Earth Science: Cells – Meiosis
Meiosis is a process in eukaryotic organisms that divides the nucleus and cell and reduces a diploid chromosome set with two sets of chromosomes to a haploid set with only one set of chromosomes.
In the more complex plants and animals, this process occurs when sperm and egg cells mature. In contrast to mitosis, meiosis involves two steps: meiosis I (reduction division) and meiosis II (meiotic division just like mitosis).
Meiosis I
During meiosis I, homologous chromosomes form pairs before the chromosome number is reduced. The process of meiosis can be divided into the following phases. The first step is prophase. Chromosomes coil up tightly. An undivided connection, the centromere, links the two sister chromatids of each chromosome. The homologous chromosomes align in parallel, forming a group of four chromatids (a tetrad).
Exchanges of genetic material between corresponding chromatid sections may occur at this time (see in focus). In metaphase I, tetrads align at the center of the longitudinal spindle axis and the nuclear membrane disappears. The homologous chromosomes segregate in anaphase I. As the centromere remains intact, entire chromosomes travel to either pole. In telophase I, the chromosomes begin to uncoil. The nuclear membrane and nucleoli form.
Homologous chromosomes are divided during this reduction division, and chromosomes from the mother and the father are distributed randomly; thus a diploid set of chromosomes has turned into a haploid set.
Meiosis II
When chromosomes have been reduced after meiosis I, meiosis II initiates the second division, which is similar to mitosis. Here, sister chromatids of the now haploid chromosome set segregate into two separate gametes with only one chromatid each, making four gametes in total from the original single cell.
After fertilization, the chromatids double through DNA synthesis, resulting in a new diploid chromosome set.
RECOMBINATION THROUGH SEXUAL REPRODUCTION
The chromosomes inherited from the parents are distributed randomly during the first reduction division of meiosis I. This lead s to a new combination or recombination of the genetic material.
There are two different types of combination known as interchromosomal recombination and intrachromosomal recombination. The first one involves entire chromosomes that are recombined during anaphase I. The latter involves an exchange of corresponding sections between chromatids (crossing-over).