II 



NUCLEIC ACID CHEMISTRY CELLULAR LOCALIZATION 26 1 



types of nucleic acids are built on the same pattern, being both made of a large 

 number of subunits, called nucleotides; everyone of these nucleotides can be 

 broken down, on hydrolysis, to orthophosphoric acid, a pentose or deoxypentose 

 sugar and a purine or pyrimidine base. 



Hydrolysis of a nucleic acid usually yields 4 different bases, 2 purines (guanine, 

 adenine) and 2 pyrimidines (cytosine and thymine or uracil according to the 

 nature of the nucleic acid). It is for this reason that the view that nucleic acids are 

 composed of 4 nucleotides (and are thus tetranucleotides) has held so long; but 

 such a simple hypothesis is no longer tenable, since we now know for certain that 

 many DNA's contain a fifth base (5-methylcytosine). Furthermore, the molar 

 proportions of the various bases vary considerably from one nucleic acid to another 

 (Chargaff, 1951). There is excellent evidence, mainly addviced and discussed by 

 Chargaff (1951), that DNA's extracted from different species are distinct entities; 

 and there is even increasing evidence that one and the same organ, calf thymus 

 for instance, contains at least 2 different DNA's (Chargaff^/ al., 1953; Brown 

 and Watson, 1953; Bendich, 1952) distinguished by the molar ratios of their 

 constituent bases and their metabolic stability. 



The situation is not quite so clear in the case of RNA, because this type of 

 nucleic acid is much more labile and difficult to isolate in a pure and native state 

 than DNA: recent evidence from Chargaff 's laboratory (Elson, Trent and Char- 

 gaff, 1955) indicates little difference in the composition of the bases between RNA's 

 isolated from different organs and even from different species. 



From a straight chemical viewpoint, two important differences have now been 

 found between DNA and RNA: while RNA contains the usual pentose (/-ribose, 

 DNA is made of a deoxypentose, namely (/-2-deoxyribose. 



HOCHa / \ OH HOCHa / \ OH 



OH H 



</-deoxyribose 



Secondly, whereas RNA possesses uracil as one of its main pyrimidine bases, 

 DNA contains instead one of its methyl derivatives, thymine. 



The rest of the two molecules {i.e. H3PO4, guanine, adenine and cytosine) is 

 common to both nucleic acids and the resemblance between them is further in- 

 creased by the fact that many DNA's and RNA's present striking regularities in 

 their molar base composition: for instance, the total of the keto compounds 

 (guanine + thymine or uracil) is equal to the sum of the amino compounds 

 (adenine + cytosine; Elson and Chargaff, 1955). Such regularities are, as we 

 shall soon see, of importance in assessing a structure to the nucleic acids. 



[b) Physical properties and tnolecular structure 



Physical studies have shown that nucleic acids are macromolecular compounds 

 consisting of a very large number of individual nucleotides. In the case of DNA 



Literature p. zgg 



