124 GENERAL BIOCHEMISTRY 



The biohigical roles of the ribonucleic acids are still not clear. It is 

 known that the synthesis of proteins by cells is accompanied by syn- 

 thesis of ribonucleic acid. It has been reported that, if protein syn- 

 thesis is blocked, synthesis of ribonucleic acid can still occur. But ap- 

 parently the converse is not true, since blocking the synthesis of 

 ribonucleic acid blocks that of protein too. Hence it is believed that 

 ribonucleic acid is somehow involved in protein formation, but there 

 is as yet no conclusive support for any mechanism proposed. 



Much of the above structural information applies equally well to the 

 deoxyribonucleic acids. In these compounds the second hydroxyl 

 group of the sugar is replaced by a hydrogen atom. This difference 

 reduces the opportunity for branching of the chain, and there is no 

 evidence of branches. A spiral form is assigned to the sugar phosphate 

 chain with the bases distributed as in the related compounds. How- 

 ever, at least some of the deoxyribonucleic acids contain more than 

 four different bases. Adenine and guanine are the only known purines, 

 but cytosine, thymine, 5-methylcytosine, and 5-hydroxymethylcytosine 

 are all known to occur, though perhaps not all in the nucleic acid 

 from any single source. 



The problems of isolation and separation again are difficult and 

 complicated by the instability of these macromolecules, which are 

 nevertheless more stable than those of the ribonucleic acids. Although 

 sensitive biological tests have been devised, they are still of limited 

 application and so far have contributed little to the general structural 

 study of deoxyribonucleic acids. 



Extensive investigation assigns the deoxyribonucleic acids, through 

 their nucleoproteins, a primary role in heredity. It is now believed 

 that the structmes of these compounds contain information chemically 

 transmitted to daughter cells and fixing the character of these cells. 

 Since detailed structural information is still largely lacking, the cur- 

 rent theories of mechanism have no real basis except perhaps that of 

 possibility. Certainly the deoxyribonucleic acids are important, since 

 alteration of these compounds inside cells leads to inability to multiply 

 or to death. 



The infectious agents of the virus group also contain nucleic acids, 

 sometimes of the ribose and sometimes of the deoxyribose type. 

 Viruses specifically attack particular host cells and appear to be in- 

 nocuous for all species outside their individual host ranges. Well- 

 known diseases are produced in all groups of the animal kingdom, 

 including mammals, of course. Plant viruses may be equally wide- 

 spread and certainly are of great agricultural importance. Even 

 microorganisms are parasitized by viruses. Much study has been de- 



