Meiotic Recombination

Meiotic Recombination

Meiotic recombination is essential for proper chromosome segregation and generating genetic diversity. It involves the exchange of DNA between homologous chromosomes during meiosis I prophase.

Stages of Prophase I

• Leptotene: Chromosomes condense and begin to pair
• Zygotene: Homologs begin synapsis; synaptonemal complex forms
• Pachytene: Full synapsis; crossing over occurs
• Diplotene: SC disassembles; chiasmata become visible
• Diakinesis: Chromosomes fully condensed for metaphase I

DSB Formation by Spo11

The topoisomerase-like protein Spo11 creates ~150-200 programmed double-strand breaks (DSBs) genome-wide in yeast. Only a fraction (~5-10%) of these mature into crossovers; the rest are repaired as non-crossovers.

Crossover Interference

Crossover interference is the phenomenon where the occurrence of one crossover reduces the probability of another nearby. This helps ensure even spacing of crossovers across chromosomes. The mechanism involves the synaptonemal complex and several protein complexes.

Obligate Chiasma

Each chromosome pair must have at least one crossover (obligate chiasma) to ensure proper segregation at anaphase I. Without chiasmata, homologs may segregate randomly, leading to aneuploidy.

CO vs NCO Repair

DSBs can be repaired as crossovers (CO) with reciprocal exchange, or non-crossovers (NCO) via synthesis- dependent strand annealing (SDSA). MLH1/MLH3 mark sites destined for crossover resolution.