RNA polymerases (represented by the blue circle in the diagram to the left) cover an area about 10 nucleotides long where they pry the two strands of DNA apart and hook together the RNA nucleotides as they base pair along the DNA template. Specific sequences of nucleotides along the DNA mark the initiation and termination sites, where transcription of a gene begins and ends. Transcription progresses at a rate of about 60 nucleotides per second. If a certain protein is needed in large amounts, the same gene can be transcribed simultaneously by several molecules of RNA polymerase. In eukaryotes, RNA processing occurs in the nucleus before the transcribed mRNA leaves to go to the ribosomes. During RNA processing ribozymes remove introns from the transcribed portion. Introns are segments of the DNA that are not part of the gene. The parts that remain are called exons because they are ultimately expressed. Following transcription and RNA processing, mRNA moves through the nuclear pores to the ribosomes in the cytoplasm. Because it carries the instructions from the nucleus to the cytoplasm it is called messenger RNA or mRNA for short.

During translation, the instructions contained in the sequence of the nitrogen bases of the mRNA are used to direct the sequence of amino acids assembled into a polypeptide and eventually a protein. The sites of translation are the ribosomes, complex particles with many enzymes and other agents that facilitate the orderly linking of amino acids into polypeptide chains.

The information in the mRNA base sequence is in a triplet code. [How many triplets can there be? 64] Each three letter unit of the molecule is called a codon. All 64 mRNA codons have been determined. Sixty one codons correspond to a particular amino acid. The remaining three are stop codons which signal polypeptide termination. The one codon that stands for methionine is also a start codon signaling polypeptide initiation.

The DNA code is universal. The same codons stand for the same amino acids in most bacteria, protists, plants, and animals. [What does this suggest? This illustrates the remarkable biochemical unity of living things and suggests that all living things have a common ancestor.]

Translation begins when the two subunits of the ribosome bind to the mRNA strand and translation is initiated at the start codon (AUG) thus establishing the reading frame [the red dog ate the cat]. The ribosome reads along the mRNA strand in one direction. The ribosome binds with two codons of the mRNA at any one time. The ribosome relies on the transfer RNA molecules to bring it the correct amino acids which it adds to the polypeptide chain.

The tRNA molecules are ultimately responsible for translating the base code of the messenger RNA. tRNA' s use energy to form a high energy bond with an amino acid forming tRNA~complex. Each type of tRNA bonds to a specific amino acid and carries it to the ribosome. The tRNA has an anticodon on its lower portion that varies according to the type of amino acid it carries.

We said already that the ribosome binds with two codons at a time. It does this at the P site and A site. The P site is the trailing site and has a tRNA and the growing polypeptide chain. The A site is the leading site and it is here that the tRNA with the complementary anticodon will bring the appropriate amino acid. Once at the A site, the tRNA's amino acid is joined to the growing polypeptide chain. This releases the tRNA that was carring the chain from the P site.

The tRNA that was in the A site moves to the P site allowing the ribosome to move forward a distance of one codon. Now the A site is again empty for the tRNA with the complementary anticodon to arrive with the next amino acid.

The released tRNA's are recycled and this process continues (elongation) until the ribosome encounters a stop codon on the mRNA (termination). This codon does not code for an amino acid. Instead it causes a release factor to bind to the termination codon in the A site. This releases the polypeptide from the tRNA in the P site by adding a water molecule to it. In some cases the polypeptide will have to be modified before it can perform its function or it may have to bind with another polypeptide to complete its quaternary structure.

A single ribosome can make an average sized polypeptide in less than a minute. Typically, a single mRNA is used to make many copies of a polypeptide simultaneously by several ribosomes working at the same time. We call these clusters polyribosomes.

[Protein Targeting - The synthesis of all polypeptides begins in the cytoplasm. The first 20 amino acids of the polypeptide, called the signal sequence, tell the ribosome to either stay in the cytoplasm or to attach itself to an ER. Other signal sequences target proteins for the mitochondria or chloroplasts after the proteins are released from the ribosomes.]

