THE CHEMICAL BASIS OF HEREDITY 25 



synthetic strains of virus, each with the RNA of one strain and the 

 protein of the other. When a plant is infected with one of these 

 synthetic strains, new progeny virus particles are produced. These 

 particles can then be classified by the type of infection they produce 

 as either TMV, HRV, or something atypical. The results of such an ex- 

 periment, diagrammed in Figure 1.10, show clearly that the RNA alone 

 determines the type of virus progeny produced. A similar conclusion 

 has been drawn from infectivity studies with isolated RNA from a 

 number of animal viruses, notably those producing polio and en- 

 cephalitis. 



One may conclude from these experiments that two classes of macro- 

 molecules, the ribonucleic acids and the deoxyribonucleic acids, carry 

 hereditary information in particular systems, the transformable bacteria 

 and the viruses. What reasons are there to consider these findings of 

 general significance and applicable to higher forms? 



Nucleic acids in higher forms 



There is a considerable body of information available concerning the 

 localization of nucleic acids in specific regions within cells. An electron 

 micrograph of a cell is shown in Plate I. DNA is located exclusively 

 within the nucleus and is entirely a chromosomal constituent (except in 

 special circumstances to be discussed later). The importance of 

 chromosomes as carriers of hereditary material was proposed in the last 

 century on the view that the meticulous mechanics of their segregation 

 at cell division must reflect the tremendous importance of each chromo- 

 somal element in the life of the cell. Subsequently, with the develop- 

 ment of genetics and the correlations between genetic and cytological 

 observations, these inferences of the early cytologists were abundantly 

 confirmed (see Chapter 4). More recently, with the cytochemical 

 localization of DNA in the chromosomes, it was proposed that DNA it- 

 self might represent an essential constituent of hereditary material. 

 Without constituting any proof of the genetic role of DNA, this cyto- 

 chemical evidence could certainly be taken as consistent with it. 



Further indirect evidence came with the demonstration that the total 

 DNA per cell is constant for each species. This is true irrespective 

 of the tissue examined except that sex cells, the egg and sperm, which 

 have half the number of chromosomes of other cells also have half the 

 DNA content (Table 7.1), and special cells with double or triple the 

 usual chromosome count have a corresponding increase in DNA con- 

 tent. The constancy of DNA is impressive because of its generality. 

 Nonetheless, it should be pointed out that a particular class of chromo- 



