Repair ionizing type

Repair of human-cell DNA as observed in our assay after UV (254-nm) irradiation differs from ionizing-type repair in two essential aspects. First, the time involved for maximum incorporation of BrlJra is 18-20 hr. Second, the size of the repaired regions (as determined by sensitivity to 313-nm radiation) appears to be equivalent to about 25 BrlJra residues (100 nucleotides). Gradient profiles for a typical UV experiment with... 

Figure 8 shows a typical result in an analysis of DNA repair in human skin fibroblasts after treatment with EMS and subsequent incubation in BrdUrd. The J(l/Afw ) curve generated by this procedure is qualitatively identical to a y-ray curve cf. Fig. 4). Quantitatively, there are more repaired lesions observed with this dose (10 2m) of EMS than with 10 krad of y-rays. Apparently, a number of small lesions are induced and quickly repaired with a few nucleotides and little BrUra insertion, presenting a target of minimal size for the 313-nm radiation. We would thus classify EMS as a typical ionizing-type DNA-damaging agent in terms of after-treatment repair events.

Chemical agents that damage DNA can be classified into two major groups on the basis of the type of repair observed after treatment—those that induce ionizing-type repair and those that induce UV-type repair. One agent, 4-NQO, induces repair activity of both types. 

Another cumulative effect of radiation can be an irreversible alteration of DNA sequences. If part of a DNA molecule is ionized, its molecular chain may be broken. Chain breaks are repaired in the body, but after a serious rupture, the repaired unit may have a different sequence. This type of changed sequence is a genetic mutation. Altered DNA sequences in the reproductive organs are transmitted faithfully, thus passing on the genetic mutations to fiature generations. Because these effects are cumulative, individuals of childbearing age need to be especially carefial about radiation exposure. 

Ionizing radiation, such as gamma rays and x-rays, causes complex types of DNA damage that are difficult to repair. Including double-strand and single-strand breaks and cross-links that may lead to leukemia and cancers of many organs. 

DNA-repalr systems are responsible for maintaining genomic fidelity In normal cells despite the high frequency with which mutational events occur. What type of DNA mutation Is generated by (a) UV Irradiation and (b) Ionizing radiation Describe the system responsible for repairing each of these types of mutations In mammalian cells. Postulate why a loss of function In one or more DNA-repalr systems typifies many cancers. 

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