, the information in mRNA is used to make a protein.

Transcription: Making RNA

The first step in using DNA to direct the making of a protein is transcription, the process that "rewrites" the information in a gene in DNA into a molecule of mRNA. In eukaryotic organisms, transcription occurs inside the nucleus; in prokaryotic organisms, it takes place in the cytoplasm. Transcription begins when an enzyme called RNA polymerase binds to the beginning of a gene on a region of DNA called a promoter. A promoter is a specific sequence of DNA that acts as a "start" signal for transcription. After RNA polymerase binds to a promoter, the enzyme starts to unwind and separate the double helix’s two strands, exposing the DNA’s nitrogen-containing bases. Like DNA replication, transcription uses DNA bases as a template for making a new molecule (RNA). In transcription, however, only one of the two strands of DNA serves as a template.

Once a protein of the DNA double helix has separated, RNA polymerase moves along the bases of the template strand like a train on a track, always in the same direction. The enzyme reads each nucleotide and pairs it with a complementary RNA nucleotide. In eukaryotic cells, the RNA nucleotides are found in the nucleus; in prokaryotic cells, they are in the cytoplasm. Transcription follows the same base-pairing rules as DNA replication except that uracil, rather than thymine, pairs with adenine. When the RNA nucleotides are added, they are linked together with sugar-to-phosphate covalent bonds. As RNA polymerase works its way down the strand, a single strand of RNA grows and dangles off the enzyme like a tail. Behind RNA polymerase, the two strands of DNA close up by forming hydrogen bonds between complementary bases, re-forming the double helix.

Transcription proceeds at a rate of about 60 nucleotides per second until the RNA polymerase reaches a stop signal on the DNA called a terminator. A terminator is a sequence of bases that tells the RNA polymerase to stop adding nucleotides. At this point the enzyme detaches from the DNA and releases the RNA molecule into the cell for the next stage of gene expression. Transcription manufactures three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Messenger RNA is an RNA copy of a gene used as a blueprint for a protein. When a cell needs a particular protein, a specific mRNA is made. Messenger RNA is appropriately named because it carries hereditary information from DNA and delivers it to the site of translation. During translation, mRNA serves as a template for the assembly of amino acids.

The function of tRNA and rRNA differs from that of mRNA. Transfer RNA acts as an interpreter molecule, translating mRNA sequences into amino acid sequences; ribosomal RNA plays a structural role in ribosome's, the organelles that function as the sites of translations. You will learn more about these two molecules when you learn about translation.

After transcription is completed in eukaryotes, a messenger RNA molecule must be processed before it can serve its role in building a protein. Many eukaryotic genes are split into coding regions called exons, which are interrupted by non-coding regions called introns. Both exons and introns are transcribed into mRNA. Before mRNA leaves the nucleus, the introns are cut out. The exons are joined to form a single molecule of mRNA, which leave’s the nucleus through a pore, and enters the cytoplasm.

After transcription, the genetic message is ready to be translated from the language of RNA to the language of proteins. The instructions for building a protein are written as a series of three-nucleotide sequences called codons. Each codon along the mRNA stand either corresponds to an amino acid or signifies a stop signal. Amino acids bond together to form a protein chain.

From trial-and-error experiments, biologists worked out which codons correspond to which amino acids. In 1961, Marshall Nirenberg, an American biochemist, deciphered the first codon by making artificial mRNA that contained only the base uracil. When the mRNA was added to a test tube containing amino acids, it was translated into a polypeptide made of the amino acid phenylalanine. From this, Nirenberg learned that codon UUU is the instruction for the amino acid phenylalaine. Soon, more elaborate techniques enabled scientists to decipher codons consisting of more than one kind of base.

The genetic code is nearly universal. With few exceptions, it is the same in all organisms. For example, the codon GUC codes for the amino acid caline in bacteria, in eagles, in dogs, and in your own cells. Thus, it appears that all life forms had a common evolutionary ancestor with a single genetic code. The only exceptions biologists have found to this rule are in the way cell organelles (such as mitochondria and chloroplasts) and a few microscopic protists (ciliates such as Paramecium) read stop codons.

Translation: Making Proteins

The equipment for translation is located in the cytoplasm, where a cell keeps its supply of transfer RNA (tRNA). A tRNA molecule is a single strand of RNA folded into a compact shape with three loops. One of the loops has a three-nucleotide sequence called an anticodon. It is called an anticodon because the three-nucleotide sequence is complementary to one of the 64 codons of the genetic codes. This enable tRNA to bind to mRNA through hydrogen bonding. In most organisms, there is not tRNA molecule with

An anticodon complementary to the codons UAG, UAA, or UGA, which is why these codons act as stop codons. Opposite the anticodon on a tRNA molecule is a site at which the molecule carries an amino acid. The amino acid that a tRNA molecule can carry corresponds to a particular codon.

