The Molecular Basis of Mutation 



399 



by G in a transition resulting from a mistake 

 in replication. Since the BU in GjBU 

 is usually in the keto form at the time of the 

 next replication, it usually accepts A as its 

 complement, resulting in the G to A transi- 

 tion. Therefore the pyrimidine BU is ex- 

 pected to produce purine transitions in both 

 directions ( A <-* G ) . 



Errors involving BU can be studied in 

 vitro. The copolymer of A and BU, dABU , 

 can be made and used as primer-template in 

 an extended 30 to 100% synthesis with 

 TPPP, </APPP, and JGPPP in the substrate. 

 Under these conditions, G is incorporated at 

 the rate of one G residue per 2000 to 25,000 

 A and T residues. (On the other hand. 

 dAT does not incorporate G. ) Using 

 </GP*PP, JAPPP, and JBUPPP as a sub- 

 strate for the dABU primer-template, an 

 extended synthesis is performed and a near- 

 est-neighbor analysis is carried out on the 

 product, thus permitting detection of any 

 BUG, GG, and AG dinucleotide sequences. 

 The primer-template presumably has a strict 

 alternation of BU and A. Consequently, 

 when a G residue appears in the product, it 

 is expected to be attached to a BU residue, 

 and to produce the BUG dinucleotide se- 

 quence. However, all three sequences for 

 the G residue are found. In fact, G is in- 

 corporated next to BU or G (the BUG and 

 GG dinucleotide sequences) with about the 

 same frequency but less often next to A (the 

 AG sequence). Because sequences other 

 than BUG are found, the results cannot be 

 explained only by mistakes in replication 

 made by the BU in the primer-template. It 

 should be noted that the behavior of BU in 

 dABU in vitro may or may not be identical 

 to that of BU present in native DNA in vivo. 

 2-Aminopurine induces point mutations. 

 In its normal tautomeric form, it can pair 

 with T (by two H bonds) or with C (by one 

 H bond). In its rare tautomeric form, it 

 can also bond with C (by two H bonds). 



As a consequence, pyrimidine transitions 

 T <r* C can eventually result. 



A considerable amount of work has been 

 carried out to identify the particular base 

 changes occurring in point mutation. 1 ' 1 One 

 technique is to produce a mutation with a 

 mutagen whose action is expected to produce 

 a specific transition or transversion. Evi- 

 dence that the change expected has occurred 

 can subsequently be obtained by studying 

 the reversional mutation rate induced by 

 chemical mutagens expected and not ex- 

 pected to cause the reverse transition or 

 transversion. 



Mutation 



It is easy to identify the completed transi- 

 tion, transversion, or whole nucleotide 

 change as mutant. But at what point in a 

 series of changes should one consider that 

 a mutation has first been produced? 



We consider the mutation accomplished 

 when, as in one of the mechanisms discussed, 

 A is permanently changed to A'. One might 

 object to this answer on the basis that A' 

 may never be reproduced in a future replica- 

 tion; however, to be considered mutant the 

 product of a novel change need not be repli- 

 cated or transmitted. The novel product. 

 A', need only be more or less permanently 

 different from A in one or more of five ways 

 (as identified by five different operational 

 procedures) : 



1 . A' may have a different chemical com- 

 position 



2. It may have a different rate of change 

 to a new chemical or physical form 



3. It may not specify T at all or not to the 

 same extent as A did 



4. It may change the phenotypic effect of 

 the cistron in which it is located 



16 See E. Freese (1963), and E. B. Freese and 

 E. Freese (1964). 



