Viruses: Bacterial, Animal, and Plant 



401 



all are subsequently capable of reverse muta- 

 tion to, or near, the r+ phenotype. 



Although the mutational spectra (see p. 

 200) for 5-bromo uracil, for other chemical 

 mutagens, and for spontaneous mutants are 

 all different at the nucleotide level, we cannot 

 specify, with any certainty, the exact chemical 

 basis for the induced mutations. This is so 

 because there are a number of possible 

 metabolic paths through which the mutagen 

 may be producing its effect. It is clear that 

 the chemical basis for mutagenic action is 

 best studied when the pathway between muta- 

 gen and gene is the shortest possible.^ In 

 this connection, just as it is preferable to 

 expose sperm rather than any other cell of 

 that organism to a chemical mutagen, clearly 

 it is more desirable to treat phage or trans- 

 forming DNA directly, rather than indirectly, 

 via its host. 



What is the molecular basis of mutationl 

 Since the genetic material is a linear array of 

 nucleotides, consider how mutation might 

 involve single whole nucleotides. Loss or 

 gain of a single whole nucleotide might be 

 expected to result from breaking the nucleo- 

 tide string backbone at two or more places, 

 followed by deletion of a whole nucleotide or 

 its insertion in a new position. This might 

 occur especially frequently after exposure to 

 a physical mutagen which ionizes, and would 

 involve, at least occasionally, only the already 

 formed "old" gene material. However, 

 single whole nucleotide change may also be 

 produced by chemical mutagens without in- 

 volving breakage. It has been suggested ^ 

 that chemical mutagens like the acridines 

 insert themselves between the nucleotides of 

 a chain which is subsequently to replicate. 

 A molecule of a chemical mutagen, inter- 

 calated this way between bases that are linear 

 neighbors, could spread the chain 3.4A, and 

 result in the addition of an entire nucleotide 

 at this position to the complementary chain 



2 As noted by I. H. Herskowitz (1955). 



3 By L. S. Lerman (1961). 



made next. The possibility also exists that 

 an unbound nucleotide or other normal sub- 

 stance might intercalate with similar results. 

 This mechanism would involve changes in 

 the new genes formed. 



Before discussing the mechanisms possible 

 for intra-nucleotide changes, consider what is 

 known about the chemical behavior and 

 mutagenicity of certain chemicals. Free T4 

 phage is known to be permeable to certain 

 small molecules, such as nitrous acid (HNO2) 

 and hydroxylamine (NH2OH), both of which 

 are mutagenic. Nitrous acid removes NH2 

 from, or deaminates, purines and pyrimi- 

 dines. Thus, when cytosine is deaminated it 

 is converted to uracil and adenine is con- 

 verted to hypoxanthine (the structural for- 

 mulae for these compounds are shown in 

 Figure 35-5), while deaminated guanine be- 

 comes xanthine. Hydroxylamine acts in the 

 reverse manner from nitrous acid, by adding an 

 amino group, for example, to the 2 — C atom 

 of cytosine, forming a molecule that may 

 function like thymine. Such chemicals and 

 others probably act as mutagens by producing 

 intranucleotide changes. 



Such mutagens may ultimately cause one 

 purine to be substituted by another purine 

 (A <-> G) or one pyrimidine for another 

 (T <-)•€). Replacement by another base 

 of the same kind (purine or pyrimidine) can 

 be called transition, while replacement of a 

 purine by a pyrimidine or the reverse (for 

 example, A <-^> C or T ^-^ G) can be called 

 transversion^ Both transitions and transver- 

 sions would act at the subnucleotide level. 



What sequence of events may be involved 

 in transition and transversion? A particular 

 base pair T : A exposed to a mutagen may 

 become T : A' (see Figure 44-1). Suppose, 

 at the time of chain separation A' specifies C 

 (instead of T), and at the next division C acts 

 normally to specify G. The net result is that 

 the original A strand eventually produces a 

 granddaughter strand carrying G, so that 

 •* Following the terminology of E. Freese. 