A cell’s cytoplasm also contains thousands of ribosome’s, the protein-making factories of the cell. A ribosome, is composed of two sub-units, which are bound together only when they are involved in translation.

A ribosome has three binding sites that play important roles in translation. One binding site hold mRNA so that its codons are accessible to tRNA molecules. The other two binding sites recognize tRNA. The A site holds a tRNA molecule that is carrying its specific amino acid. The P site holds a tRNA molecule that is carrying its specific amino acid attached to the growing protein chain. These two bindings sites are next to each other on the ribosome.

Assembling the Protein

Translation begins when mRNA binds to the smaller ribosomal sub-unit in the cytoplasm. The mRNA in now oriented so that the "start" codon, a codon that signals the beginning of a protein chain, is sitting in the P site. Research has shown that in most cases, the start codon has the sequence AUG. A tRNA molecule with the anticodon UAC can bind to the start codon, carrying with it modified form of the amino acid methionine. A functional ribosome is formed when the mRNA, the two ribosomal sub-unit, and the first tRNA bind together.

Once a complete ribosome has been formed, the codon in the vacant A site is ready to receive the next tRNA. A tRNA molecule with the complementary anticodon arrives and binds to the codon, carrying its specific amino acid with it. Now, both the A site and the P site are holding amino acids. Next, an enzyme helps form a peptide bond between the adjacent amino acids, forming the first link of the protein chain. Afterward, the tRNA in the P site detaches and moves away from the ribosome, leaving behind its amino acid. The tRNA in the A site moves over to fill the vacant P site, with the protein chain in tow. Because the anticodon remains attached to the codon, the tRNA molecule and mRNA molecule move across the ribosome into the P site as a unit. As a result, a new codon is present in the A site, ready to receive the next tRNA and its amino acid. Amino acids are carried to the A site and bonded to the protein chain until the end of the mRNA sequence is reached. At this point, a stop codon is encountered for which, as you know, there is no anticodon. With nothing to fit into the empty A site in the ribosome, the ribosome complex falls apart and the newly made protein is released into the cell.

5. Explain how genes are regulated in prokaryotes.

In order to survive, bacteria must be able to adjust to changes in their environment, such as fluctuations in available nutrients. For example, when the amino acid tryptophan is not present in the environment, the bacterium Escherichia coli must manufacture it from another compound. Later, when tryptophan is present in the environment, the bacterium stops making the amino acid. By being able to adjust its metabolism to changes in its environment, a bacterium is saved from wasting its energy and resources on producing a substance that is readily available.

Scientists first studied gene expression in bacteria. An example of gene regulation that is well understood at the molecular level is found in E. coli. When you consume a milk product, the disaccharide lactose (milk sugar) is soon present in your intestinal tract and available to the E. coli living there. Before a bacterium can absorb the lactose, it must first make beta-galactosidase, an enzyme that cleaves lactose into glucose and galactose.

As with typtophan, it is in a bacterium’s best interest to focus its energy on using available nutrients. Therefore, E. coli should make beta-galactosidase only when lactose is present. To understand how this happens, you must first understand the basic mechanism that controls gene regulation in prokaryotes – the operon. An operon is a cluster of genes that codes for proteins with related functions. The gene for beta-galactosidase is a part of a group of genes called the lac operon. The lac operon is divided into several different regions. One region is the promoter, the RNA polymerase binding site that signals the beginning of a gene. Another region is made of structural genes, the genes that code for polypeptides. Between the promoter and the structural genes is a region of DNA called the operator. Because of its position in the operon, the operator is able to control RNA polymerase’s access to the structural genes; it acts like a switch, turning the operon on or off.

What determines whether the lac operon in the "on" or "off" mode? When no molecules are bound to the operator, the lac operon is switched on; RNA polymerase can bind to the promoter, move across the operator, and transcribe the structural genes. The lac operon is switched off when a protein called a repressor is bound to the operator. A bound repressor creates a barrier, preventing RNA polymerase from transcribing the structural genes (although RNA polymerase can still bind to the promoter).

Transcription can resume when the repressor is removed by a molecule called an inducer. In the case of the lac operon, the inducer is allolactose, a molecule that is made from lactose when it enter the cell. When allolactose is present in the cell, it binds to the repressor and changes the shape of this protein. As a result, the repressor falls off the operator. Now, RNA polymerase has access to the structural genes. The bacterial cell can begin building the proteins it needs to metabolize lactose.

