Practice question

Segregation patterns distinguish meiosis from mitosis through reductional division where homologous chromosomes rather than sister chromatids separate. After S phase, each chromosome consists of two sisters held by cohesin containing meiosis-specific kleisin Rec8. During prophase I homologs pair and cross over forming bivalents. At metaphase I, homologous kinetochores achieve bipolar attachment but sister kinetochores exhibit monopolar orientation mediated by monopolin complex in yeast and fused geometry plus MEIKIN-MoA1 in mammals, causing both sisters of each homolog to attach to same pole. Shugoshin SGO2 paired with PP2A protects centromeric Rec8 from separase cleavage by dephosphorylation. Upon APC/C-Cdc20 activation, securin degrades, separase cleaves arm Rec8 distal to chiasmata, dissolving link between homologs while centromeric cohesion persists. Consequently, maternal and paternal homologs composed of two chromatids segregate to opposite poles in anaphase I, halving chromosome number. Nondisjunction at meiosis I produces disomic gametes causing trisomies such as Down syndrome. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.