REPRODUCTION

The Function of the Nucleus within the Cell

The Nucleus controls the activities of all the parts of the cell. Those parts include:

Cell Membrane - Surrounds both animal and plant cells separating the cytoplasm from the surrounding environment. The organelles have membranes as well that separate their contents from the surrounding cytoplasm.

In addition to a cell membrane all plant cells have a primary cell wall and some have a secondary cell wall that forms inside the first. The cell wall lends support to the plant cells.

Nucleus - The nucleus is a large organelle near the center of eukaryotic cells. Its contents are separated from the rest of the cytoplasm by a double membrane called the nuclear envelope. Pores in the nuclear envelope allow large molecules to pass into and out of the nucleoplasm. DNA is contained in a threadlike material called chromatin located within the nucleus. Chromatin is nondistinct in non-dividing cells but condenses into chromosomes at the time of cell division. DNA directs protein synthesis making the nucleus the control center of the cell because it is a cell's proteins help determine its structure and function.

Nucleolus - One or more nucleoli are present in the nucleus. Nucleoli are specialized parts of chromatin in which ribosomal RNA is produced from DNA located there. Ribosomal RNA is necessary to the formation of small ribosomes that function in the cytoplasm.

Ribosomes - formed in the nucleolus as two subunits. The two subunits are not assembled until they reach the cytoplasm. Ribosomes are the site of protein synthesis in the cytoplasm. Free ribosomes manufacture proteins for use in the cell. Ribosomes attached to the Endoplasmic reticulum make proteins for export.

Endoplasmic Reticulum - The ER forms a membranous system of tubular canals throughout the cytoplasm and continuous with the nuclear membrane. The ER serves as a transport system to the Golgi apparatus. There are two types of ER. Smooth ER - (no ribosomes attached) produces steroid hormones and is involved in the detoxification of drugs, including alcohol. The smooth ER also synthesizes lipids. Rough ER - (ribosomes attached) specializes in protein synthesis. The Rough ER has attached ribosomes that make proteins (such as digestive enzymes) and a vast surface area for exporting them from the cell. The proteins enter the interior space of the rough ER. Then a vesicle pinches off and carries the protein to the Golgi apparatus.

Golgi Apparatus - Composed of a stack of about a half-dozen or more saccules (flattened vacoules) which provide a vast surface area from which proteins can be packaged and exported. Vesicles occur at the edges of the saccules. The Golgi apparatus functions in the packaging, storage, and distribution of molecules produced by the ER. Finally molecules are often packaged in secretory vesicles. These move to the cell membrane and discharge their contents.

Vacuoles - A vacuole is a large membrane enclosed sac. A vesicle is a small vacuole. Vacuoles are more prominent in plant cells. Vacuoles in plant cells are filled with watery fluid, which gives added support to the cell. Most of the central area of the plant cell is occupied by a vacuole. Most often vacuoles are storage areas. Plant vacuoles contain water, sugar, salts and pigments responsible for the many colors of flowers and some leaves. Some vacuoles contain toxic substances to protect the plant from predacious animals.

Mitochondria - Mitochondria are bounded by a double membrane. The inner membrane is folded to form little shelves called cristae. Mitochondria produce ATP. All cells use ATP energy to synthesize molecules and many use it to carry out specialized functions such as muscle contraction and nerve impulse conduction. Mitochondria are called the powerhouses of the cell because they convert the chemical energy of glucose products into the chemical energy of ATP molecules. In the process, mitochondria use up oxygen and produce carbon dioxide and water. Because gas exchange is involved, it is said that mitochondria carry on aerobic cellular respiration.

Chloroplasts - Plant cells also have organelles called chloroplasts that use the energy from the sun to make glucose (sugar)

The two major functions of proteins: Structure and Metabolism.

Protein macromolecules sometimes have a structural function. For example, in humans, the protein keratin makes up hair and nails, while collagen is found in all types of connective tissue, including ligaments, cartilage, bones, and tendons. The muscles contain proteins, which account for their ability to contract.

Some proteins are enzymes, necessary contributors to the chemical workings of the body. Enzymes speed up chemical reactions; they work so quickly that a reaction that normally takes several hours or days without an enzyme takes only a fraction of a second with an enzyme.

