Thymine and Uracil-thymine Dimers and Deoxyribonucleic Acid Synthesis in Mammalian Cells Irradiated with Ultraviolet Light

The effects of ultraviolet light (UV) irradiation on the rate of DNA replication in synchronized Chinese hamster ovary (CHO) cells were investigated. A technique for measuring semiconservative DNA replication was employed that involved growing the cells in medium containing 5-bromodeoxyuridine and subsequently determining the amount of DNA that acquired hybrid buoyant density in CsCl density gradients. One of the advantages of this technique was that it allowed a characterization of the extent of DNA replication as well as rate after irradiation. It was found that while there was a dose-dependent reduction in the rate of DNA replication following UV-irradiation, doses of up to 10 J/m2 (which produce many dimers per replication) did not prevent the ultimate replication of the entire genome. Hence, we conclude that dimers cannot be absolute blocks to DNA replication. In order to account for the total genome replication observed, a mechanism must exist that allows genome replication between dimers. The degree of reduction in the rate of replication by UV was the same whether the cells were irradiated at the G1-S boundary or 1 h into S-phase. Previous work had shown that cells in early S-phase are considerably more sensitive to UV than cells at the G1-S boundary. Experiments specifically designed to test for reiterative replication showed that UV does not induce a second round of DNA replication within the same S-phase.

Section: Introductionmentioning

confidence: 99%

Effects of ultraviolet irradiation on the rate and sequence of DNA replication in synchronized Chinese hamster cells

Meyn¹,

Hewitt²,

Thomas³

et al. 1976

“…The delays observed for cells exposed in mitosis or G1 may be the consequence of characteristic sensitivities of these two stages, or they may be the result of difficulties in passing through the DNA synthetic phase. Progression through the S phase depends upon the rate of DNA synthesis, and UVL inhibits this process severely (18)(19)(20). Studies on cell progression following UVL irradiation with various cell lines have established that cells exposed in G1 move to S without any delay; that is, no G1-S block was observed (3,4,6).…”

Section: Discussionmentioning

confidence: 99%

Ultraviolet Light-Induced Division Delay in Synchronized Chinese Hamster Cells

Han¹,

Sinclair²,

Chen³

1971

The age-dependent, ultraviolet light (UVL) (254 nm)-induced division delay of surviving and nonsurviving Chinese hamster cells was studied. The response was examined after UVL exposures adjusted to yield approximately the same survival levels at different stages of the cell cycle, 60% or 30% survival. Cells irradiated in the middle of S suffered the longest division delay, and cells exposed in mitosis or in G(1) had about the same smaller delay in division. Cells irradiated in G(2), however, were not delayed at either survival level. It was further established, after exposures that yielded about 30% survivors at various stages of the cycle, that surviving cells had shorter delays than nonsurvivors. This difference was not observed for cells in G(2) at the time of exposure; i.e., neither surviving nor nonsurviving G(2) cells were delayed in division. The examination of mitotic index vs. time revealed that most cells reach mitosis, but all of the increase in the number of cells in the population can be accounted for by the increase of the viable cell fraction. These observations suggest strongly that nonsurviving cells, although present during most of the experiment, are stopped at mitosis and do not divide. Cells in mitosis at the time of irradiation complete their division, and in the same length of time as unirradiated controls. Division and mitotic delays after UVL are relatively much larger than after X-ray doses that reduce survival to about the same level.

“…The response of mammalian cells to ultraviolet light (UVL) has been extensively studied recently in both synchronous and asynchronous cultures. The effects studied include cell survival (1,2,3,4,5), inhibition of DNA synthesis and induction of thymine dimers (6,7,8,9,10), chromosomal aberrations (11,12) and the repair of UV-induced damage (13,14,15,16). Photochemical events related to cell lethality are comparatively well known in bacteria (17) but not in mammalian cells.…”

Section: Introductionmentioning

confidence: 99%

Sensitivity of Synchronized Chinese Hamster Cells to Ultraviolet Light

Han¹,

Sinclair²

1969

The age response for lethality of Chinese hamster cells to ultraviolet light shows that they are resistant in G(1), sensitive as they move into and through the S phase and resistant again in G(2) and mitosis. Survival curves determined at different times in the cycle reveal that mitotic cells are the most resistant fraction, much more resistant than S cells, and more resistant than either G(1) or G(2) cells. The extent to which the age response is ilfluenced by nucleic acid and protein synthesis was investigated by using inhibitors of these processes. In the presence of inhibitors of DNA or protein synthesis added to G(1) cells before exposure, cell survival neither declines to the minimum survival of S cells nor rises subsequently to the resistance of G(2) cells. If, before exposure, DNA synthesis is arrested in the middle of S, when survival is at a minimum, the subsequent rise in survival during G(2) is not prevented. However when cycloheximide is added before exposure, during the middle of S, this rise is prevented. When actinomycin D, an inhibitor of RNA synthesis is added prior to exposure the age response is affected only slightly. Postirradiation treatment of G(1) and mid-S cells with inhibitors of DNA or protein synthesis maintains survival at a level characteristic of the age of the cells.