When lactose is not present in the intestinal tract, allolactose is not produced. Therefore, there is nothing to change the shape of the repressor, which remains bound to the operator. As a result, transcription of the lac operon is blocked and the structural genes remain unexpressed. By producing the enzymes only when the nutrient is available, the bacterium avoids wasting its energy making proteins it does not need.

6. Compare gene regulation in eukaryotes and prokaryotes.

Like prokaryotic cells, eukaryotic cells must continually turn certain genes on and off in response to signals from their internal and external environments. After operons were discovered in the 1960s, molecular biologists studying eukaryotes expected to find similar mechanisms for regulating gene expression. However, operons have not been found in eukaryotic cells. Instead, genes with related functions are often scattered among different chromosomes. This is just one of the many differences between a prokaryotic genome and a eukaryotic genome. In addition, a eukaryotic cell contains much more DNA than a prokaryotic cell, and most eukaryotes are multi-cellular organisms made of specialized cells. Different cell types produce different proteins. For example, the gene that enables a red blood cell to produce hemoglobin is also present in all the other cell types, but it is unexpressed. Gene expression in eukaryotes must involve mechanisms that account for differences such as these. Once a eukaryotic genome is available to be expressed, what kinds of mechanisms regulate its expression? As with prokaryotes, gene expression can be regulated before, during, or after transcription. However, because a eukaryotic cell has a nuclear envelope that physically separates transcription from translation, there are more opportunities for regulating gene expression. Control mechanisms have been found to occur after the mRNA leaves the nucleus, before translation, and even after the protein is functional.

Eukaryotic cells contain a chromosome puff, unlike p[prokaryotic cells. A chromosome puff is a region of intense transcription that forms when DNA loops out from the chromosome, perhaps making the genes in that region more accessible to RNA polymerase. Chromosome puff’s sometimes tend to change locations. The main mechanism that regulates gene expression in prokaryotes is the operon, while in eukaryotes the mechanism is the enhancer. An operon is a cluster of genes that codes for proteins with related functions and the enhancer is a sequence of nucleotides that, when activated by specific signal proteins, aids in exposing the RNA polymerase binding site of a specific gene. Both of these mechanisms can be turned on and off by proteins. An enhancer is different from the operon in that it is a sequence of nucleotides while the operon is a cluster of genes. Both the mechanisms transcribe RNA polymerase The enhancer is activated by a protein while the operon is deactivated by a protein called a repressor which can later be token of by an inducer so transcription can begin again. This is different in prokaryotic cells because prokaryotic cells contain an operator which is a piece of DNA between the promoter, which starts as a start signal, and the structural genes, where the RNA polymerase is transcribed to. Eukaryotic cells also has enhancer regions in the DNA which an acceptor protein binds to, to activate transcription instead of deactivating it. Not much is known about gene regulation in eukaryotes, and certainly more is known about gene regulation in prokaryotes.

7. Explain how mutations can cause cancer.

Of all the health problems that can afflict humans, few are as mysterious as cancer. Cancer is a term used to indicate a disease characterized by abnormal cell growth. Normally, cell growth is a highly regulated process that is controlled by chemical and physical signals that inhibit or stimulate cell division. A cell that has become cancerous does not respond to the signals that stop cell division. As a result, cancerous cells, divide without stopping.

Health problems begin when a cell that has turned cancerous evades the body’s immune system, which normally destroys cancerous cells. The Cancerous cell then proliferates, forming a mass of cells called a tumor. Tumors are classified as either benign or malignant. A benign tumor does not invade surrounding tissues. They usually cause few problems and can be surgically removed. Tumors described as malignant, on the other hand, are very harmful. A malignant tumor spreads into other tissues and interferes with organ functions. The most devastating property of a malignant tumor is that its cells are able to break free of the tumor and enter blood and lymph vessels. These cancerous cells are then carried to new locations in the body and form new growths. The spread of malignant cells beyond their original site is called metastasis.

Cancer is a disease in which cells escape the factors, still largely unknown, that regulate normal cell growth. As a consequence, the cells multiply out of control, crowding out, invading and destroying other tissues. Cancer is often considered a group of diseases rather than a single disease because, with few exceptions, any one of the 200 or more cell types in the human body can become malignant. The behavior of the cells and the prognosis of the illness depend on the type of cells that have become malignant.

