Chemical Nature of Genes 



263 



typical DNA). Uracil's structure is shown 

 in Figure 19-3. The two purines commonly 

 found in DNA, adenine and guanine, are 

 also common in ribotides. In RNA the base 

 plus sugar combination is called a ribonu- 

 cleoside or riboside. Ribosides are joined 

 together by phosphates joined both at the 

 3' and 5' positions of the sugar just as in 

 DNA; consequently Figure 19-9 can repre- 

 sent a polyribotide if an O is added at each 

 2' position (making each sugar D-ribose), 

 and if among the bases usually present uracil 

 is substituted for thymine. It should be 

 noted that RNA also absorbs ultraviolet 

 light of 2600 A but can be removed from 

 the chromosome by treatment with ribo- 

 nucleases or RNases. 



In summary, typical chromosomes con- 

 tain two nucleic acids, DNA and RNA. 

 These normally occur in combination with 

 protein to form nucleoproteins (deoxyribo- 

 nucleoprotein and ribonucleoprotein) in 

 which DNA and RNA occur as polynucleo- 

 tides (polydeoxyribotides and polyribotides). 

 Each polynucleotide is built of (mono-) 

 nucleotides (deoxy- and ribotides, respec- 



tively), which in turn are composed of phos- 

 phates joined at 5' of nucleosides (deoxy- 

 ribo- and ribosides ) . These nucleosides are 

 made up of a pentose (2'-deoxy-D-ribose 

 and D-ribose) joined to a pyrimidine (usu- 

 ally cytosine or thymine and cytosine or 

 uracil) or to a purine (usually adenine or 

 guanine). A portion of this terminology is 

 summarized in Figure 19-10. 



Although the RNA in chromosomes pos- 

 sesses neither the proper quantitative varia- 

 tion nor the constancy expected of ordinary 

 chromosomal genes, it does have the same 

 linear organization as DNA. Moreover, 

 some viruses composed primarily of ribo- 

 nucleoprotein (influenza, poliomyelitis, and 

 other encephalitic viruses; plant-attacking 

 viruses such as the tobacco mosaic virus; and 

 certain bacteria-attacking viruses) possess 

 genetic properties but do not contain DNA. 

 Since DNA rather than protein is favored 

 as being the genetic chemical under typical 

 chromosomal conditions, it is reasonable to 

 consider RNA rather than the protein to be 

 the chemical basis of genetic specification in 

 these particular viruses. 



SUMMARY AND CONCLUSIONS 



This chapter is an initial attempt to throw some light on the chemical nature of the 

 genetic material. The search for chemical substances with properties of the genetic 

 material has led to a consideration of the protein found in chromosomes, but the avail- 

 able evidence does not actively support such a primary role for protein. 



It is hypothesized that DNA either is or, at least, is intimately associated with the 

 genetic material in chromosomes in view of the following: the localization of DNA; 

 its quantity and distribution in mitosis, meiosis, and fertilization; its quantity in poly- 

 ploid and polynemic situations; the parallel between DNA absorption and the muta- 

 genicity of ultraviolet light; the maintenance of molecular integrity; and its long, linear, 

 unbranched arrangement. It is also hypothesized that RNA may assume the genetic 

 role in certain DNA-free viruses. Some details of the chemical nature of DNA and 

 RNA are presented. 



Subsequent chapters will aim to further test the hypothesis that DNA (and RNA 

 in special cases) is typically either the genetic material or intimately associated with 

 it. Our ultimate objective is to determine the chemical units of the genetic material — 

 chemical units corresponding to the genetic units of replication, mutation, recombina- 

 tion, and function. 



