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 molecu
Ref: Hunter, Cold Spring Harb Perspect Biol 2015, Meiotic Recombination in Pachytene. Alberts 7th ed., Chapter 21.