Meiosis is a specialized cell division for sexual reproduction. It is the process by which a single parent cell produces gametes, such as sperm and eggs. Its primary purpose is to reduce the chromosome number by half, ensuring that when two gametes combine during fertilization, the offspring maintains the correct number of chromosomes characteristic of the species.
Meiosis I: The First Reduction Division
Meiosis begins with Meiosis I. This stage is considered a “reductional division” because it halves the number of chromosomes in the resulting cells. Before Meiosis I, the cell undergoes DNA replication, so each chromosome consists of two identical sister chromatids.
During Meiosis I, homologous chromosomes, which are pairs of chromosomes inherited one from each parent, separate from each other. Crossing over occurs, where segments of genetic material are exchanged between these homologous chromosomes. This exchange creates new combinations of genes, contributing to genetic diversity. Following the separation of homologous chromosomes, the cell divides, resulting in two daughter cells, each with half the original number of chromosomes, but with each chromosome still composed of two sister chromatids.
Meiosis II: The Second Division
Meiosis II follows Meiosis I, without DNA replication. This second division is an “equational division” because the chromosome number does not change from the end of Meiosis I. The haploid cells from Meiosis I, with chromosomes composed of two chromatids, proceed into Meiosis II.
In Meiosis II, sister chromatids, the identical halves of a replicated chromosome, separate. This process is similar to what occurs in mitosis. As sister chromatids pull apart to opposite poles of the cell, each chromatid is now considered an individual chromosome. This results in cells, each containing a haploid set of chromosomes, with each chromosome consisting of a single chromatid.
The Outcome of Meiosis: Two Divisions, Four Cells
Meiosis involves two distinct cell divisions: Meiosis I and Meiosis II. From a single parent cell, this two-step process produces four genetically distinct haploid cells. Each of these final cells contains half the number of chromosomes of the original parent cell, and each chromosome is now a single chromatid.
The two divisions of meiosis are important for two main reasons. Firstly, they generate genetic variation through processes like crossing over in Meiosis I and the random alignment of homologous chromosomes, known as independent assortment. These mechanisms ensure that each gamete produced is genetically unique. Secondly, by halving the chromosome number, meiosis maintains the correct chromosome count across generations. When a haploid sperm and a haploid egg fuse during fertilization, they restore the diploid chromosome number in the new individual.