10 : 5/ Cellular Events Produced by Ionizing Radiations 199 



Instead of trying to use the word "gene" in discussing radiation 

 damage, many investigators now describe their results in new terms like 

 cistron, recon, and muton. The cistron is based on experiments in the 

 so-called "cis" and "trans" configurations. The trans configuration 

 corresponds to having two mutations, one on each member of a pair of 

 chromosomes. If no normal offspring are formed, the two mutations 

 are said to be noncomplementary. 



The cis configuration consists of both mutations on the same chromo- 

 some and a normal (that is, a so-called "wild-type") chromosome for 

 the other member of the homologous pair. The cis configuration 

 forms a control. In order to be able to use this analysis, the cis con- 

 figuration must correspond to normal individuals. This shows that 

 both mutations are recessive when compared to the normal. If, in 

 addition, the trans configuration showed the two mutations to be 

 noncomplementary, then they must block the same function. Under 

 these circumstances, the two mutations are said to be in the same 

 cistron. Each cistron in turn is made up of smaller chromosomal regions 

 defined in terms of crossover frequency. 



By studying relative crossover frequencies for different mutations, 

 and from a knowledge of the chromosome length, one can estimate the 

 minimum separation for crossing over. The unit of length for this 

 minimum separation is called the recon. 



Likewise, the critical length of the chromosome which must be altered 

 for a mutation to occur is called the muton. Experiments with viruses 

 support the idea that the muton and recon are both about 20 A long, 

 although the recon is probably shorter than the muton. These results 

 are in accord with the view that genetic mutations induced by ionizing 

 radiation occur due to ionizations in a small critical volume. 



Studies of the variation of mutation rate with dosage for higher animals 

 have been interpreted in terms of the critical volume target theory. 

 These data led to a volume whose diameter lay between 70 and 80 A. 

 At one time, when the gene was believed to be a structural unit, these 

 figures were discarded as being a factor of 100 to 1,000 times too small. 

 All theory suggested that if the critical volume differed from the gene, 

 it should be larger because of the influence of ionizations in the water 

 (nucleoplasm) surrounding the chromosome. This point of view is 

 presented in Reference 2 at the end of this chapter. It represented the 

 views commonly held in 1952. 



It is now apparent that the best estimates of the critical volume, a 

 sphere about 60 A in diameter, are larger than the recon or the muton. 

 Thus, apparently, chromosomes are sensitive both to direct hits and to 

 ones very close by but are not altered by ionizations more than about 

 3 uifji (30 A) away. 



