CELL DIVISION (MITOSIS AND MEIOSIS): CELL PHYSIOLOGY

CELL DEVISION: MITOSIS AND MEOSIS

1. CELL DEVISION SURENDRA K JOGPAL NURSING TUTOR

2. CELL DEVISION Most cells of the human body undergo cell division, the process by which cells reproduce themselves. The two types of cell division—somatic cell division and reproductive cell division

3. CELL DEVISION A somatic cell ( soma body) is any cell of the body other than a germ cell. A germ cell is a gamete (sperm or oocyte) or any precursor cell destined to become a gamete. In somatic cell division, a cell undergoes a nuclear division called mitosis and a cytoplasmic division called cytokinesis to produce two identical cells, each with the same number and kind of chromosomes as the original cell. Somatic cell division replaces dead or injured cells and adds new ones during tissue growth. Reproductive cell division is the mechanism that produces gametes, the cells needed to form the next generation of sexually reproducing organisms. This process consists of a special twostep division called meiosis, in which the number of chromosomes in the nucleus is reduced by half.

4. CELL CYCLE The cell cycle is an orderly sequence of events by which a somatic cell duplicates its contents and divides in two. Human cells, such as those in the brain, stomach, and kidneys, contain 23 pairs of chromosomes, for a total of 46. One member of each pair is inherited from each parent. The two chromosomes that make up each pair are called homologous chromosomes When a cell reproduces, it must replicate (duplicate) all its chromosomes to pass its genes to the next generation of cells. The cell cycle consists of two major periods: interphase, when a cell is not dividing, and the mitotic (M) phase, when a cell is dividing

5. A. Interphase During interphase the cell replicates its DNA Interphase is a state of high metabolic activity; it is during this time that the cell does most of its growing. Interphase consists of three phases: G1, S, and G2. The S stands for synthesis of DNA. Because the G-phases are periods when there is no activity related to DNA duplication, they are thought of as gaps or interruptions in DNA duplication.

6. 1. G1 Phase The G1 phase is the interval between the mitotic phase and the S phase. During G1, the cell is metabolically active; it replicates most of its organelles and cytosolic components but not its DNA. Replication of centrosomes also begins in the G1 phase. Virtually all the cellular activities happen during G1. For a cell with a total cell cycle time of 24 hours, G1 lasts 8 to 10 hours. However, the duration of this phase is quite variable. It is very short in many embryonic cells or cancer cells. Cells that remain in G1 for a very long time, perhaps destined never to divide again, are said to be in the G0 phase. Most nerve cells are in the G0 phase. Once a cell enters the S phase, however, it is committed to go through cell division.

7. 2. S Phase The S phase, the interval between G1 and G2, lasts about 8 hours. During the S phase, DNA replication occurs. As a result, the two identical cells formed during cell division will have the same genetic material.

8. 3. G2 Phase The G2 phase is the interval between the S phase and the mitotic phase. It lasts 4 to 6 hours. During G2, cell growth continues, enzymes and other proteins are synthesized in preparation for cell division, and replication of centrosomes is completed. When DNA replicates during the S phase, its helical structure partially uncoils, and the two strands separate at the points where hydrogen bonds connect base pairs.

9. B. MITOTIC PHASE The mitotic (M) phase of the cell cycle consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) to form two identical cells. The events that occur during mitosis and cytokinesis are plainly visible under a microscope. NUCLEAR DIVISION: MITOSIS Mitosis is the distribution of two sets of chromosomes into two separate nuclei. The process results in the exact partitioning of genetic information. For convenience, biologists divide the process into four stages: prophase, metaphase, anaphase, and telophase. However, mitosis is a continuous process; one stage merges directly into the next

10. 1. Prophase During early prophase, the chromatin fibers condense and shorten into chromosomes that are visible under the light microscope. The condensation process may prevent entangling of the long DNA strands as they move during mitosis. Because longitudinal DNA replication took place during the S phase of interphase, each prophase chromosome consists of a pair of identical strands called chromatids. A constricted region called a centromere holds the chromatid pair together.

11. 1. Prophase At the outside of each centromere is a protein complex known as the kinetochore, Later in prophase, tubulins in the pericentriolar material of the centrosomes start to form the mitotic spindle, a football-shaped assembly of microtubules that attach to the kinetochore. As the microtubules lengthen, they push the centrosomes to the poles (ends) of the cell so that the spindle extends from pole to pole. The mitotic spindle is responsible for the separation of chromatids to opposite poles of the cell. Then, the nucleolus disappears and the nuclear envelope breaks down.

