. Cell heredity. Cytogenetics. 38 CELL HEREDITY > - â c FIGURE 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 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
. Cell heredity. Cytogenetics. 38 CELL HEREDITY > - â c FIGURE 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 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
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