DNA is used and copied thousands of times with very little change It is found in every cell of every organism, deoxyribonucleic acid is the only molecule known that is able to replicate itself and correct errors, thereby allowing cell division. DNA provides the directions for the building of new cells and for the repair of worn cells. DNA is most often described as a double helix. DNA closely resembles a twisted ladder. Sugar and phosphate molecules form the backbone of the ladder, while the nitrogen bases form the rungs. Nitrogen bases from one spine of the ladder are connected by weak hydrogen bonds to the nitrogen bases on the other side of the ladder. Complementary base pairing describes the behavior of the nitrogen bases. In DNA cytosine always pairs with guanine and adenine always pairs with thymine. A genes meaning to the cell is encoded in its specific sequence of the four bases. The linear order of bases encoded in a gene specifies the amino acid sequence of a protein, which then specifies that protein's function in the cell. The genetic code is contained in 46 seperate chromosomes in your body.

This Diagram shows how proteins are produced.

Protein Synthesis is a very complicated process. Here is a short summary:

After a signal to start making a protein a strand of mRNA is made from the DNA. That strand of mRNA leaves the nucleus through one of the pores in the nuclear membrane. At the ribosome that strand of mRNA provides the instructions to the ribosome for how the protein should be assembled. Once assembled the protein travels through the ER and then the Golgi Body for packaging. Lastly it will leave the cell through the membrane headed to where it is needed. Some proteins are also made for use in the cell.

Mutation

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 bases (C, G, A, and T) in the double helix, the mutagen changes the genetic code. The shift in a single base 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 (Positive Mutations), the vast majority of mutations produce adverse effects (Negative Mutations). Some mutations have no effect on the functioning of the cell (Neutral Mutations)

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.

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 (Neutral Mutation). Other substitutions 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. 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.

The Cell Cycle and Mitosis

The Major events in the cell cycle are Interphase, Mitosis, and Cytokinesis

Interphase - Chromosomes are not visible because they are uncoiled

Overview of the major events in mitosis.

Prophase - The chromosomes coil. The nuclear membrane disintegrates. Spindle fibers (microtubles) form.

Metaphase - The chromosomes become aligned at the equator.

Anaphase - The chromatids separate; the number of chromosomes doubles.

Telophase - One complete set of chromosomes at each pole. The chromosomes uncoil. The nucleus reforms. The spindle apparataus disassembles.

Cytokinesis is the final stage of the cell cycle which seperates the two nucleii into two daughter cells.

(The essential fact about mitosis is that at metaphase each duplicated chromosome lines up on the metaphase plate and the two sister chromatids separate and move to opposite poles of the cell. • Since the two sister chromatids are the same this ensures that each of the unique DNA molecules in the cell are transmitted to the next generation.)

Cancer cells have characteristics indicating a severe failure in the control of gene expression.

Normal cells only divide about 50 times, but cancer cells enter the cell cycle over and over again and never fully differentiate. This process is also called anaplasia. In the body, cancer cells produce a tumor, which invades and destroys neignboring tissue. The nondifferentiated cells are disorganized and do not function as they should.

In tissue culture, normal cells (shown left and immediately below) grow in only one layer because they adhere to the glass and stop dividing when they make contact with their neighbors.

Cancer cells cells (shown left and immediately below) have lost this contact inhibition and grow in multiple layers. Cancer cells (left) also exibit abnormal (larger) nuclei.

To support their growth cancer cells release a growth factor that causes neighboring blood vessels to branch into the cancerous tissue, a process called vascularization.

(d) In a process called metastasis, cancer cells detach from the tumor and spread around the body. Cancer cells produce hydrolytic enzymes which enable them to invade underlying tissues. After travelling through the blood vessels or the lymphatic vessels, cancer cells start new tumors elsewhere in the body. This is why early detection is so important in treating cancer. If the cancer is found before metastasis the chance of curing it is much greater.

The danger signals that can indicate the possibility of a cancerous tumor are:

unusual discharge or bleeding,

a lump or thickening of the breast or anywhere,

a sore that does not heal,

change in bowel or bladder habits,

persistent cough or hoarseness,

persistent indigestion or difficulty in swallowing

or a change in a wart or mole.

