Teach yourself genetics / by R. J. Berry.

  • R. J. Berry
Date:
[1965], ©1965
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Teach yourself genetics / by R. J. Berry. Source: Wellcome Collection.

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    RADIATION GENETICS 141 and independent of the spatial or temporal proximity of other ionizations or clusters. However, it has recently been shown that more densely ionizing rays (such as fast neu¬ trons) are much more effective in the production of chromo¬ some breaks than those producing less dense ionizations. Now considerable restitution can take place between recently broken chromosome or chromatid ends : Muller has suggested that one ionization is sufficient to produce a break, that normally most of the breaks rejoin, but that with densely ionizing rays, it is logical to assume that more than one break may occur in a small locality. This is now generally accepted. Such multiple breaks would have a much smaller chance of restitution than single breaks, and hence, the net yield of breaks would be much higher, 12.4 Modification of the genetical effects of radiation Considerable advances in our understanding of the genetical effects of radiation have come in the last few years with the realization that there are various factors which can modify the extent of genetical radiation damage. These underline the point already made in Chapter 7, that muta¬ tion is to a large extent dependent on cell metabolism. For example, lack of oxygen before and during radiation pro¬ tects against genetical damage (both point and chromosomal changes). Thus, only about a third as many aberrations are observed following X-irradiation of bean root-tips in nitrogen rather than in air. It seems that the presence of oxygen in some way increases the number of breaks, possibly by facilitating the production of free peroxide radicals. Certain chemicals can modify radio-sensitivity. Thus, treatment before irradiation with cyanides or azides increases the frequency of mutations (possibly due to the inhibition of certain enzyme systems which destroy peroxides), while the alkaloid colchicine (which inhibits chromosomal movement) greatly reduces the frequency of chromatid aberrations. In general, chromosomes show markedly different radio-