98 CONTROL MECHANISMS IN CELLULAR PROCESSES 



for one particular amino acid, so that the cell must contain 20 dif- 

 ferent such molecules corresponding to the different amino acids 

 (Schweet et al, 1958a; Berg and Ofengand, 1958). According to 

 present theories, the soluble RNA carries activated amino acids onto 

 the ribosomal RNA template and functions as an "adapter" ( Hoag- 

 land, 1959) for the amino acid on an appropriate site of the tem- 

 plate. Nothing is known about the origin of this RNA and it is 

 possible that it has been overlooked in experiments with autoradi- 

 ography. In Neurosjwra, there was no indication of a soluble RNA 

 being present free in centrifuged cells (Zalokar, 1960b), where it 

 should not be sedimented, but it is possible that in a living cell it is 

 loosely bound to the ribosome fraction. Recent reports (Venka- 

 taranan and Lowe, 1959) indicate, however, that an appreciable 

 fraction of ribonucleoprotein becomes soluble in alcohol after treat- 

 ment with cold trichloracetic acid. Since the preparations used in 

 autoradiographv were subjected to these chemicals, it cannot be 

 excluded that soluble RNA was removed. 



Chemistry of RNA Synthesis. All the evidence presented for the 

 synthesis of RNA from DNA has been rather indirect, and a direct 

 proof of such a svnthesis can be obtained only by discovering the 

 chemical mechanism of RNA synthesis. The immediate precursor 

 for RNA is believed to be a nucleotide. The appropriate nucleotides 

 are presumed to be assembled on a DNA template and bound into 

 an RNA molecule. Ochoa and his collaborators (Grunberg-Manago 

 et al., 1955) discovered an enzyme which is able to build polynucleo- 

 tides from nucleoside diphosphates. A primer was probably needed 

 (Mil and Ochoa, 1957), but the polynucleotides built reflected the 

 original proportion of nucleotides in the reaction mixture (Ochoa, 

 1957 ) . It is not probable that the action of such an enzyme by itself 

 would be sufficient to produce information-carrying molecules. But 

 in conjunction with a DNA template, the enzyme may perform the 

 function of linking the nucleotides into a polynucleotide chain. 



It was believed for a while that RNA synthesis requires concomi- 

 tant protein synthesis (Spiegelman et al, 1955). Later, by using 

 chloramphenicol to inhibit protein formation, it was demonstrated 

 that RNA can be formed in the absence of protein formation, but a 

 complete set of free amino acids is still necessary ( Gale and Folkes, 

 1953; Pardee and Prestidge, 1956; Yeas and Brawerman, 1957; Gros 

 and Gros, 1958 ) . Tlie RNA thus formed seemed to differ from nor- 