 

Use examples to outline the roles of initiators and promoters in carcinogenesis

Carcinogenesis - the development of cancer

Initiators are generally mutagens (radiation, X rays, chemicals), compounds that cause changes in the DNA in a cell that helps bring on cancerous growth in the future. [A papilloma viral infection , the cause of genital warts may lead to cervical cancer. Cigarette smoke contains chemical carcinogens and causes lung cancer.]

Promoters speed up the expression of cancerous growth by stimulating the cell to multiply. It is also possible that a promoter only provides the environment that causes mutated cells to form a tumor. [There is some evidence to suggest that a diet rich in saturated fats and cholesterol is a cancer promoter. Considerable time may elapse between initiation and promotion. This is why cancer is more often seen in older than younger individuals.]

 

Demonstrate a knowledge of how a virus can bring about carcinogenesis

A virus infection can also introduce an oncogene or an enhancer into a cell, making it cancerous. For example, a papilloma viral infection, the cause of genital warts, may lead to cervical cancer.

The following terms are often used to describe a cancerous tumor.

1) Benign - slow growth rate, contain more differentiated cells, do not metastisize

2) Malignant - aggressive, grow rapidly, contain more undifferentiated cells, metastisize

To download the powerpoint we watched in class that summarizes mitosis, DNA, and cancer click: www.bios.niu.edu/johns/bios103/Mitosis.ppt

Asexual Reproduction

In many simple organisms, reproduction is not a very complicated thing. It generally involves only one organism. The resulting offspring often have the exact same genetic information as the parent. This type of reproduction in which one parent is involved in the production of an identical offspring is called asexual reproduction.

 

The main ways that asexual reproduction can take place are:

 

Binary Fission: A situation in which the parent cell splits in half producing two identical cells. Binary fission is the form of asexual reproduction used by most prokaryotes (bacteria) and protists to reproduce. This process results in the reproduction of a living cell by division into two equal or near-equal parts.

 

Fragmentation is another way to reproduce asexually. The parent breaks into different fragments, which eventually form new individuals. This process is exemplified by certain flatworms known as planarians. This type of reproduction would also occur in molds, yeast, and mushrooms, all of which are part of the Fungi family of organisms. These organisms produce tiny filaments called Hyphae. When a piece of hyphae breaks off and grows into a new individual, this is called fragmentation.

 

In regeneration, when an animal that is capable of regeneration loses a body part, it can grow a replacement part. If the lost body part contains enough genetic information from the parent, it can regenerate into an entirely new organism. Echinoderms are examples of animals that use regeneration.

 

Budding: When conditions are favorable. yeast cells can reproduce through budding. Once a copy of the genetic material is made, a bud begins to form outside the body of the yeast cell. It continues to grow larger until, eventually, it breaks away to form a new individual cell.

 

Vegetative reproduction is form of duplication using only mitosis also called vegetative propagation or vegetative multiplication. It is a process by which new plant "individuals" arise or are obtained without production of seeds or spores. Example, a new plant grows out of the root or a shoot from an existing plant. Vegetative reproduction produces only genetically identical offspring since all divisions are by mitosis. Offspring are called clones meaning that each is an exact copy of the original organism. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony. This method of reproduction is rapid and effective allowing the spread of an organism. It is both a natural process in many plant species and one utilized or encouraged by horticulturists to obtain quantities of economically valuable plants.

 

Advantages of Asexual Reproduction

 

Asexual reproduction can be very advantageous to certain animals. For instance, animals that remain in one particular place and are unable to look for mates would need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually. The cloned offspring are more likely to succeed in the same stable areas as their parents.

 

For photos of different species undergoing asexual reproduction go to: Asexual Reproduction

Human-assisted Cloning

Dr. Najeeb Layyous' Web Site

Meiosis

Sexual reproduction requires two parents who randomly contribute half their genetic material two a new offspring resulting in genetic diversity through a new combination of the genetic material that exists in the species. Genetic diversity ensures survival of a species in a changing environment because some individuals should be born better equipped to meet the challenges. Overview of the major events in meiosis.

The object of meiosis is to generate a random assortment of chromosome combinations in cells that have half the normal number of chromosomes. In a diploid organism that means the formation of haploid cells that carry a random collection of chromosomes. The only way to produce cells with half their normal number of chromosomes is to divide twice without allowing DNA replication to occur twice.

Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material.

Meiosis comprises two successive nuclear divisions with only one round of DNA replication.

