Cell division is a fundamental process that allows organisms to grow, repair tissues, and reproduce. Mitosis and meiosis are the two primary forms. While both processes involve the division of a cell’s nucleus and its genetic material, meiosis includes several unique occurrences that distinguish it from mitosis, serving a specific purpose in sexual reproduction.
Homologous Chromosome Pairing and Crossing Over
One distinct event in meiosis is the pairing of homologous chromosomes, termed synapsis, which does not occur in mitosis. During prophase I, homologous chromosomes—one inherited from each parent—align, forming bivalents or tetrads. This association allows for crossing over.
Crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes. Segments of DNA are swapped, leading to new combinations of alleles. These exchange points, visible as chiasmata, link the homologous chromosomes. This recombination increases genetic variation in the resulting cells, a mechanism absent in mitotic division.
Independent Assortment of Chromosomes
Another event exclusive to meiosis is the independent assortment of chromosomes during metaphase I. Homologous chromosome pairs align randomly at the cell’s equatorial plate. The orientation of each homologous pair is independent, meaning paternal and maternal chromosomes from different pairs can align on either side of the metaphase plate.
This random alignment ensures each daughter cell receives a mix of maternal and paternal chromosomes. For humans, with 23 pairs of chromosomes, independent assortment alone can lead to over 8 million different combinations in the gametes. This mechanism contributes to the genetic diversity observed in sexually reproducing organisms, a complexity not seen in mitosis where individual chromosomes align independently.
Separation of Homologous Chromosomes
A defining characteristic of meiosis, contrasting with mitosis, is the separation of homologous chromosomes during anaphase I. In this phase, homologous chromosomes, each still composed of two sister chromatids, move to opposite poles. This differs from mitosis, where sister chromatids separate and move to opposite poles during anaphase.
This separation in meiosis I reduces the chromosome number by half, a process known as reductional division. Each resulting cell becomes haploid, containing one chromosome from each homologous pair, though each chromosome still consists of two chromatids. Mitosis, by contrast, maintains the chromosome number, producing diploid daughter cells.
Two Rounds of Division and Resulting Cells
Meiosis uniquely involves two successive rounds of cell division, Meiosis I and Meiosis II, without intervening DNA replication. Meiosis I separates homologous chromosomes, forming two haploid cells. Meiosis II then proceeds, much like mitosis, where sister chromatids within these haploid cells separate.
This two-step division yields four genetically distinct haploid daughter cells. These cells, such as sperm or egg cells, contain half the number of chromosomes of the original parent cell. In contrast, mitosis consists of a single round of division, producing two genetically identical diploid daughter cells. These processes contribute to the genetic variation in meiotic cells.