Meiosis is a form of cell division necessary for sexual reproduction in most organisms. This process takes a single diploid parent cell (containing two sets of chromosomes) and divides it to produce haploid gametes (possessing only one set). The goal of meiosis is to reduce the total number of chromosomes by half, ensuring offspring inherit the correct amount of genetic material after fertilization. Meiosis is a two-part division process, designated as Meiosis I and Meiosis II.
Meiosis I: The Reductional Division
Meiosis I is recognized as the reductional division because its function is to decrease the chromosome number of the cell. The process begins with a diploid cell (2n) and concludes with two haploid (n) daughter cells. This reduction is achieved by separating the homologous chromosomes, which are pairs inherited one from each parent. This separation of homologous pairs fundamentally distinguishes Meiosis I from Meiosis II, where the sister chromatids separate.
Prophase I: The Stage for Genetic Mixing
Prophase I is the first and longest stage of Meiosis I, where events for genetic variation occur. Chromosomes begin to condense, becoming visible as distinct structures. A defining event is synapsis, where homologous chromosomes align and pair up along their lengths.
This tight pairing forms a structure known as a tetrad or bivalent, containing four chromatids. Within the tetrad, crossing over takes place, which is the exchange of genetic material between non-sister chromatids. Crossing over involves the breakage and rejoining of DNA segments, resulting in a reciprocal exchange of genes.
The points where this exchange occurs are called chiasmata. These chiasmata link the homologous chromosomes together, which is necessary for their proper separation later in Meiosis I. The resulting recombinant chromosomes carry a new combination of alleles from both parents.
This recombination shuffles the genetic material, making crossing over a major source of genetic diversity within a species. The final part of Prophase I involves the further condensation of chromosomes and the disintegration of the nuclear envelope.
Completing the First Division
The cell enters Metaphase I, where the tetrads move and align along the cell’s equatorial plane, the metaphase plate. Unlike mitosis, where single chromosomes line up, here the homologous pairs align side-by-side.
Independent assortment occurs during this alignment; the orientation of each homologous pair on the metaphase plate is random and independent of the other pairs. This random arrangement is the second major source of genetic variation created during meiosis.
The next stage is Anaphase I, marked by the separation of the homologous chromosomes. Spindle fibers pull one entire chromosome (still composed of two sister chromatids) from each pair toward opposite poles. The sister chromatids remain attached at their centromeres and do not separate during this stage.
Telophase I begins as the separated chromosomes arrive at the opposite poles. The chromosomes may partially decondense, and a nuclear envelope may re-form around the two sets of chromosomes.
Immediately following or concurrently with Telophase I, Cytokinesis occurs, which is the physical division of the cytoplasm. This results in two separate haploid daughter cells, each containing one chromosome from each original homologous pair. These cells then enter a brief resting period called Interkinesis before proceeding to Meiosis II.