218 



CELL HEREDITY 



Treatment 



Urea 



FIGURE 8.9. A diagram of the breakage of the sugar-phosphate backbone of DNA 

 by alkylating agents or X-rays. 



nucleotide pairs and producing stable mutants which would not revert. 

 Evidence exists for such occurrences within the structure of the gene. 

 Other conceivable consequences of breaks include substitution of groups 

 of base pairs, rearrangements of groups of base pairs yielding inversions 

 and terminal translocations, and duplications and insertions of one or 

 more base pairs. Such rearrangements have thus far been observed only 

 at grosser levels of chromosome structure than the gene itself. 



These chromosomal aberrations are caused by many agents including 

 some which will not induce mutations in Neurospora or Drosophila. It 

 is not known whether the breaks that cause them are in the backbone 

 of the polynucleotide chain of DNA or whether they involve separation 

 at sites in between the DNA molecules in the chromosome. But they 

 may resemble intragenic point mutations because they may often be ac- 

 companied by position effects. In Drosophila, and presumably in other 

 organisms, genes confronted with a new chromosomal environment often 

 fail to act, or act differently, or are unstable. When they are returned to 

 the original chromosomal environment, they behave as they originally 

 did, demonstrating that the kind of mutation that involves a new base 

 order has not occurred within them. This points up the suprastructural 

 influences which pervade the genotype, and we may ask whether some 

 unstable genes may not be the expression of chromosomal rearrange- 

 ments. McClintock has shown that in maize there are mutagenic factors 

 that may move from site to site on a chromosome and from one chromo- 

 some to another. When they do so, they change the action of neighbor- 

 ing genes and may even break the chromosome. This calls to mind epi- 



