38 



CELL HEREDITY 



0.15 



> 0.10 - 



■c 0.05 



FIGURE 2.2. The rate of pro- 

 duction of necrotic mutonts de- 

 tected by thie infection of tobocco 

 leaves with the RNA of tobacco 

 mosaic virus which had been ex- 

 posed to nitrous acid (after 

 Mundry and Gierer, 1958, Z. 

 Verebungslehre, 89:614). 



12 3 4 5 



Minutes Exposed to Nitrous Acid 



of mutant.s would increase with time. But in the nitrous acid experiments 

 the rate of production of mutants is constant, and so we conclude 

 that one chemical reaction is sufficient to cause a mutation. This 

 does not mean that every oxidative deamination of a base in RNA 

 causes a mutation, but rather that when a suitable base is changed 

 the mutation takes place. Nor does it mean that all mutations are 

 caused by a single modification of a single base in the polynucleotide 

 chain. As we will see later, mutations may occur in various ways, 

 including even gross rearrangements of many bases in fairly large seg- 

 ments of the chromosome. 



The fact that the change of one base can cause a mutation points up 

 an interesting conceptual situation. The unit that is being studied in 

 mutation is at least as small as a single nucleotide, but the gene must be 

 considerably larger than that. In order to possess the specificity neces- 

 sary to account for gene action, the gene as a functional unit of genetic 

 material must consist of many nucleotides; it is only in a polynucleotide 

 that different base sequences can be achieved. The smallest mutable 

 unit is, therefore, smaller than the unit of function, and only a part of it. 

 From this consideration alone it is apparent that genetic material con- 

 sists of at least two kinds of units, namely, functional units (genes) and 

 mutable subunits of which the genes are composed. 



Subdivision of the genetic material may be studied by still other opera- 

 tions. For example, we saw that certain genes are often linked in 

 transformation but that, nonetheless, they will sometimes be incor- 

 porated separately. We may ask: What is the smallest unit that can be 

 separated from the neighbors to which it is linked? This question must 