12. 1. Prophase

13. 2. Metaphase During metaphase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle . This midpoint region is called the metaphase plate

14. 3. Anaphase During anaphase, the centromeres split, separating the two members of each chromatid pair, which move toward opposite poles of the cell. Once separated, the chromatids are termed chromosomes. As the chromosomes are pulled by the microtubules of the mitotic spindle during anaphase, they appear V-shaped because the centromeres lead the way, dragging the trailing arms of the chromosomes toward the pole.

15. 3. Anaphase

16. 4. Telophase The final stage of mitosis, telophase, begins after chromosomal movement stops. The identical sets of chromosomes, now at opposite poles of the cell, uncoil and revert to the threadlike chromatin form. A nuclear envelope forms around each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks up.

17. 4. Telophase

18. B. CYTOPLASMIC DEVISION: CYTOKINESIS Division of a cell’s cytoplasm and organelles into two identical cells is called cytokinesis . This process usually begins in late anaphase with the formation of a cleavage furrow, a slight indentation of the plasma membrane, and is completed after telophase. The cleavage furrow usually appears midway between the centrosomes and extends around the periphery of the cell. Actin microfilaments that lie just inside the plasma membrane form a contractile ring that pulls the plasma membrane progressively inward. The ring constricts the center of the cell, like tightening a belt around the waist, and ultimately pinches it in two. Because the plane of the cleavage furrow is always perpendicular to the mitotic spindle, the two sets of chromosomes end up in separate cells. When cytokinesis is complete, interphase begins . The sequence of events can be summarized as: G1 S phase G2 phase mitosis cytokinesis

19. B. CYTOPLASMIC DEVISION: CYTOKINESIS

20. REPRODUCTIVE CELL DEVISION In the process called sexual reproduction, each new organism is the result of the union of two different gametes (fertilization), one produced by each parent. If gametes had the same number of chromosomes as somatic cells, the number of chromosomes would double at fertilization. Meiosis , the reproductive cell division that occurs in the gonads (ovaries and testes), produces gametes in which the number of chromosomes is reduced by half. As a result, gametes contain a single set of 23 chromosomes and thus are haploid (n) cells . Fertilization restores the diploid number of chromosomes.

21. Meiosis Unlike mitosis, which is complete after a single round, meiosis occurs in two successive stages: meiosis I and meiosis II. During the interphase that precedes meiosis I, the chromosomes of the diploid cell start to replicate. As a result of replication, each chromosome consists of two sister (genetically identical) chromatids, which are attached at their centromeres. This replication of chromosomes is similar to the one that precedes mitosis in somatic cell division.

22. A. Meiosis-I Meiosis I begins once chromosomal replication is complete, consists of four phases: prophase I, metaphase I, anaphase I, and telophase I. Prophase I It is an extended phase in which the chromosomes shorten and thicken, the nuclear envelope and nucleoli disappear, and the mitotic spindle forms. Two events that are not seen in mitotic prophase occur during prophase I of meiosis. First, the two sister chromatids of each pair of homologous chromosomes pair off, an event called synapsis . The resulting four chromatids form a structure called a tetrad. Second, parts of the chromatids of two homologous chromosomes may be exchanged with one another. Such an exchange between parts of non-sister (genetically different) chromatids is termed crossing-over . This process, among others, permits an exchange of genes between chromatids of homologous chromosomes. Crossing-over results in genetic recombination —that is, the formation of new combinations of genes—and accounts for part of the great genetic variation among humans and other organisms that form gametes via meiosis.

23. A. Meiosis-I In metaphase I, the tetrads formed by the homologous pairs of chromosomes line up along the metaphase plate of the cell, with homologous chromosomes side by side. During anaphase I, the members of each homologous pair of chromosomes separate as they are pulled to opposite poles of the cell by the microtubules attached to the centromeres. The paired chromatids, held by a centromere, remain together. Telophase I and cytokinesis of meiosis are similar to telophase and cytokinesis of mitosis. The net effect of meiosis I is that each resulting cell contains the haploid number of chromosomes because it contains only one member of each pair of the homologous chromosomes present in the starting cell.

24. A. Meiosis-II meiosis II, also consists of four phases: prophase II, metaphase II, anaphase II, and telophase II. These phases are similar to those that occur during mitosis; the centromeres split, and the sister chromatids separate and move toward opposite poles of the cell. In summary, meiosis I begins with a diploid starting cell and ends with two cells, each with the haploid number of chromosomes. During meiosis II, each of the two haploid cells formed during meiosis I divides; the net result is four haploid gametes that are genetically different from the original diploid starting cell.