94 



MUTATIONS 



Figure 17. 



incorporated into DNA we should get a base pair change. This can 

 arise in two ways (Fig. 17). Either bromouracil, as a deoxynucleotide 

 triphosphate, pairs with a guanine base already present in DNA, and 

 the bromouracil nucleotide then gets incorporated into the DNA. Later, 

 when this DNA pair duplicates again, BU chooses its normal comple- 

 mentary base which is adenine. In this way a "mistake in incorpora- 

 tion" occurs. 



The other possibility is that bromouracil first gets incorporated by 

 pairing with adenine and then when the DNA duplicates again, this 

 bromouracil pairs by mistake with guanine, in which case we get 

 a mistake in replication. 



Auerhach: But you had another one where it goes — 

 Freese: No, these are the two possibilties. In the one case, when 

 the bromouracil makes a mistake while it goes in, the original guanine- 

 cytosine pair gets mutated, eventually, into an adenine-thymine pair, 

 while, when bromouracil is first incorporated and then makes the mis- 

 take in pairing, we go in the opposite direction. Thus we can change 

 both base pairs. 



