Meiosis is a type of cell division that occurs only in eukaryotes (organisms with membrane-bound cell organelles). The process of meiosis is exhibited by higher forms of organisms that reproduce sexually. In plants, meiosis is observed after spore production; whereas in animals, meiosis takes place during gamete (sperm and egg) formation. Meiosis produces daughter cells with half the number of chromosomes as the parent cell.
A significant difference between meiosis and other types of cell division like mitosis or binary fission is that, in meiosis, the parent cell divides and produces four gametes that are not capable of further division; whereas in other types of cell division, the parent cell produces identical daughter cells, which can undergo further division on their own.
The process of meiosis takes place in two stages, namely meiosis I and meiosis II. During meiosis I, a cell is divided into two, and in meiosis II, even further division takes place, resulting into a total of four haploid cells. Before the start of the process, the parent cell goes through a stage of preparation called the interphase. During this phase, the parent cell synthesizes more DNA (deoxyribonucleic acid) and proteins, thereby increasing the overall size and mass of the cell.
Each chromosome is made up of two identical chromatids, known as sister chromatids. The pair of chromosomes within a cell is called homologous chromosomes. Thus, there are a total of four chromatids in a cell, which are collectively called a tetrad.
- During prophase I, the homologous chromosomes begin to condense and come towards each other by a process called synapsis.
- After synapsis, crossing over takes place.
- Crossing over is the exchange of equivalent sections of chromatids between the homologous chromosomes.
- The chromosomes begin to thicken and move away from the nuclear envelope.
- In metaphase I, the tetrads get aligned at the center of the cell, at the equatorial plane.
- The centromeres, a region in the chromosome where the chromatids are held together are located at the opposite poles.
- The homologous chromosomes separate during this stage.
- The chromosomes migrate to the opposite poles of the cell.
- The sister chromatids remain together at this stage.
- The chromosomes continue to migrate towards the poles.
- Both the poles have haploid number of chromosomes.
- Condensation of the chromosomes and cytokinesis (division into two cells) takes place.
- A nuclear envelope starts forming.
- Two daughter cells with a haploid chromosome number are formed.
Meiosis II Stages
Meiosis II takes place in the same manner as mitosis. The two daughter cells produced at the end of meiosis I (in most cases) undergo further division and produce four daughter cells at the end of meiosis II.
- The nuclei and nuclear membrane are separated.
- The chromosomes start moving towards the equatorial plane.
- The chromosomes still have the chromatids attached by the centromere.
- The chromosomes are aligned at the equatorial plane prior to separation.
- The centromeres are oriented towards the opposite poles.
- The sister chromatids held at the centromere are separated by the spindle fibers.
- Chromatid pairs begin to move towards the poles.
- Four nuclei (two each in a daughter cell) are formed by the process of cytokinesis.
- Each of the four nuclei develops a nuclear envelope.
- Four daughter cells or gametes are formed.
The resulting four gametes contain half the number of chromosomes, and are therefore called haploids, each having a single set of chromosome. During the process of fertilization, two gametes fuse, which results in the doubling of the number of chromosomes. The fusion of gametes leads to the production of a zygote that has the same chromosome number as that of the parents. The haploids are unique and distinct from each other due to the crossing over process in meiosis I. Thus, meiosis leads to genetic variation and diversity.