Practice question

Double Holliday junctions arising during DSBR can yield crossover if cleaved by resolvases such as Mus81 or Yen1. Somatic cells limit crossovers to avoid loss of heterozygosity and translocations. RecQ helicase family members including yeast Sgs1 and human BLM cooperate with topoisomerase IIIα and Rmi1/Rmi2 to dissolve double Holliday junctions. Helicase migration converges junctions into single hemicatenane structure decatenated by topoisomerase. This dissolution pathway produces exclusively non-crossover products, preventing interchange of flanking markers, while resolution by structure-specific nucleases randomly generates crossover and non-crossover classes, explaining hyper-recombination in Bloom patients.