The early and late modes of DNA replication in ultraviolet irradiated Syrian hamster embryo cells.

The nature of DNA replication in UV irradiated Syrian hamster embryo cells (HEC) was investigated by measuring the size distribution of nascent daughter strand DNA. During the early mode nascent strands are made in smaller pieces than in nonirradiated cells. The late mode begins when nascent strands recover to normal size. This was observed in HEC 5 h post-UV. When the late mode is operational, nascent strands elongate to parental size in greater than 2 h, whereas less than 3 h are required during early mode function. Evidence from split dose experiments demonstrates that the recovery of the size of nascent strands is not due to enhanced gap filling. Furthermore, pyrimidine dimers are probably recognized differently by the replication complex during early and late mode DNA synthesis. The late mode of replication could account for the ability of HEC to survive UV irradiation even though they are inefficient in both excision and postreplication repair.