110 Information Storage and Neural Control 



complementary-RNA (C-RNA) strand then dissociates from the 

 DNA, and moves to the vicinity of nuclear or cytoplasmic ribo- 

 somes where it serves as a template for protein synthesis. Mean- 

 while, the DNx^ strands probably spontaneously return to the 

 double stranded form. 



Messenger-RNA (C-RNA) rapidly becomes radioactive when 

 cells are incubated with radioactive uridine or orthophosphate. 

 To account for the high turnover of this species of RNA, it has 

 been suggested that it is very unstable and that possibly it is 

 degraded after it has fulfilled its function in protein synthesis (4). 

 The fact that the bulk of the cellular RNA differs markedly in 

 composition from the DNA of a given organism and also from the 

 C-RNA suggests that C-RNA is present at very low concentrations 

 in the cell. As commonly isolated, C-RNA has a sedimentation 

 coefficient of only 9 to 12S, but this may be due to the fact that the 

 9 to 12S molecules represent degraded messenger-RNA chains. 



The first evidence for the existence of messenger-RNA came 

 from the experiments of Volkin and Astrachan (76). Using isotope 

 labeling, Volkin and Astrachan were able to show that in bacterial 

 cells infected with a bacteriophage, such as T-2, there was a high 

 turnover in a minor RNA fraction. This RNA fraction had an 

 apparent nucleotide composition which corresponded to that of 

 the DNA of the phage and was markedly different from that 

 of the host DNA. The experiments of Volkin and Astrachan 

 were subsequently confirmed and extended by Nomura and 

 co-workers (49). 



In 1960, Rich demonstrated that it was possible to form a 

 specific and complementary helical complex involving a synthetic 

 DNA strand (polydeoxyribothymidylic acid) and a synthetic RNA 

 strand (polyriboadenylic acid) (53). Schildkraut et al. (62) em- 

 ployed density gradient centrifugation experiments to show that 

 a hybrid complex between polydeoxyguanylic acid and poly- 

 ribocytidylic acid was also possible. 



In the same year, it was shown by several laboratories (5, 79, 

 80, 23, 71) that an RNA polymerase enzyme was present in 

 bacteria and in animal cell nuclei. The purified enzyme could 

 catalyze a net synthesis of new RNA froin uridine triphosphate, 

 adenosine triphosphate, guanosine triphosphate, and cytidine 



