Meiosis is a specialized type of cell division that occurs in organisms undergoing sexual reproduction for the formation of gametes. This process is distinct from mitosis, the cell division used for growth and repair, because it involves two sequential divisions instead of one. The term “reduction division” is applied to meiosis because the total number of chromosomes in the resulting daughter cells is precisely halved compared to the original parent cell.
Chromosome Number and Ploidy
Every organism that reproduces sexually has a characteristic number of chromosomes in its somatic, or body, cells. These cells are described as diploid (2n), meaning they contain two complete sets of chromosomes, one inherited from each parent. Within a diploid cell, chromosomes exist in pairs called homologous chromosomes, which are approximately the same size and carry genes for the same traits.
The haploid number, represented as n, is exactly half the diploid number and represents a single set of chromosomes. This state is the genetic condition of gametes, or sex cells, which are produced through meiosis. For example, human body cells are diploid with 46 chromosomes (2n = 46), while human gametes are haploid with 23 chromosomes (n = 23).
Meiosis I: The Reduction Phase
Meiosis I is the phase that defines meiosis as a reduction division. This division separates meiosis from mitosis because it involves the separation of homologous chromosomes rather than sister chromatids. Before Meiosis I begins, the cell duplicates its DNA, resulting in each chromosome consisting of two identical strands called sister chromatids.
During Meiosis I, the homologous chromosome pairs align side-by-side in the center of the cell, and then the entire pair separates. One chromosome moves to one pole and its partner moves to the opposite pole. Because the homologous pairs move into separate cells, the chromosome number is instantaneously halved. The two resulting cells are haploid (n) because they contain only one chromosome from each original pair, though each chromosome remains duplicated.
Meiosis II: Maintaining the Reduced State
Meiosis II begins without any further DNA replication. This second division is structurally similar to mitosis and is often referred to as an equational division because the chromosome number does not change. Its main function is to separate the sister chromatids that still make up each chromosome.
During Meiosis II, the two sister chromatids of each chromosome finally separate and move to opposite poles of the cell, becoming individual, unduplicated chromosomes. The two cells that entered Meiosis II divide again, resulting in a total of four daughter cells. Since the cells entering this stage were already haploid, the cells exiting Meiosis II remain haploid.
Biological Significance of Halving the Genome
The halving of the chromosome number is an evolutionary necessity for sexually reproducing species. The primary consequence of reduction division is the stable maintenance of a species-specific chromosome count across generations. If gametes were produced with the full diploid number, the fusion of two such cells during fertilization would result in a zygote with double the chromosome count (4n).
This doubling would occur with every successive generation, quickly leading to an unsustainable increase in genetic material and significant genetic instability. Meiosis prevents this outcome by ensuring that gametes carry only the haploid number (n). When a haploid sperm cell fuses with a haploid egg cell, the resulting zygote immediately restores the correct diploid number (2n) for the species.