Delay In DNA Synthesis Research Articles

The resistance of Micrococcus radiodurans to ultraviolet radiation II. Action spectra for killing, delay in DNA synthesis, and thymine dimerization

Micrococcus radiodurans cells have been irradiated at different wavelengths of ultraviolet radiation. The relative efficiencies of these radiations to kill the cells, delay DNA synthesis, and induce thymine dimers in the cellular DNA have been determined. The shape of the action spectra indicates that (1) killing and delay in DNA synthesis must be at least in part the result of lesions other than the thymine dimer, and (2) possibly deoxycytidine and protein damage are involved in these two effects of ultraviolet radiation.

Effects of ultraviolet radiation on mammalian cells I. Induction of chromosome aberrations

A quasidiploid (22 chromosomes) clonal cell line of the Chinese hamster was used for investigations of the effects of UV light. The cells grown in monolayer were less than 3 μ thick, permitting ready penetration of UV light. By means of autoradiography, the average generation time was determined as 24 h, which included 8 h for S (DNA synthesis), 2–3 h for G 2 (post-synthesis interphase), and 13–14 h for G 1 (pre-synthesis interphase) and M (mitosis). Severe cellular damage was induced by UV, resulting in cell death, sterility, delay in DNA synthesis and mitosis, and chromosome aberrations. Various kinds of damage appear to depend both on wavelength and dose. All types of chromatid and chromosome aberrations were induced in the hamster cells by UV at various wavelengths. They are qualitatively similar to those induced by ionizing radiations and probably have the same mode of production through chromosome breakage and rejoining. They can be induced at all stages of the cell cycle, irrespective of DNA synthesis. Within the dose-range tested, aberration frequency increased with incident UV dose. UV-induced chromosome aberrations are non-photoreactivable. Aberration frequency is wavelength-dependent and reaches a broad maximum between 2400 and 2800 Å. Thymidine analog pre-treatment of these cells significantly increases their UV sensitivity as measured by chromosome aberrations. These results suggest that DNA is a site of UV injury which leads to chromosome aberrations, but the possibility cannot be excluded that chromosomal structural proteins also play an important role in the induction of aberrations. Chromosome demolition can be induced most effectively by UV at 2800 Å, but post-treatment with a protein inhibitor (puromycin) following irradiation with UV at 2650 Å produces the same effect. The phenomenon is interpreted as the result of damage to proteins of the cell.

SOME BIOCHEMICAL ASPECTS OF THE POSTIRRADIATION MODIFICATION OF ULTRAVIOLET-INDUCED MUTATION FREQUENCY IN BACTERIA

Postirradiation treatments blocking RNA or protein syntesis lower markedly the ultraviolet-induced mutation frequency (reversion of tryptophan requirement) in Escherichia coli strain WP2. Treatments of less than 20 min reduce the mutation frequency to a minimal level without significant modification of the time or rate of subsequent synthesis of RNA, DNA, and protein. Therefore the mutation frequency decline process apparently is an active process, probably enzymatically mediated, and not dependent on delay in postirradiation macromolecular synthesis. The progressive loss of susceptibility of the potential mutation to these treatments is closely correlated with the progression of ribonucleic acid snythesis in the culture. A relation exists between the amoumt of RNA synthesized at the time of chloramphenicol addition, the relative rate of DNA synthesis in the presence of chloramphenicol, and the induced mutation frequency. Apparently only when the RNA and protein necessary for genetic replication are synthesized prior to chloramphenicol addition will DNA be formed. When the RNA and protein involved have not been formed, the potentiality for mutation is lost through the active decline process. Decline in induced mutation frequency is promoted by 5-hydroxyuridine after a considerable lag. This decline is correlated with the doubling of RNA in the culture, suggesting thatmore » the analogue causes the decline through incorporation into the RNA. However, loss of susceptibility to the 5-hydroxyuridine-promoted reduction in mutation frequency occurs prior to measurable RNA synthesis and is correlated with the mutation stabilization process. Icubation with 5hydroxyuridine of sufficient duration to prevent all sensitive mutations does not delay DNA synthesis. Several lines of evidence indicate that DNA synthesis is the terminal event in ultraviolet-induced mutation. However, delay of DNA synthesis does not modify ultraviolet-induced mutation frequency in the absence of the active decline process unless drastic treatments are employed which grossly alter cellular metabolism. Loss of mutation photoreversibility is correlated with DNA synthesis in the culture. Dinitrophenol (but not chloramphenicol) blocks the loss of mutation photoreversibility. (auth)« less

Effect of X-Radiation on DNA Metabolism in Various Tissues of the Rat: II. Recovery after Sublethal Doses of Irradiation

The effect of 100 and 400 r of total-body x irradiation on DNA synthesis in thymus, spleen, and small intestine of the rat has been studied over extended periods. The incorporation of thymidine-2-C/sup 14/ into DNA during 1 hour was used as a relative measure of DNA synthesis. Following an initial period of inhibition the tissues recovered their ability to synthesize DNA. The time for this recovery depended on the tissue and on the radiation dose. In terms of specific activity of the isolated DNA, i.e., the relative rate of DNA synthesis, the maximum rate of recovery after 100 r for small intestine, spleen, and thymus was at 2, 1, and 3 days, respectively. After 400 r the maximum specific activities were observed at 1, 6, and 4 days for the same tissues, the spleen in this case recovering later than the thymus. Since there was an extenstve initial loss of DNA from the irradiated organs, the recovery in terms of total activity of the DNA was less pronounced than that of the specific activity. In all cases the specific acttvity exceeded the control value at the time of maximum recovery, usually by a factor of approximately 2. For thymusmore » after 400 r this overshoot was ten times as great as the control value, indicating an extremely active regeneration. It has been concluded that the effect of radiation on the DNA metabolism of the three tissues studied can be considered qualitatively the same, differing only in the degree of acute cell death, in the duration of the delay of DNA synthesis in the surviving cells, and in the rate of recovery resulting from accelerated cell replication during the period of regene ration. (auth)« less