Four stages can be described for each nuclear division.

* Interphase: Before meiosis begins, genetic material is duplicated.

* First division of meiosis

Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. (Crossing-over can occur during the latter part of this stage.)

Metaphase 1: Homologous chromosomes align at the equatorial plate.

Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

* Second division of meiosis: Gamete formation

Prophase 2: DNA does not replicate.

Metaphase 2: Chromosomes align at the equatorial plate.

Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.

Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.

Sexual Reproduction

In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population. New gene combinations for the species are the advantage of sexual reproduction. The disadvantage is that sexual reproduction requires organisms to find a mate (consuming time and energy, and increasing predation risk) and it is more complicated.

Gametes - In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by meiosis. The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes). In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete.

Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid organisms/cells have only one set of chromosomes, abbreviated as n. Chromosomes that carry the same genes are termed homologous chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent. There are 44 autosomes and 2 sex chromosomes in the human genome, for a total of 46. (2n=46) (n=23)

Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

Assisted Reproductive Technologies

Biomedical technology involves the application of engineering and technology principles to the domain of living or biological systems. Usually biomedical denotes a greater stress on problems related to human health and diseases. Biomedical engineering combined with Biotechnology is often called Biomedical Technology or Bioengineering. Biological engineers are similar to biologists in that they study living organisms. They are engineers because they have a practical design aim in mind - they use research to create usable tangible products. In general, biological engineers attempt to 1) mimic biological systems in order to create products or 2) modify and control biological systems so that they can replace, augment, or sustain chemical and mechanical processes.

 

Genetic technologies involve changing the genes in a living cell. There are two types of genetic modification: non-inheritable genetic modification (somatic) and inheritable genetic modification (germline). Non-inheritable genetic modification changes the genes in cells other than egg or sperm cells. Diseases caused by defective genes could be treated by modifying the genes in affected cells. These changes are not passed to future children. Applications of this sort (such as gene therapy) are being pursued in clinical trials, and are generally considered to be socially acceptable.

 

Reproductive technology is a term for all current and anticipated uses of technology in human and animal reproduction, including:

    * artificial insemination

    * artificial wombs

    * cloning (see human cloning for the special case of human beings)

    * cryopreservation of sperm, oocytes, embryos

    * embryo testing

    * embryo transfer

    * genetic engineering

    * hormone treatment to increase fertility

    * in vitro fertilization

          o intracytoplasmic sperm injection

    * in vitro parthenogenesis

    * preimplantation genetic diagnosis (PGD)

    * reprogenetics

    * sperm selection

    * Testicular sperm extraction (TESE)

 

Assisted reproduction or assisted reproductive technology (ART) is sometimes used as a term for fertility treatment using reproductive technology.

Contraception may also be viewed as a form of reproductive technology, as it enables people to control their fertility.

Many issues of reproductive technology have led to ethical issues being raised, since it often alters the assumptions that lie behind existing systems of sexual and reproductive morality.

 

There are risks and benefits associated with biotechnology. For example, the removal of hemophilia or other serious disorders from the gene pool is a benefit because people would no longer suffer from a chronic condition. An example of a risk is going too far in selecting the genetic makeup of future children.

 

Possible risks:

- Relying on eugenics, or selecting the genetic makeup of future children. This practice may give people the power to control some personal traits, such as having blond hair or being tall. Taken to an extreme, this could eliminate some traits.

- Using biotechnology before exploring other options, particularly in reproductive medicine. For example, technology enables scientists to implant an egg from one woman into the uterus of another. But it may not be a good idea to use this technique before trying less extreme techniques first.

 

Possible benefits:

- Eliminating genetic diseases. For example, geneticists think it may be possible to eliminate genetic diseases such as Tay-Sachs through careful and methodical screening programs.

- Screening unborn babies. This refers to screening for genetic disorders either before a pregnancy takes place or in the early months of a pregnancy. More information would give prospective parents more options in dealing with their infants' problems.

- Treating diseases. For example, scientists are working on ways to insert cells from embryos into cancerous cells as a way to stop the growth of cancer.

 

Biotechnology is a powerful tool and scientists have had to consider many ethical issues surrounding it. As a result, the new field of bioethics has emerged. Bioethics is the study of the ethical implications of biological research and applications, especially in medicine; it involves examination of the benefits and the risks of biotechnology.