Meiosis

During meiosis, the parent cell loses half of its chromosomes, resulting in four gametes. Sexual reproduction is dependent upon the production of egg and sperm cells. In pregnancy, the sperm and egg merge to form one cell, leading to the restoration of the number of chromosomes in the offspring.

Diploid cells are the ones that start meiosis as they contain two copies of each chromosome. Two separate nuclear division cycles follow one round of DNA replication in the parent cell. Four haploid cells are formed when one diploid cell divides.  Each cell has half the number of chromosomes of the diploid parent cell.

Definition of Meiosis in Biology

Two gametes having a haploid set of chromosomes with  complete genetic information fuse to form a diploid cell. This diploid cell  divides into four new individual haploid cells as the number of chromosomes is reduced by half. This process of division where the number of chromosomes reduce to half of parent cell is called meiosis. During the lifecycle of sexually-reproducing organisms, meiosis results in the haploid phase, while fertilisation results in the diploid phase. Meiosis occurs during the process of gametogenesis in plants and animals. Four haploid gametes develop from this process.

Features

Meiosis is characterised by the following features:

• DNA replication occurs only once during meiosis, which has two consecutive cycles of nuclear and cellular division
• During the S phase, chromosomes multiply and produce identical sister chromatids. This is when meiosis begins
• Recombination and pairing of homologous chromosomes are key features of the meiotic process
• Meiosis II ends with the formation of four haploid cells

Process of meiosis

Nine stages of meiosis can be distinguished. They are divided into two groups based on the first time a cell divides (meiosis I) and the second time it divides (meiosis II).

The following are the phases of meiosis:

Meiosis I

1. Interphase:

• This is the most important step of cell division. Cells have to go through interphase
• Two identical sets of chromosomes are created in the cell when the DNA is copied
• Outside nucleus clear cytoplasm called centrosome is present which contains two centrioles.  These structures are necessary for cell division
• In interphase, this centrosome is surrounded by microtubules
• During this phase, the cell replicates its DNA and prepares for mitosis

2. Prophase I:

• During the copying process, chromosomes come together into X-shaped structures that can be visualised through a microscope
• Identical genetic information is contained in each sister chromatid of a chromosome
• In order to pair chromosomes together, both copies of chromosomes come together
• Through a process called recombination or crossing over, the chromosome pairs exchange bits of DNA
• By the final stages of Prophase I, the nucleus membrane in the cell has disintegrated, releasing the chromosomes
• During meiosis, microtubules and other proteins form a meiotic spindle, which extends between centrioles within the cell

3. Metaphase I:

• The chromosome pairs are lined up next to each other along the equator of the cell
• As the meiotic spindles extend from the centrioles, they are at opposite poles of the cell
• One meiotic spindle fibre attaches to each pair of chromosomes

4. Anaphase I:

• Spindle fibres contract, as a result, one of the chromosomes is pulled to one pole of the cell and the other to the other one by the meiotic spindle
• During meiosis I, the sister chromatids remain together. As opposed to mitosis and meiosis II, this occurs inside the cell

5. Telophase I and cytokinesis:

• After moving to the opposite poles of the cell, the chromosomes complete a round trip
• A complete set of chromosomes is found at each pole of the cell
• Each set of chromosomes forms nuclei by forming a membrane around them. As a result two nuclei are formed
• In a pinching action, the single cell divides into two chromosome-containing daughter cells, each with a separate nucleus. Cytokinesis is a term used to describe this process

Meiosis II

6. Prophase II:

•   Two new daughter cells have half the number of chromosomes but the amount of DNA is still double
• The X-shaped structures can be seen under a microscope in each of the two daughter cells
•  Daughter cell releases its chromosomes once the membrane around its nucleus dissolves
• Each centriole duplicates itself
• Once more, the meiotic spindle is formed

7. Metaphase II:

• Each of the two daughter cells is lined up end-to-end with its chromosomes (pair of sister chromatids) along the equator
• In each of the daughter cells, the centrioles are now located at the opposite poles
• In the meiotic process, each sister chromatid is attached to a meiotic spindle fibre

8. Anaphase II:

• During the meiotic process, the meiotic spindle constrict. This splits the centromere and  pulls the sister chromatids to opposite poles
• Individual chromosomes are now formed from the separated chromatids

9. Telophase II and cytokinesis:

• As the chromosomes reach the opposite poles of the cell, they complete their move
• A complete set of chromosomes is found at each pole of the cell
• Each set of chromosomes forms two new nuclei via the formation of a membrane
• Even though meiosis is now complete, cell division cannot be completed without another round of cytokinesis
• During cytokinesis, four granddaughter cells are produced, each with half a set of chromosomes
• Males have four types of cells, all of which are sperm cells
• Females have one egg cell and three polar bodies (small cells that do not develop into eggs)

Conclusion

Based on the cell theory, cells develop from pre-existing cells. The process of dividing cells into two daughter cells having the same genetic material is known as cell division. It is not merely during the synthesis of the mature organism that cells divide, but throughout their existence.