By Carolyn Csanyi, Updated Aug 30, 2022
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Cell division occurs in two fundamental modes—mitosis and meiosis—across plants, animals, protists, and fungi. In animals, mitosis supplies growth, repair, and tissue maintenance by producing daughter cells that are genetic replicas of the parent.
Meiosis, on the other hand, is essential for sexual reproduction. It generates haploid gametes—eggs and sperm—that fuse to form a new individual with a balanced chromosome complement.
Synapsis is the precise alignment of homologous chromosome pairs during prophase I of meiosis. This pairing, absent in mitosis, allows the chromosomes to exchange segments—a process called crossing‑over—that increases genetic variability in sexually reproducing organisms.
Meiosis reduces the chromosome number by half, producing haploid cells that preserve the correct number of chromosomes in offspring. Human somatic cells are diploid (46 chromosomes, 23 pairs). Each pair consists of a maternal and a paternal homolog. After two meiotic divisions, gametes contain 23 single chromosomes.
Each gamete carries a unique combination of maternal and paternal chromosomes. This genetic diversity enables populations to adapt to changing environments. Synapsis further enhances variability through recombination of sister chromatids during crossover.
Prior to meiosis, homologous chromosomes replicate, forming two sister chromatids held together at centromeres. The nuclear envelope dissolves, and chromosomes condense. During prophase I, the two chromatids of each homologous pair align along their lengths, forming the synaptonemal complex—a protein‑RNA scaffold.
Aligned chromatids may interlock; fragments of one chromatid can detach and reattach to the opposite chromatid, effectively swapping genetic material. This crossing‑over enriches genetic variation beyond the random assortment of chromosomes.
As meiosis I progresses, homologous pairs align at the metaphase plate. In anaphase I, the synapsis breaks, and the homologs separate to opposite poles. Telophase I yields two haploid cells, each containing one chromatid from every homologous pair, now bearing recombined DNA.
Meiosis II divides the two haploid cells from meiosis I, separating sister chromatids. The outcome is four haploid gametes in males and one large ovum plus three polar bodies in females. The two principal sources of genetic variability are independent assortment during both meiotic divisions and recombination during synapsis. In humans, the theoretical number of distinct gametes from 23 chromosome pairs is 8,324,608.
