Which substage of prophase I is characterized by crossing over?
Pachytene represents longest stable meiotic prophase stage where fully synapsed bivalents thicken and crossing over implemented. Molecular hallmark is homology-directed repair of programmed double-strand breaks introduced by Spo11 topoisomerase-like transesterase in leptotene that generates 3 prime single-strand overhangs via MRE11-mediated resection. Overhangs invade homologous duplex mediated by meiosis-specific recombinases DMC1 and RAD51, forming D-loop intermediates. Decision between non-crossover and crossover fate controlled by ZMM proteins MSH4-MSH5 heterodimer stabilizing joint molecules and MLH1-MLH3 heterodimer functioning as resolvase cleaving Holliday junctions as crossover. At least one obligate crossover per homolog pair ensures physical connection via chiasma required for correct biorientation at metaphase I. Pachytene therefore concentrates recombination machinery within synaptonemal complex central region. Defects preventing crossover formation cause achiasmate chromosomes, trigger aneuploidy, and underlie human trisomy risk increasing with maternal age. 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.
Ref: Hunter, Cold Spring Harb Perspect Biol 2015, Meiotic Recombination in Pachytene. Alberts 7th ed., Chapter 21.