title: Pyrimidine dimers block simian virus 40 replication forks.
abstract: UV light produces lesions, predominantly pyrimidine dimers, which inhibit DNA replication in mammalian cells. The mechanism of inhibition is controversial: is synthesis of a daughter strand halted at a lesion while the replication fork moves on and reinitiates downstream, or is fork progression itself blocked for some time at the site of a lesion? We directly addressed this question by using electron microscopy to examine the distances of replication forks from the origin in unirradiated and UV-irradiated simian virus 40 chromosomes. If UV lesions block replication fork progression, the forks should be asymmetrically located in a large fraction of the irradiated molecules; if replication forks move rapidly past lesions, the forks should be symmetrically located. A large fraction of the simian virus 40 replication forks in irradiated molecules were asymmetrically located, demonstrating that UV lesions present at the frequency of pyrimidine dimers block replication forks. As a mechanism for this fork blockage, we propose that polymerization of the leading strand makes a significant contribution to the energetics of fork movement, so any lesion in the template for the leading strand which blocks polymerization should also block fork movement.
mesh_terms: D000818:Animals; D002460:Cell Line; D004261:DNA Replication / Q000528:radiation effects; D004279:DNA, Viral / Q000235:genetics* / Q000528:radiation effects / Q000648:ultrastructure; D008854:Microscopy, Electron; D011740:Pyrimidine Dimers / Q000502:physiology* / Q000528:radiation effects; D013539:Simian virus 40 / Q000235:genetics*; D014466:Ultraviolet Rays
chemical_list: D004279:DNA, Viral; D011740:Pyrimidine Dimers