MUTATION AS A CHEMICAL PROCESS 219 



somes such as the prophages of bacteria and especially their relative, 

 the F factor, which may insert itself in different places on the bacterial 

 chromosome, which it breaks. 



In practice, it is often difficult to distinguish the position effects of 

 chromosomal rearrangements from point mutations, as Stadler, the emi- 

 nent student of mutation in maize, has pointed out. It is a matter of 

 historical interest that the Dutch botanist, De Vries, who was the first 

 serious student of mutation, actually investigated a result of chromosome 

 rearrangement, rather than what we would now call mutation. We have 

 seen how the more sensitive recombination techniques of microbial gene- 

 tics have recently permitted studies to be made of alterations within 

 the fine structure of genes. Thus in some cases, it is clear that point 

 mutations and not the position effects of chromosomal rearrangments are 

 being investigated. 



Still other chromosomal modifications resemble point mutations. In 

 diploid organisms, as we have also seen, it is often necessary to dis- 

 tinguish mutation from certain exchanges occurring between homologous 

 chromosomes. Some "mutations ' may actually be due to miscopying 

 under the influence of another allele. Or they may be due to unequal 

 crossing over. 



MUTATIONS INDUCED BY RADIATION 



High energy radiations are among the best agents for the production of 

 mutations not restricted to sites within the gene. X-rays, for example, 

 frequently induce chromosome breaks. They act on matter by knocking 

 electrons out of the orbits of atoms that absorb their quanta. This 

 results in positively charged particles and free electrons which move at 

 high speed. The latter, in turn, knock other electrons free; eventually 

 when their high energy has been dissipated, the free electrons attach to 

 other atoms. The positively and negatively charged particles which re- 

 sult from these processes are arranged in an ionization track. Each 

 ionization causes a chemical alteration of the charged molecule which 

 becomes chemically exceedingly reactive. The quantitative relationship 

 existing between X-rays and mutation fits this physical picture well and 

 has given rise to what is called the target theory. This theory assumes 

 that the gene is a target and that, when a sufficient number of ionizations 

 (referred to as hits) occur within it, a mutation results. Figure 8.10 

 shows a curve relating X-ray dose to the number of mutants induced 

 in a large population. Its linearity indicates that one hit is sufficient 



