Ultraviolet-Light Irradiation of Mouse L Cells: Effects on Cells in the DNA Synthesis Phase

Abstract

Damage produced by low exposures of ultraviolet-light (UV) irradiation exerts a specific effect on mouse L cells during their passage through their first S phase after irradiation. Previous results showed that irradiation of a synchronized cell population occupying late G1 to early S phase with 40, 100, or $200 {\rm ergs}/{\rm mm}^{2}$ of UV resulted in mitotic delays of 1-2, 4, or 10 hours. In the present work these delays are shown to be chiefly due to delays in cell progression through S phase. Most of the cells undergoing mitotic delay are able to form colonies. However, when irradiated cells occupying S phase are immediately incubated in the presence of 2 mM caffeine their ability to form colonies is rapidly reduced. This concentration of caffeine does not show any killing effect on cells irradiated in G2 phase during a corresponding period of incubation. Conversely, caffeine has little effect on cells irradiated in late G1 or early S phase if the cells are incubated in its absence for 10 hours befo...