Gaps and forks in DNA replication: Rediscovering old models.

Most current models for replication past damaged lesions envisage that translesion synthesis occurs at the replication fork. However older models suggested that gaps were left opposite lesions to allow the replication fork to proceed, and these gaps were subsequently sealed behind the replication fork. Two recent articles lend support to the idea that bypass of the damage occurs behind the fork. In the first paper, electron micrographs of DNA replicated in UV-irradiated yeast cells show regions of single-stranded DNA both at the replication forks and behind the fork, the latter being consistent with the presence of gaps in the daughter-strands opposite lesions. The second paper describes an in vitro DNA replication system reconstituted from purified bacterial proteins. Repriming of synthesis downstream from a blocked fork occurred not only on the lagging strand as expected, but also on the leading strand, demonstrating that contrary to widely accepted beliefs, leading strand synthesis does not need to be continuous.