Pyrimidine Moieties in Aiiimals, Plants, and Bacteria 103 



demonstrated by conjugation or transduction. No hybrid formation 

 was detected between the DNA oi E. coli K-12 and that oi Salmonella 

 typhimurium. The latter bacteria mate but transduction from one 

 to the other occurs only to a very limited extent, if at all. 



Aside from the taxonomic importance of this technique, it offers 

 a rational approach to the study of genetic compatibility where 

 genetic exchanges have not been demonstrated. In addition, the 

 technique has found application in connection with the problem 

 of information transfer between DNA and RNA. The latter 

 experiments will be discussed in the next section of this paper. 



THE RIBONUCLEIC ACIDS (RNA) 



General Characteristics 



The ribonucleic acids (RNA), which mediate the transfer of 

 genetic information between DNA and proteins (Fig. 1), differ 

 chemically from DNA in several ways (35): 1) The sugar com- 

 ponent of RNA is ribose, instead of deoxyribose (Fig. 2); 2) Uracil 

 (U), instead of thymine, is the 6-keto-pyrimidine base in RNA 

 (Fig. 3); 3) RNA is a single stranded, flexible polynucleotide coil 

 unlike DNA which is rather stiff and double-stranded; and 4) Most 

 RNA molecules are much shorter in length than DNA. Also, 

 RNA is less stable in alkaline solutions than is DNA. 



Four classes of RNA are known: l)transfer-RNA, 2) ribosomal- 

 RNA, 3) messenger or informational-RNA, and 4) virus-RNA. 



Transfer-RNA 



Transfer-RNA (T-RNA or S-RNA) consists of a family of 

 molecules which function in the activation of amino acids and in 

 the transfer of the activated amino acids to the ribosomal tem- 

 plates so that they can be linked together to form proteins. Prob- 

 ably, a different and characteristic transfer-RNA molecule is 

 required for each of the twenty amino acids. Yeast T-RNA 

 specific for the activation of the amino acid, valine, has recently 

 been obtained by Stephenson and Zamecnik (70) in highly 

 purified form (65-80 per cent). Holley et al., (31) have partially 

 purified the alanine, valine, and tyrosine T-RNA of yeast and 

 have studied the oligonucleotide content of ribonuclease digests. 



