214 CELL HEREDITY 



that once in a while some other base is incorporated opposite 5-bromo- 

 uracil during replication. The mutation would then be completed when 

 that base, let us sav guanine, chose its normal partner, cytosine, at the 

 next replication. It seems clear that DNA carrying the base analogue 

 cannot function to give the mutant phenotype before the mutation is 

 genetically completed. Recent studies with synchronously replicating 

 bacteria show that two replications after incorporation are required be- 

 fore 5-bromouracil-induced mutants can be detected. 



As long as DNA containing 5-bromouracil continues to replicate, it 

 will continue to mutate; such mutability in a growing population would, 

 however, soon disappear unless the proportion of genes with 5-bromo- 

 uracil were maintained at a high level by continued provision of the 

 analogue. Since this hypothesis of incorporation and copy-error assumes 

 that 5-bromouracil sometimes chooses as a partner guanine rather than 

 adenine, it must sometimes replace cytosine at incorporation, with the 

 consequences shown in case 2 of Figure 8.6. We must further consider 

 that base analogues mav be incorporated by exchange reactions in the 

 absence of replication. 



In any event, we may conceive of some mutations as consisting of at 

 least two steps. The first involves the incorporation of an unusual base 

 in a certain position on one strand of the double helix of D\A, and 

 the second consists of replication on this template. The base may be 

 unusual in either of two ways. First, it may not normally be synthesized 

 by the cell but may require the action of some agent such as ultraviolet 

 light for its production. In this event, the unusual base can be in- 

 corporated only during the short time it is present in the cell. This 

 initial incorporation would not constitute the complete mutation; it 

 would be followed by a replication which involves the introduction of a 

 novel but normally synthesized base at the affected site in the new 

 strand, just as was the case with 5-bromouracil shown in Figure 8.6. 

 Alternatively, the incorporated base may be unusual only because, al- 

 though it is normally synthesized by the cell, it occupies a novel posi- 

 tion in that, in the parental DXA, another base was present at that site. 

 Errors in incorporation of this sort might occur with increased chance in 

 the presence of agents which break the hydrogen bonds of DXA and 

 separate the strands. Temperature does this, and heat shock is known 

 to induce mutation. Furthermore, certain chemicals of the right dimen- 

 sions and groupings might insert themselves between the two chains and 

 separate them. A large imbalance in the proportions of nucleotide pre- 

 cursors might also engender this kind of error. In any event, replication 

 would be needed, first for the incorporation of the unusual base, and 

 again for the stabilization of the new pattern by the inclusion of a 

 complementary base in the next generation of polynucleotide strands. 



