Meiosis is a specialized cell division fundamental to sexual reproduction. It produces gametes: sperm and egg cells. Meiosis reduces the chromosome number by half. This halving is essential for maintaining the correct chromosome count during fertilization.
Meiosis I: The First Division
Meiosis begins with Meiosis I, where homologous chromosomes separate. Before this, DNA replicates, forming chromosomes with two sister chromatids. Prophase I starts Meiosis I, as chromosomes condense.
During Prophase I, homologous chromosomes pair in synapsis. This allows crossing over, exchanging genetic material between non-sister chromatids. This creates new gene combinations, contributing to genetic variation. In Metaphase I, paired homologous chromosomes (tetrads) align centrally. Their random orientation enhances genetic diversity.
Anaphase I separates these homologous chromosomes, pulled to opposite poles by spindle fibers. Unlike mitosis, sister chromatids remain attached. Each pole receives a haploid set of chromosomes, each with two chromatids. Meiosis I concludes with Telophase I and cytokinesis, where chromosomes decondense, nuclear envelopes may reform, and the cell divides into two haploid daughter cells.
Meiosis II: The Second Division
The two haploid cells from Meiosis I proceed to Meiosis II, which resembles mitosis. This second division separates sister chromatids. No DNA replication occurs between Meiosis I and Meiosis II.
Prophase II begins with chromosome recondensation. Spindle fibers form in each daughter cell. In Metaphase II, sister chromatids, attached at their centromeres, align along the metaphase plate.
Anaphase II splits centromeres, allowing sister chromatids to separate and move to opposite poles. Each separated chromatid is now an individual chromosome. Telophase II and cytokinesis complete the meiotic process. Chromosomes decondense, nuclear envelopes reform around chromosome sets, and cells divide, resulting in four haploid daughter cells, each containing unreplicated chromosomes.
The Purpose of Meiosis
Meiosis plays an important role in sexual reproduction by ensuring offspring receive the correct number of chromosomes. It achieves this by reducing the chromosome number by half. When a haploid sperm fuses with a haploid egg during fertilization, the original diploid chromosome number is restored in the resulting zygote. Without this reduction, the chromosome number would double each generation, leading to genetic abnormalities.
Meiosis is a source of genetic variation. Two primary mechanisms contribute to this diversity. Crossing over, in Prophase I, exchanges genetic material between homologous chromosomes, creating recombinant chromosomes. Independent assortment during Metaphase I, where homologous chromosome pairs randomly align, ensures each gamete receives a unique combination of paternal and maternal chromosomes. These processes create unique genetic combinations in offspring, important for evolution.