Cell Division

Types

Two types of cell divisions occur in eukaryotes: mitosis and meiosis.

1. Mitosis

Here, the daughter cells receive the exact copies of the parent cell chromosomes in the diploid form. It is a part of the cell cycle. A eukaryotic cell cycle consists of two main phases: 1) interphase and 2) mitotic (M) phase and an alternative G0-phase.

Interphase is the preparatory stage when a cell gets ready to divide and start the cell cycle. It is the largest part of the cell cycle involving three sub-phases: G1, S, and G2.

Interphase is followed by the mitotic (M) phase involving mitosis and cytokinesis. Mitosis includes the step or phases prophase, prometaphase, metaphase, anaphase, and telophase, also known as karyokinesis. The nuclear division is followed by the cytoplasmic division (cytokinesis) forming two daughter cells.

2. Meiosis

It is the other way eukaryotic cells divide. It is a form of cell division that forms sex cells or gametes that fuse to form the zygote, giving rise to a new individual. Meiosis has two cycles of cell division, Meiosis I, and Meiosis II. The result is four daughter cells with a haploid (n) number of chromosomes.

Unlike eukaryotes, single-celled bacteria and archaea undergo binary fission, a type of cell division similar to mitosis. It requires replication of parent cell DNA, segregation, and ultimately splitting of the parent cell through the assembly of the septal (Z-ring) around the middle of the cell. It helps to pinch the cell into half, forming two daughter cells.

Some microorganisms like cyanobacteria reproduce by budding, a form of reproduction where the parent cell forms first forms a small extension, which increases in size until it becomes a separate organism.

Cell division is essential because mitosis and meiosis serve critical functions in higher organisms.

Reproduction

The primary purpose of cell division in living organisms is reproduction. Life on Earth would cease to exist without a way to reproduce. Trillions of cell divisions occur subsequently in a controlled manner to produce a complex, multicellular human being. Similarly, single-celled organisms like bacteria and amoebas use cell division to make copies of their own.

Growth

For any multicellular organism to grow, its cells need to divide. Living things divide not by increasing the cell size but by increasing the number or volume of the cell. For this reason, adults have more cells compared to their young ones. However, cell division continues till an organism dies. Then, it may not contribute to the cell number increase, but repair damaged or worn-out cells. If such cells are not removed or replaced, they may cause severe life-threatening diseases like cancer.

Repair

Have you ever thought about the reason behind healing our wounds after an injury? It is because damaged cells are replaced with newer ones through cell division. As discussed, cell division through mitosis also helps to repair old and damaged cells in our body by replacing them with newer versions.

Genetic Variation

Cell division by meiosis brings in genetic diversity in all sexually reproducing organisms. Since meiosis allows the mixing up of chromosome parts (recombination) through crossing over, it helps to produce genetic variation in the offspring. Thus, individuals made from egg and sperm cells from the same parents look vastly different from each other.

Genetic variants are sometimes also found to help in evolution. Such organisms are more environmentally fittest to survive than organisms that divide by mitosis.