GENETIC CONTROL OF CELL INTEGRATION 313 



possible to obtain separately, in pure form, polyadenylic acid and poly- 

 uridylic acid, both of which have been extensively studied, and a number 

 of other synthetic products as well. These polymers have been extremely 

 valuable in physical chemical studies. One question of importance from 

 the biological point of view concerns the properties and stability of 

 single-stranded nucleic acids. With the synthetic polymers it has been 

 shown that they form hydrogen-bonded double-stranded helices similar to 

 those of DNA, with the bases mainly lying in the inside and the sugar- 

 phosphate backbone on the outside of the helical molecule. Further- 

 more, three-stranded configurations have been observed, and have pro- 

 vided the basis for speculation that RNA may be synthesized in the cell 

 as a single-stranded polymer in contact with the double-stranded DNA. 



The fact that cellular RNA seems only to be single-stranded is an im- 

 portant finding in view of the in vitro demonstration that the single- 

 stranded molecules are very unstable in their physical configuration. 

 Their single-strandedness might be an evidence of their need for a 

 specific binding site for activation, or even that different binding sites 

 might make possible different activities. 



It is not established just what is the cellular role of polynucleotide 

 phosphorylase, but there is little known as yet of any other RNA- 

 synthesizing enzymes. It seems likely that some RNA may be replicated 

 by copying from pre-existing RNA, in view of the genetic role of viral 

 RNA. With the recent discovery of a single-stranded DNA in the bac- 

 terial virus, 0X174, more information may soon become available con- 

 cerning possible mechanisms of synthesis and stabilization of single- 

 stranded nucleic acids within cells. 



Only recently have most investigators begun to take into account the 

 known heterogeneity of RNA in their experimental design. With the 

 growing awareness that different classes of RNA are found within cells, 

 performing different functions and localized at different sites, it has be- 

 come evident that studies of total RNA have limited significance for most 

 questions. Consequently, it is only in recent experiments that attempts 

 have been made to distinguish among classes of RNA; these have begun to 

 shed some light on questions of RNA synthesis and function. 



Recent studies with E. coli have demonstrated that RNA can be syn- 

 thesized in the absence of protein synthesis, by uncoupling the two sys- 

 tems with chloramphenicol, provided that catalytic amounts of all amino 

 acids are present. However, this RNA synthesized in the presence of 

 chloramphenicol is unstable, perhaps owing to the absence of its com- 

 panion protein. The requirement of small amounts of all amino acids 

 for RNA synthesis suggests that an amino acid-nucleotide complex may be 

 an intermediate in RNA synthesis as it is in protein synthesis. 



