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
- 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.
- 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:
- Interphase: When a cell is not dividing.
- Mitotic phase: When a cell is dividing.
- 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 G1 phase, 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.
Mitotic (M) Phase:
- 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