Mitosis and meiosis are two distinct forms of cell division. Mitosis allows a somatic cell, or non-sex cell, to divide into two genetically identical daughter cells for growth and tissue repair. Meiosis is a specialized division that occurs only in germ cells to produce gametes, such as sperm and egg cells, necessary for sexual reproduction. Meiosis halves the chromosome number and introduces genetic diversity, functions absent in mitosis.
Synapsis and Genetic Recombination
The most fundamental difference occurs early in the first meiotic division, during Prophase I. This stage is defined by synapsis, where homologous chromosomes physically pair up with one another. Homologous chromosomes are the two similar chromosomes a cell inherits, one from each parent.
The tight alignment of these homologous pairs forms a structure known as a bivalent or a tetrad, containing four sister chromatids. This pairing is mediated by the synaptonemal complex. This physical association between homologous chromosomes does not occur during mitosis, where chromosomes align independently.
Once the tetrad is formed, crossing over, or genetic recombination, takes place. Non-sister chromatids exchange segments of genetic material. This physical exchange occurs at points called chiasmata and results in recombinant chromosomes that are a unique mix of parental DNA. This exchange is the primary mechanism for generating genetic diversity in sexually reproducing organisms, a function bypassed in mitosis, which produces only genetic clones.
The Reduction Division of Homologous Chromosomes
Following synapsis and recombination, the next unique event occurs in Metaphase I, where the paired homologous chromosomes align at the cell’s center. Unlike mitosis, where individual chromosomes line up singly, in Meiosis I, the chromosomes align as pairs. This paired alignment sets the stage for the reduction division.
During Anaphase I, the entire homologous chromosomes separate and move toward opposite poles of the cell. The sister chromatids of each chromosome remain attached throughout this separation. This separation of homologous pairs, rather than the separation of sister chromatids, halves the chromosome number, changing the cell’s ploidy from diploid (two sets of chromosomes) to haploid (one set of chromosomes).
Mitotic anaphase involves the splitting of the centromere and the separation of sister chromatids, which move to opposite poles. Because the original chromosome number is maintained, mitosis is known as an equational division. The unique separation of homologous chromosomes during Meiosis I earns it the designation of a reduction division.
The Outcome of Two Sequential Divisions
Meiosis is characterized by two sequential rounds of nuclear division, Meiosis I and Meiosis II, following a single round of DNA replication. Mitosis, conversely, involves only one round of division. Meiosis II is structurally similar to mitosis, as it involves the separation of the remaining sister chromatids.
The outcome of this two-stage process is the production of four daughter cells, which is double the two daughter cells produced by mitosis. These four cells are haploid, meaning they contain only one complete set of chromosomes, half the number of the original parent cell. This haploid state is necessary for gametes, so that when two gametes fuse during fertilization, the species’ characteristic diploid number is restored.
Because of the crossing over and the independent assortment of homologous chromosomes during Meiosis I, the four resulting haploid cells are genetically unique from the parent cell and from each other. The single division of mitosis results in two daughter cells that are genetically identical to the parent cell and to each other. These four genetically distinct, haploid cells are the specialized product of meiosis, ensuring genetic variation within a population.