Cell Division (Somatic & Reproductive Cell Division)

CELL DIVISION
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
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)
In somatic cell division, a cell undergoes a nuclear division called mitosis and a cytoplasmic division called cytokinesis to produce two genetically 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 two step division called meiosis, in which the number of chromosomes in the nucleus is reduced by half.
Somatic Cell Division: The cell cycle is an orderly sequence of events in 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 or homologs; they contain similar genes arranged in the same (or almost the same) order.
When examined under a light microscope, homologous chromosomes generally look very similar.
The exception to this rule is one pair of chromosomes called the sex chromosomes, designated X and Y.
In females the homologous pair of sex chromosomes consists of two large X chromosomes; in males the pair consists of an X and a much smaller Y chromosome. 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
The period between two cell divisions is known as the cell cycle, which has two phases: mitosis (M phase) and interphase.
Interphase
This is the longer phase and three separate stages are recognized: G1, S, and G2
During interphase the cell replicates its DNA & also produces additional organelles and cytosolic components.
1) First gap phase (G1) – the cell grows in size and volume.
This is usually the longest phase and most variable in length.
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.
For a cell with a total cell cycle time of 24 hours, G1 lasts 8 to 10 hours.
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. E.g. Most nerve cells are in the G0 phase 2) S (synthesis) Phase......................

1. CELL DIVISION UNIT-I Presented By Ms. Pranali A. Chandurkar Assistant Professor DRGIOP, Amravati

2. CELL DIVISION 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 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) In somatic cell division, a cell undergoes a nuclear division called mitosis and a cytoplasmic division called cytokinesis to produce two genetically 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 two step division called meiosis, in which the number of chromosomes in the nucleus is reduced by half.

3. Somatic Cell Division

4. The cell cycle is an orderly sequence of events in 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 or homologs; they contain similar genes arranged in the same (or almost the same) order.

5. When examined under a light microscope, homologous chromosomes generally look very similar. The exception to this rule is one pair of chromosomes called the sex chromosomes , designated X and Y. In females the homologous pair of sex chromosomes consists of two large X chromosomes; in males the pair consists of an X and a much smaller Y chromosome. 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 The period between two cell divisions is known as the cell cycle, which has two phases: mitosis (M phase) and interphase.

6. CELL CYCLE

7. Interphase This is the longer phase and three separate stages are recognized: G1, S, and G2 During interphase the cell replicates its DNA & also produces additional organelles and cytosolic components. 1) First gap phase (G1) – the cell grows in size and volume . This is usually the longest phase and most variable in length. 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. For a cell with a total cell cycle time of 24 hours , G1 lasts 8 to 10 hours . 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. E.g. Most nerve cells are in the G0 phase.

8. 2) S (synthesis) Phase The S stands for synthesis of DNA: the chromosomes replicate forming two identical copies of DNA Once a cell enters the S phase, however, it is committed to go through the rest of the cell cycle. The S phase, the interval between G1 and G2 , lasts about 8 hours. During the S phase, DNA replication occurs . As a result of DNA replication, the two identical cells formed during cell division later in the cell cycle will have the same genetic material.

9. 3) Second Gap Phase (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. Once a cell completes its activities during the G1, S, and G2 phases of interphase, the mitotic phase begins.

10. Mitotic Phase The mitotic (M) phase of the cell cycle, which results in the formation of two identical cells , consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) The events that occur during mitosis and cytokinesis are plainly visible under a microscope Nuclear Division: 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 seamlessly into the next.

11. 1. Prophase: During early prophase , the chromatin fibers condense and shorten into chromosomes that are visible under the light microscope Each prophase chromosome consists of a pair of identical strands called chromatids . A constricted region called a centromere holds the chromatid pair together . Later in prophase, tubulins centrosomes start to form the mitotic spindle , a football-shaped assembly of microtubules 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. 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.

13. 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

14. 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 thread-like chromatin form . A nuclear envelope forms around each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks up.

15. Cytoplasmic Division ( 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. cytokinesis occurs: the cytosol, intracellular organelles and plasma membrane split forming two identical daughter cells. When cytokinesis is complete, interphase begins The sequence of events can be summarized as G1 S phase G2 phase mitosis cytokinesis

16. Reproductive Cell Division

17. In sexual reproduction, each new organism is the result of the union of two different gametes (fertilization), one produced by each parent. 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 . Meiosis occurs in two successive stages: meiosis I and meiosis II.

18. Meiosis produces four daughter cells , not two, all different from the parent cells and from each other. This is the basis of genetic diversity and the uniqueness of each human individual. 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.

19. Meiosis I Meiosis I, which begins once chromosomal replication is complete, consists of four phases: prophase I, metaphase I, anaphase I, and telophase I Prophase I 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 . 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 called crossing-over. This process, among others, permits an exchange of genes between chromatids of homologous chromosomes.

20. In metaphase I, the tetrads (group of four) 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. (Recall that during mitotic anaphase, the centromeres split and the sister chromatids separate.) 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

21. Meiosis II The second stage of meiosis, 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.

22. Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis Prophase II Metaphase II Anaphase II Telophase II & Cytokinesis

23. THANK YOU….