Type Repair

Human-cell DNA, as visualized on sucrose gradients by the technique of McGrath and Williams (1966), under the conditions of our experiments has a control weight-average molecular weight Mw) of 1-2 X 10 daltons. After 10 krad of Co y-rays (zero time, ice temperature), the Mw is 3 X 107 after 60 min of incubation at 37°C, the DNA returns to its control (unirradiated) Mw (Fig. 3). If we perform this same experiment with BrdUrd in the medium during and after irradiation and then subject the cells to several doses of 313-nm radiation, we can derive an estimate of the number and size of the repaired regions. If we plot the 

FIGURE 3. DNA strand rejoining with time after 10 krad of Co y-rays, as observed on alkaline sucrose gradients. 

FIGURE 4. Relationship of the reciprocal of the weight-average molecular weight (lIMw) to the dose of 313-nm radiation after DNA damage by 10 krad of Co y-rays and repair in BrdUrd. 

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Backings are usually considered to be a treatment of last resort, used only for tapas which are structurally insecure and would not be adequately supported with patch-type repairs. One problem with a complete backing is that it obscures previous repairs which may be historically significant, particularly if the repair material is tapa applied in a native cultural context. 

In some cases, reformatting the drive can solve the problems described in the preceding paragraph. In other cases, reformatting only brings the drive back to life for a short while. The bottom line is that read and write problems usually indicate that the drive is going south and should be replaced soon. Never expect a Band-Aid -type repair like reformatting to cover a major trauma problem like disk failure. 

Failure Type Repair Material and Substratum Failure ... 

Prompt gamma measurements Medium-high Up to 10000 n-type repairable Possible absorber for X-rays, risk of neutron damage... 

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... 

This curvilinear response to 313-nm radiation in these experiments, contrasted to that seen in y-ray experiments, is an indication of target size for the 313-nm radiation. The number of BrUra residues per repaired region in the y-ray experiments present a small target for 313-nm radiation. Saturation is impossible even at extremely high doses of 313-nm. In the case of UV-type repair, however, saturation is possible due to the extensive excision and BrUra substitution that has occurred in the average repaired region. 

A. lonizing-Type Repair after Chemical Damage... 

ICR-170 is an acridine with one nitrogen mustard group. It has been employed as an antitumor agent (Creech et al., 1972). In our repair assay, this agent induced repair events similar although not identical to those induced by jV-acetoxy-AAF (Fig. 10). ICR-170 differed from jV-acetoxy-AAF in that the size of the repaired regions with ICR-170 was smaller (approximately ten BrUra residues) than those with jV-acetoxy-AAF (approximately 25 BrUra residues). Nevertheless, ICR-170 clearly induces UV-type repair in normal cells and, like jV-acetoxy-AAF, defective repair in XP cells (see Section VI). 

Vn. XERODERMA PIGMENTOSUM AND UV-TYPE REPAIR AFTER CHEMICAL DAMAGE TO DNA... 

The results with JV-acetoxy-AAF in XP cells led us to examine the response of XP cells to ICR-170, since UV-type repair was observed in normal cells after treatment with this agent. As expected, XP cells showed defective repair of lesions induced by ICR-170 (Fig. 10). 

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. 

Chemicals that induce UV-type repair in normal cells induce lesions that are essentially irreparable in XP cells. 

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