118 RADIOBIOLOGY 



way, but since a respectable fraction of the x-ray energy is dissipated 

 in amounts similar to those of ultraviolet light photons, it is not im- 

 probable that it sometimes does. 



Since nucleic acids have been shown to include double-stranded struc- 

 tures as the major component of the genetic apparatus, it is conceivable 

 that one strand could be affected without at all altering the other strand. 

 Then, when copying of both strands ensues, if the copying mechanism 

 reaches an ionized base it could make an error by inserting one of the 

 other three bases, thereby producing a mutation. 



2. Ultraviolet-light inactivation and reactivation 



The formation of standard chemical bonds across the two strands of 

 the DNA would surely provide adequate impedance to DNA replication, 

 thereby inducing death of the organism at the time of its attempt to 

 replicate the DNA. This picture is supported by the finding that UV- 

 inactivated DNA does not separate into two strands on prolonged heat- 

 ing, indicating a "sewing together" of the strands. In addition, dimeriza- 

 tions within a strand, as discussed above, can lead to mutation, and some 

 of these may be lethal mutations in that they affect an indispensable 

 function of the DNA. 



Two types of reactivation of UV inactivation have been discovered. 

 The first type is called photoreactivalion. If UV-inactivated organisms 

 are subsequently exposed to visible light, a substantial fraction of the 

 organisms is found to be viable again. In microbial systems, this has 

 been shown to be due to the light supplying the energy needed for a 

 photoreactivating enzyme to work. This enzyme has been greatly puri- 

 fied, and it is found to work in the test tube. If DNA is treated with 

 UV, as mentioned above, it no longer separates on being heated, but if 

 it is then treated with the photoreactivating enzyme (in the light), its 

 separability is restored. 



The second type of reactivation is potentially more instructive, al- 

 though its promise has not yet been realized. Bacteriophages inactivated 

 by UV have been shown to be able to cooperate to produce live progeny. 

 Specifically, cells infected with two or more UV-inactivated phages are 

 found to yield viable phage progeny, even though each phage, by itself, 

 could produce no progeny. Presumably, the phage DNA which is in- 

 jected into the cells can undergo a reaction which allows the DNA copy- 

 ing mechanism to produce one viable copy by copying the good parts of 

 the two UV-damaged phages. Such an interaction should provide us 

 with great insight into the functioning and replication of DNA, but thus 

 far there has been no really successful theory of reactivation in multiple 

 infection or, as it is normally called, multiplicity reactivation. 



