meiosis

Meiosis occurs only in germ cells and consists of Meiosis I (reductional division) and Meiosis II (equational division), following a single S phase of DNA replication. In Meiosis I, homologous chromosomes pair up during prophase I (synapsis) via the synaptonemal complex, and crossing over occurs at chiasmata, allowing recombination of genetic material between maternal and paternal homologs. This is a key source of genetic diversity along with independent assortment. Homologous chromosomes then separate at anaphase I, reducing the chromosome number from diploid (2n) to haploid (n), though each chromosome still has two sister chromatids. Meiosis II resembles mitosis: sister chromatids separate, producing four haploid daughter cells total.

Clinical significance: Errors in meiosis are a major cause of chromosomal abnormalities. Nondisjunction during Meiosis I or II can lead to aneuploidy, such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and sex chromosome abnormalities like Klinefelter syndrome (XXY) or Turner syndrome (XO). Maternal nondisjunction (especially Meiosis I errors) increases with advanced maternal age and is the most common cause of these trisomies. Robertsonian translocations and other structural chromosome errors can also arise during meiotic recombination. Understanding meiosis is essential for grasping the basis of genetic disorders, oogenesis and spermatogenesis timing (oocytes arrest in Meiosis I prophase until puberty, then arrest again in Meiosis II metaphase until fertilization), and the mechanisms underlying genetic diversity and inheritance patterns tested on USMLE Step 1.