[The information in the mRNA base sequence is in a triplet code. Each three letter unit of the molecule is called a codon.]

1) A tRNA molecule with the anticodon GCU would be carrying which amino acid

2) Determine the sequence of amino acids produced by this DNA sequence:

GGAGTTTTCGCT.

(See KEY on bottom of last page for answers)

Mutations are inheritable changes in the genetic material of an organism. Mutations may take place in any cell. Cosmic rays, X rays, ultraviolet radiation, and chemicals that alter the DNA are called mutagenic agents (or mutagens). By changing the arrangement of the nucleotides in the double helix, the mutagen changes the genetic code. The shift in a single nucleotide will lead to the production of a new protein from the instructions. The new protein has a different chemical structure and, in most cases, is incapable of carrying out the function of the required protein. Without the required protein, cell function is impaired, if not completely destroyed. Although some mutations can, by chance, improve the functioning of the cell, the vast majority of mutations produce adverse effects.

Ex. Occasionally, X rays will break the backbone of the DNA molecule. Special enzymes will repair the break but the spliced segment may not get placed in the proper position. The misplaced segment may alter the entire library of genetic information.

Germ cell mutations occur in sex cells, such as eggs and sperm. They do not affect the organism itself but are passed on to offspring. Somatic mutations take place in body cells. They are passed on to daughter cells through mitosis.

Chromosome mutations often occur during cell division. Deletion occurs when a piece of chromosome breaks off. All the information on that piece is lost. Inversion occurs when a piece breaks from a chromosome and reattaches itself to the chromosome in the reverse orientation. Translocation occurs when a broken piece attaches to a nonhomologous chromosome. Another kind of chromosomal mutation, called nondisjunction, occurs when a replicated chromosome pair fails to seperate during cell division. When nondisjunction occurs, one daughter cell recieves an extra copy of a chromosome, and the other daughter cell lacks that chromosome entirely. In humans, for example, if nondisjunction occurs during sperm formation, one sperm cell may have 22 chromosomes, and the other may have 24. If one of these gametes combines with a normal egg, the zygote will have either 45 (monosomy) or 47 (trisomy) chromosomes. [An extra chromosome 21, for example, results in Down syndrome, a disorder characterized by mental retardation, a fold of skin above the eyes, and weak muscles. Klinefelter syndrome results from the trisomic genotype XXY. Klinefelter individuals may be mentally retarded and have low fertility. Turner syndrome is a monosomic condition with the genotype XO. An XO female is characterized by immature physical development, sterility, and a webbed neck.]

Gene mutations arise from mistakes in DNA replication. When one nitrogen base is substituted for another, added, or deleted a point mutation has occured. The addition or deletion of a nitrogen base is a point mutation called a frameshift mutation (WHY?). Very serious. The substitution of a nucleotide will sometimes have no effect because of the redundancy of the genetic code. Other substitutions called missense mutations lead to the production of a different protein because one amino acid has been changed. One well known genetic mutation is a human disorder called sickle-cell anemia. See Figure 24.17 on page 479. This genetic disorder affects the structure of the oxygen-carrying molecule found in red blood cells. The alteration of a single nitrogen base causes valine to replace glutamate as the sixth amino acid in one of the protein chains. Even this slight change has devastating consequences. The red blood cell assumes a sickle shape and is unable to carry an adequate amount of oxygen. To make matters worse the sickle-shaped cells clog the small capillaries, starving the body's tissues of oxygen.

If a base substitution results in a stop codon [what would happen?] transcription is terminated and an incomplete polypeptide results. These are called nonsense mutations. A substitution having this effect may be the cause of the X-linked clotting disorder known as [?] hemophilia. The most common type of hemophilia results from the absence or minimal presence of a particular clotting factor called factor VIII.

KEY Answer 1: Arginine. Answer 2: Proline, Glutamine, Lysine, Arginine.