The Cell Division

Cell division is the process by which a cell reproduces itself. Most cells of the human body undergo cell division. There are two types of cell division: mitosis and meiosis. In this process, there is formation of two identical daughter cells.

Interphase

·         During interphase, the cell replicates its DNA as well as produces an additional set of organelles or cytosolic components.

·         It is a high metabolic activity stage.

·         Interphase consists of three phases:

1.       G₁ phase

2.       S phase

3.       G₂ phase

G₁ phase

·         The G1 phase is the interval between the mitotic phase and the S phase.

·         This stage is metabolically active, and it replicates most of its organelles and cytosolic components but does not replicate its DNA.

·         It takes 8-10 hours to complete the G₁ phase. Some muscle cells and nerve cells do not reproduce themselves; such type of cells is in G₀ phase. If the cell dies, its function is also lost.

S phase

·         The interval between G1 and G2 and it is lasts about 8 hours. In this stage, DNA replication occurs, and two identical cells are formed.

G₂ phase

·         The G2 phase is the interval between the S phase and the mitotic phase. It lasts 4 to 6 hours.

·         In this stage, complete replication of the centrosome occurs, and protein and enzyme synthesis continues for cell division.

·         During DNA replication in S phase, its helical structure partially uncoils, and the two strands separate at the points where hydrogen bonds connect base pairs.

·         Each exposed base of the old DNA strand then pairs with the complementary base of a newly synthesized nucleotide.

·         A new DNA strand is taking shape due to the formation of chemical bonds between neighboring nucleotides.

·         The uncoiling and complementary base pairing continues until each of the two original DNA strands is joined with a newly formed complementary DNA strand.

·         The original DNA molecule has become two identical DNA molecules.

·         In mitosis, one cell with the diploid number of chromosomes (46) divides into two identical cells, each with the diploid number of chromosomes.

·         This production of cells is necessary for growth and repair of tissues.

·         The process of DNA replication enables each chromosome (in the form of chromatin) to make a copy of itself.

·         The time taken for the conversion of the duplicate is called interphase or the resting phase.

 

·         The mitotic phase results in the formation of two identical cells, which consists of a nuclear division (mitosis) and cytoplasmic division (cytokinesis). The mitotic phase is divided into four phases:

1.       Prophase

2.       Metaphase

3.       Anaphase

4.       Telophase

Prophase

·         In the early part of this phase, the chromatin becomes tightly coiled or condenses into chromosomes that are easier to see under a microscope. This process helps to prevent the entanglement of the long DNA strands.

·         Each prophase chromosome consists of a pair of identical strands called chromatids. The two chromatids are joined to each other at the centromere.

·         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 (assembly of microtubules that attach to the kinetochore).

·         Then the mitotic spindle moves to the opposite pole and separates the chromatids. The nucleolus disappears, and the nuclear envelope breaks down.

Metaphase

·         The chromatids align at the center of the spindle, attached by their centromeres.

Anaphase

·         The centromeres separate, and one of each pair of sister chromatids (termed a chromosome) migrates to each end of the spindle as the microtubules that form the mitotic spindle contract.

·         It appears V-shaped.

Telophase

·         It is the last phase of mitosis; the mitotic spindle disappears, the chromosomes uncoil, and the nuclear envelope reforms.

Cytokinesis

·         Cytoplasmic division: the contractile ring forms a cleavage furrow around the center of the cell, dividing cytoplasm into separate and equal portions.