CELL DIVISION & CELL DIVERSITY - Human Anatomy and Physiology B. Pharma 1st Semester

CELL DIVISION & CELL DIVERSITY

 

The process by which the cells reproduce themselves is called cell division.

Somatic cell:Cell of the body other than germ cell

Reproductive cell/Gamete/Germ cell: Precursor cell destined to become gamete Ex: Sperm or Oocyte

Mitosis: Process by which nuclear division in case of somatic cell takes place.

Cytokinesis:process by which the cytoplasm of somatic cell divides.

Meiosis: process by which the reproductive cells undergo division to produce gametes. 

Cell cycle: an orderly sequence of events by which a cell duplicates its contents and divides.

Cell division can be classified into 2 types

1) Somatic cell division

2) Reproductive cell division

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:This type of cell division produces gametes. This type of cell division consists of 2 step division called meiosis in which the number of chromosomes in the nucleus is reduced by half. The cells thus produced will be utilized in sexual reproduction.

A cell reproduces itself by duplicating all its contents to pass to the next generation of cells. The cell cycle consists of two steps

1) Interphase: When the cell is preparing its self for division.

2) Mitotic phase: when the cell is dividing.

Interphase:during this phase the cell replicates its DNA, produces additional cell organelles and cytosolic components and prepares itself for cell division.

Interphase consists of 3 steps/phases

a) G1 phase:  is the interval between mitotic phase and the S phase. During G1 the cell is metabolically active, it replicates most of its organelles and cytosolic components. 

b) S phase: is the interval between G1 and G2 phase during S phase the replication of DNA occurs 

c) G2 phase: is the interval between S phase and mitotic phase, during G2 cell growth continues and enzymes, proteins are synthesized for the cell division.

Once a cell completes its activities during the G1, S, and G2 phases of interphase, the mitotic phase begins.

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.

Mitosis can be divided into 4 phases

A) Prophase

B) Metaphase

C) Anaphase

D) Telophase

Prophase: During this phase chromatin fibers condense and shorten into chromosomes, each prophase chromosome consists of a pair of identical strands called chromatids. A constricted region called a centromere holds the chromatid pair together. At the outside of each centromere is a protein complex known as the kinetochore. 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.

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.

Anaphase: During anaphase, the centromeres split, separating the two members of each chromatid pair, which move toward opposite poles of the cell

Telophase: This is 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.

Cytokinesis

Cytokinesis process of division of a cell’s cytoplasm and organelles into two identical cells.

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.

Reproductive cell division

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.

Meiosis I: consist of 4 phases

A) Prophase I

B) Metaphase I

C) Anaphase I

D) 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. 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 nonsister (genetically different) chromatids is termed crossing-over. Crossing-over process permits an exchange of genes between chromatids of homologous chromosomes. Due to crossing-over, the resulting cells are genetically unlike each other and genetically unlike the starting cell that produced them. Crossing-over accounts for part of the great genetic variation among humans and other organisms that form gametes via meiosis.

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.

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.

MEIOSIS II:consists of four phases

A) Prophase II

B) Metaphase II

C) Anaphase II

D) Telophase II

These phases are similar to those that occur during mitosis

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.

CELL DIVERSITY

The body of an average human adult is composed of nearly 100 trillion cells. All of these cells can be classified into about 200 different cell types. Cells vary considerably in size and shape.

Cells may be round, oval, flat, cube-shaped, column-shaped, elongated, star-shaped, cylindrical, or disc-shaped. A cell’s shape is related to its function in the body. 

For example, a sperm cell has a long whip like tail (flagellum) that it uses for locomotion.

The disc shape of a red blood cell gives it a large surface area that enhances its ability to pass oxygen to other cells.