Pablo Hernández Rosas

During replication, the topology of DNA changes continuously in response to well‐known activities of DNA helicases, polymerases, and topoisomerases. However, replisomes do not always progress at a constant speed and can slow‐down and even stall at precise sites. The way these changes in the rate of replisome progression affect DNA topology is not yet well understood. The interplay of DNA topology and replication in several cases where progression of replication forks reacts differently to changes in DNA topology ahead is discussed here. It is proposed, there are at least two types of replication fork barriers: those that behave also as topological barriers and those that do not. Two‐Dimensional (2D) agarose gel electrophoresis is the method of choice to distinguish between these two different types of replication fork barriers.

The topology of DNA duplexes changes during replication and also after deproteinization in vitro. Here we describe these changes and then discuss for the first time how the distribution of superhelical stress affects the DNA topology of replication intermediates, taking into account the progression of replication forks. The high processivity of Topo IV to relax the left‐handed (+) supercoiling that transiently accumulates ahead of the forks is not essential, since DNA gyrase and swiveling of the forks cooperate with Topo IV to accomplish this task in vivo. We conclude that despite Topo IV has a lower processivity to unlink the right‐handed (+) crossings of pre‐catenanes and fully replicated catenanes, this is indeed its main role in vivo. This would explain why in the absence of Topo IV replication goes‐on, but fully replicated sister duplexes remain heavily catenated.