DNA topoisomerases perform topological transformations of DNA by generating a transient DNA break through which strand passage can occur. These enzymes are grouped into types I and II based on structure and mechanism: type I enzymes carry out strand passage through a reversible single-strand break, whereas type II enzymes mediate strand transport through a double-strand DNA gate. DNA topoisomerases have critical roles in DNA replication, transcription, recombination, repair, and chromatin remodeling. (Adapted from PMID:23495937.)