Viruses: Bacterial, Animal, and Plant 



403 



^ 



N- 



H 



Adenine 





CH, 



-N 



O H 

 Thymine 



H 





H 



N 

 Adenine 



^ 



O H 



Cytosine 



FIGURE 44-2. Tautomeric shift of ade- 

 nine which could change its complementary base 

 from thymine to cytosine. Upper diagram shows 

 adenine before, and lower diagram after, under- 

 going a tautomeric shift of one of its hydrogen 

 atoms. {After J. D. Watson and F. H. C. Crick.) 



nucleotide change. But at what point in a 

 series of changes should one consider that a 

 mutation has been first produced? The mu- 

 tation can be said to be accomphshed when, 

 as in one of the mechanisms discussed, A is 

 permanently changed to A'. You might 

 object to this answer on the basis that A' may 

 never be reproduced in a future rephcation. 

 But, the product of a novel change need not 

 be replicated or transmitted in order to be 

 considered mutant. Such a novel product 

 need only be more or less permanent in order 

 to qualify in this respect. A' may be demon- 

 strated to be more or less permanently 

 different from A in five ways (as identified by 

 five different operational procedures). First, 

 A' may have a different chemical composi- 

 tion; second, it may have a different rate of 

 change to a new chemical form ; third, it may 

 not specify T at all, or to the same extent as 

 A did; fourth, it may change the phenotypic 

 effect of the cistron in which it is located; 



fifth, it may affect the recombination rate of 

 itself or another recon. Operationally, then, 

 it would seem desirable to classify as a 

 mutation any one or a combination of novel 

 identifiable changes in the chemical, muta- 

 tional, replicative, phenotypical, or recombina- 

 tional properties of one or more nucleotides. 

 This new operational definition of mutation 

 includes all aspects of the older one (a novel 

 qualitative or quantitative change in the 

 genetic material). Of course, at the present 

 time, certain of the changes listed, which 

 would identify a mutant, cannot be detected 

 in specific individual nucleotides for technical 

 reasons. Even so, it would seem fruitful to 

 indicate even now all the possible operational 

 ways we may be able to identify a mutant. 

 It is also clear, from the preceding, that the 

 smallest part of the genetic material whose 

 change is mutant is presumably smaller than 

 a nucleotide (and therefore smaller than a 

 recon). Subnucleotide changes could in- 

 volve the methyl, hydroxymethyl, or other 

 groups or atoms in the base portion, as well 

 as changes in the sugar or the phosphate 

 portions of a nucleotide. Subnucleotide 

 components should not be considered to be 

 the smallest units of the genetic material 

 capable of mutation, since the nucleotide is 

 the smallest meaningful chemical unit of the 

 genetic material. It is probably more fruitful 

 to speak of subnucleotide parts as furnishing 

 a number of mutational sites within the nucleo- 

 tide. 



Until now, our discussion of viruses has 

 been restricted almost completely to DNA- 

 containing phages. (Genetic recombination 

 also occurs in the more complex vaccinia 

 virus, that causes smallpox, whose DNA is 

 probably single-stranded in the infective 

 stage.) There is another group of viruses 

 which contains no DNA but is entirely, or 

 mainly, ribonucleoprotein in content. These 

 viruses include many of the smaller animal 

 viruses (causing poliomyelitis, influenza, and 

 encephalitis, for example), at least one bac- 



