Cell Division and Cell Cycle

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Cell Division and Cell Cycle

It is the process by which a parent cell divides into two or more daughter cells.
These cells divide once approximately every 24 hours.
The duration of the cell cycle can vary with the organism and the cell type.
The two types of cell division are:

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.
Reproductive cell division: A cell undergoes a division called meiosis, in which the number of chromosomes in the nucleus is reduced by half. This mechanism produces gametes – the cells needed to form the next generation of sexually reproducing organisms.

The cell cycle is an orderly sequence of events by which a somatic cell duplicates its contents and divides into two.
Human cells contain 23 pairs of chromosomes with a total of 46 chromosomes.
One member of each pair is inherited from each parent.
The two chromosomes that make up each pair are called homologous chromosomes.
Somatic cells contain two sets of chromosomes; they are called diploid cells, denoted as 2n.
The cell cycle is divided into two basic phases:

During interphase replication of DNA takes place and additional cell organelles and cytosolic components are formed.
Interphase is a state of high metabolic activity; during this time the cell grows.
Interphase consists of three phases: G₁, S, and G₂.
G₁ phase: It is the interval between the mitotic phase and the S phase. During the G₁phase, 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. The G₁ phase lasts for 8 to 10 hours.
S phase: It is the interval between G₁ and G₂ phase, lasts about 8 hours. During the S phase, DNA replication occurs. As a result, the two identical cells are formed during cell division will have the same genetic material.
G₂ phase: It is the interval between the S phase and the mitotic phase. It lasts for 4 to 6 hours. During the G₂ phase, 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.

This is the most dramatic period of the cell cycle, involving a major reorganization of virtually all components of the cell.
Since the number of chromosomes in the parent and progeny cells are the same, it is also called equational division.
The mitotic phase of the cell cycle consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) to form two identical cells.
The process results in the exact distribution of genetic information.
It is divided into four stages:

In the early prophase, the 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 kinetochore.
In late prophase, tubulins in the pericentriolar material of the centrosomes start to form the mitotic spindle, a football shape of microtubules that attach to the kinetochore.
As the microtubules lengthen, they push the centrosomes to the ends of the cell so that the spindle extends from pole to pole.
The nucleolus disappears and the nuclear envelope breaks down.

During metaphase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle called a metaphase plate.
This arrangement of the chromosomes at the metaphase plate ensures that each nucleus after cell division will receive one copy of each chromosome.

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 appear V-shaped.

Telophase begins after the chromosomal movement gets stops.
The identical sets of chromosomes, now at opposite poles of the cell, uncoil and forms thread-like chromatin.
A nuclear envelope forms around each chromatin mass, nucleoli reappear and the mitotic spindle breaks up.

The division of a cell’s cytoplasm and organelles into two identical cells is called cytokinesis.
The process usually begins in late anaphase with the formation of the 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.
When cytokinesis is complete, the interphase begins.
The sequence of events can be summarized as
G₁ phase → S phase → G₂ phase → Mitosis → Cytokinesis