Three lines of evidence have long linked the development of cancer with changes in the genetic material. First, once a cell has become cancerous, all of its daughter cells are cancerous; in other words, cancer is an inherited property of cells. Second, gross chromosomal abnormalities, such as deletions and translocations are often visible in cancer cells. Third, most carcinogens - agents known to cause cancer, such as x-rays, ultraviolet radiation, tobacco smoke, and a variety of chemicals are also mutagens.

As long ago as 1911, a cancer-causing virus, the Rous sarcoma virus, was isolated from chicken tumors. ( In those days, a virus was defined simply as "a cell-free extract that produces a disease when it is injected into a suitable host.") Even thought other cancer-causing viruses, particularly viruses affecting laboratory mice, were gradually discovered, a viral theory of cancer was slow to emerge. For one thing, viruses could not be shown to be important in causes of human cancer. ( Even today, after years of searching, only a few rare human cancers have been linked to viruses.) For another, the "viral theory" of cancer seemed to be at odds with the "mutation theory". In addition, the fact that most of the known cancer-causing viruses, including the Rous sarcoma virus, are RNA viruses rather than DNA viruses also seemed to make these two hypothesis incompatible.

Eventually, however, evidence emerged that viruses, like mutagens, can bring about changes in the cell’s genetic makeup and that, furthermore, all known cancer-causing viruses are viruses that introduce information into host cell chromosomes. These include both DNA viruses, like SV40, and RNA retroviruses. The discovery of the role of reverse transcriptase forged the crucial link between retroviruses and the chromosomes of eukaryotic cells. The Rous sarcoma virus has been shown to be a retrovirus, and DNA segments produced by reverse transcriptase from it s RNA have been located in host cell chromosomes.

Recombinant DNA techniques have enabled molecular biologists to study some of the changes in eukaryotic chromosomes that lead to cancer. When such cells are exposed to a cancer-causing agent, such as a virus, they may undergo characteristic changes in their growth patterns and in their shape. Such cells are said to be transformed. Transformed cells can produce cancers when they are transplanted into laboratory animals. (Not the transformation has two meanings in biology: one is the introduction of new characteristics into a cell by means of DNA from another cell, as in the pneumococcus experiments of some 60 years ago; the other is the induction of cancer.)

Studies of transformed cells have uncovered a group of genes known as oncogenes (from the Greek word onkos, meaning "tumor"). Oncogenes closely resemble normal genes of the eukaryotic ells in which they are found. According to the oncogene hypothesis, cancer is caused when something goes wrong in the expression of these normal cellular genes, as a result of mutations in the genes themselves, changes in gene regulation, or both. Thus viruses can cause cancer in three different ways. First, simply by their presence in the chromosomes, viruses may disrupt the function of normal genes. Second, viruses may encode proteins needed for viral replication that also affect the regulation of cellular genes. Third, and most interesting of all, viruses may serve as vectors of oncogenes. In fact, oncogenes were first discovered when genetic analyses of cancer-causing retroviruses revealed the presence of genes that were not required by the viruses for their own multiplication. It was subsequently found that the nucleotide-sequence of these genes not only closely resemble those of normal genes of the host cell but also cause malignant transformation of the cells. With these discoveries, the "viral theory" and the "mutation theory" of cancer are no longer regarded as incompatible but rather as mutually supportive. About 50 oncogenes have been discovered so far. Their gene products that have been identified all seem to be regulatory proteins of some sort, involved with the control of either cell growth or cell division. Thus, this work not only is bringing us closer to the control of one of our oldest and ugliest enemies but also is yielding new information on the fundamental question of the regulation of cell growth.

Through many years of intense work, researchers have learned that a cell becomes cancerous when mutations occur in genes that regulate cell growth. An example of growth-regulating genes is a class of genes called the ras genes (because they were first discovered in viruses that cause tumors known as sarcomas in rats). The ras genes code for proteins that help prevent uncontrolled cell division. When there is a mutation in a ras gene, a faulty ras protein is built. As a result, the cell divides more rapidly than normal, a sign that it has become cancerous. The ras genes are examples of oncogenes. An oncogene is a gene that, when mutated, can cause a cell to become cancerous.

Researchers have found that a mutated ras gene usually contains a point mutation. For example, in a form of human bladder cancer caused by a mutated ras gene, a single G nucleotide has been replaced with a T, transforming the normal amino acid glycine into a valine. However, the ras gene is only one of several controls that the cell normally exercises over unwanted growth. All of these controls must be inactivated before cancer results. This is why most cancers occur in people over 40 years old – it takes time for an individual cell to accumulate the necessary mutations.