Direct Radiation Effects

For the direct radiation effect, the mean number of inactivated molecules of, e.g., DNA, is directly proportional to the dose. Despite all of these lesions (Figure 7-07), the DNA is functionally more stable than the two other cellular macromolecules, RNA and protein. This stability can be attributed to the following three factors:

a. The primary structure of DNA is all that is needed for transfer of information;

b. Because of the double-helical structure, DNA carries the information in duplicate;

c. There are molecular mechanisms of different complexity to undo the DNA damage thus maintaining cellular survival as well as genetic integrity.

DNA repair encompasses the molecular reactions which eliminate damaged or mismatched nucleotides from DNA. There are a variety of repair mechanisms, each catalyzed by a different set of enzymes. Nearly all of these mechanisms depend on the existence of two copies of the genetic information, one in each strand of the DNA double helix. If the sequence in one strand is accidentally changed, information is not lost irretrievably, because a complementary copy of the altered strand remains in the sequence of nucleotides in the other strand. However, incomplete or erroneous DNA repair may also lead to mutations and consequently to cancer or cell death (see Figure 7-05).

Figure 7-08. Representative dose-effect curves for survival of the bacteria Deinococcus radiodurans R1 (filled circles) and its recombination deficient mutant Rec30 (open circles), compared to survival curves of spores of Bacillus subtilis (filled triangles) and cells of Escherichia coli B/r (open triangles) following exposure to X-rays.

Figure 7-08. Representative dose-effect curves for survival of the bacteria Deinococcus radiodurans R1 (filled circles) and its recombination deficient mutant Rec30 (open circles), compared to survival curves of spores of Bacillus subtilis (filled triangles) and cells of Escherichia coli B/r (open triangles) following exposure to X-rays.

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