Fig. 3. The formation of catenations and their resolution by topoisomerase II. Catenations are a byproduct of DNA replication and they form at the points at which two replication forks collide. Removal of catenations is performed by type-II topoisomerases, which produce a double-strand break in one of the chromatids and pass the other through the break. Two resolved sister chromatids are the result of the strand-passing process and re-ligation reaction.