Meiosis is a common feature in Spermatogenesis (The formation of haploid sperm cells in the testes) and Oogenesis (The formation of haploid ova in the ovaries). Meiosis occurs in two divisions. The Reduction Division (Meiosis 1) and the Equatorial Division (Meiosis 2)
Chromosomes replicate during what is known as the Interphase, and this occurs previous to the Reduction Division.
Comparing to mitosis process stages, this eduction Division is divided into four stages, known as:
- Prophase 1
- Metaphase 1
- Anaphase 1
- Telophase 1
- Prophase 1
Chromosomes shorten and thicken. The purpose of this is so that long strands do not become entangled, thus restricting movement. The nuclear envelope breaks down and disappears along with the nucleoli. It is now that the mitotic spindle appears. The chromosomes become arranged in homologous pairs. This paring is referred to as synapses and the four chromatids of each homologous pair is known as a tetrad. Within a tetrad, portions of one cromatid are swapped with portions of another. This is referred to as the crossing over process. The reason for this is so that the daughter cells do not share identical genetic information and also that they differ from parent cells;therefore, human beings, among other species do not become clones.
With the homologues side by side, the pairs of chromosomes line up alongside the metaphase plate of the cell. The centromeres (the region where the chromatids join) form kinetochore microtubules that attach the centromere to opposite poles of the cell.
The members of each homologous pair separate; the centromeres do not split, so the paired chromatids stay together.
The paired chromatids are moved to opposite poles of the cell, and although vital for the development of humans, this phase does not occur in all species.
Following from this, there is a resting period known as the Interphase, the length of resting time varies between the cell types. It is after the Interphase that the Equatorial Division commences, and this is again divided into four stages, known as:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Prophase II
At a right angle to the first spindle, a new spindle forms. Each chromosome is made up of a chromatid pair, and this attaches to the second spindle.
The chromosomes now align along the spindle’s equator.
The chromatids are now forced apart to form two chromosomes, and each one moves to the opposing poles of the cell.
The spindle disappears and the nuclear membrane reforms. The chromosomes unravel and Cytokinesis begins. Four haploid cells, each consisting of a single chromosome are now formed, and each chromosome is genetically different. In essence, Meiosis is a complicated procedure that ensures that each individual is genetically unique.