V. MOLECULAR MECHANISM OF MUTATIONS 



247 



use one or two hydrogen bonds; in the latter case they have slightly 

 wrong distances or angles for the two sugar (C) — base (N) bonds. 

 Since the DNA backbone is not completely stiff it can accommodate such 

 distortions, at least occasionally, as a mistake. The principle by which 

 in this way transversions may come about is illustrated in Fig. 16. 



transversion 



t ransversion 



e T 



by 



pairing- 

 mistake 



A- e 



by 

 pairing- 

 mistake 



T- C 



Fig. 16. Spontaneous production of transversions by rare base pairing between 

 two purines or two pyrimidines. 



Initiated by the first pairing mistake, larger alterations might be induced 

 occasionally (Freese, 1959b). Deletions of one or more nucleotide pairs 

 might also come about when one or more nucleotides in an existing DNA 

 strand form a loop and thus do not get copied during DNA duplication, 

 and inversely insertions might come about when by the same mecha- 

 nism one or more nucleotides get copied twice (Fresco and Alberts, 1960). 

 Apart from these mistakes inherent in the structure and mode of 

 replication of nucleic acids, spontaneous mutations may be caused by a 

 variety of agents made by the cell or present in the medium. Agents of 

 this kind are base analogs, peroxides, nitrous acid, etc., all of which 

 are produced under some growth conditions and often are inactivated 

 by special enzymes like catalase, peroxidase, etc. Some organisms have 

 an especially high mutations rate (Drosophila: Demerec, 1937; bacteria: 

 Treffers et al., 1954) ; jMiyake (1959) has shown that in .S. typhimurium 



