Complex sentence: As each crossover strand reanneals to its original partner strand it displaces the original complementary strand ahead of it, causing the Holliday junction to migrate, creating the heteroduplex segments.
Split into simpler sentences: As each crossover strand reanneals to its original partner strand, it displaces the original complementary strand ahead of it. This causes the Holliday junction to migrate, creating the heteroduplex segments.