Search

DNA, released from unirradiated mouse L-cells gently lysed in a thin layer of 2% sucrose on top of an alkaline sucrose gradient, was found to sediment in a narrow band with a sedimentation coefficient of about 500S. Exposure of cells to increasing doses of X-rays (89-712 rads) continuously reduced the DNA sedimentation velocity until, after about 890 rads, the DNA appeared in a narrow peak with a sedimentation coefficient of approximately 180S. As the dose given to cells was increased beyond 890 rads, the sedimentation coefficient of the DNA released continued to decrease and the sedimentation profiles now broadened in a manner consistent with the random production of single-strand breaks in the DNA. The DNA released from unirradiated cells (500S) is thought to be loosely aggregated and only partially single stranded. It is presumed that cells exposed to low doses of radiation release DNA with marked reductions in sedimentation coefficient because single-strand breaks produced in the DNA aid the alkaline denaturation process. By using the system to be described, it has been possible to demonstrate DNA repair (rejoining of X-ray-induced single-strand breaks) during postirradiation incubation of cells given doses as low as 400 rads.

The lethal and inhibitory effects of actinomycin D (Act D) on asynchronous and synchronized populations of mouse L-cells have been studied. It has been shown that the survival curve of populations in the logarithmic phase of growth can be approximated by two exponential survival curves corresponding to a sensitive and resistant moiety. The size and sensitivity of both moieties vary during the growth of the population. As the cell population moves through logarithmic and into stationary phase, the sensitive moiety becomes smaller but more resistant whereas the resistant moiety increases in size and also becomes more resistant. This variation appears to be related to a reduced uptake of Act D and also a reduced rate of DNA and RNA synthesis. Variations in sensitivity to the drug have also been observed during the division cycle of synchronized cells with cells in the S phase showing the greatest uptake of the drug and also the greatest sensitivity. However, no direct correlation between uptake and sensitivity has been established. Actinomycin D has inhibitory effects on both RNA and DNA synthesis. RNA synthesis is inhibited rapidly but does not seem to drop to less than 5% of the control value. The inhibition of DNA synthesis appears to occur over a longer period and may reach values as low as 0.25% of control. In both cases the degree of inhibitions appears to be dependent on both the length of exposure and the concentration of the drug. Certain similarities between the response of cells to Act D and X-rays have been observed and are discussed.