88 CONTROL MECHANISMS IN CELLULAR PROCESSES 



for the structure of DN A proposed bv Watson and Crick ( 1953 ) 

 helped to explam self-replication of DNA and a coding system 

 (Gamow et al., 1956) suggested a means by which DNA could carry 

 the necessary genetic information. 



It is believed today that the basic function of genes is to store 

 and transmit information concerning the sequence of amino acids 

 in proteins. When the amino acid sequence was determined in 

 insulin (Sanger, 1956), this hypothesis became open to experimental 

 verification. Indeed it has been found that the insulin molecules of 

 different animal species differ by slight modifications of their amino 

 acid sequence (Sanger and Smith, 1957) and that hemoglobins of 

 genetically different strains have one amino acid substituted for an- 

 other in an otherwise unchanged molecule (Ingram, 1957). Many 

 laboratories are now engaged in the arduous task of finding direct 

 proof that gene mutation expresses itself in a change of amino acid 

 sequence of enzyme whose formation is controlled by the gene. 



In the meantime, cytochemical observations led Caspersson 

 (1941) and Brachet (1942) to suggest that RNA functions in pro- 

 tein synthesis. The idea found experimental support when protein 

 synthesis was obtained in vitro with preparations containing micro- 

 somes, cell particles rich in RNA (Siekevitz, 1952; Zamecnik and 

 Keller, 1954; Borsook, 1956). Evidence is continuously accumulat- 

 ing that RNA directs protein synthesis. This has become the basis 

 for the hypothesis, gradually accepted, that RNA is the primary 

 product of gene action and serves to transfer genie information to 

 the cytoplasm. Tracer experiments have shown that nuclei are par- 

 ticularly active in RNA synthesis ( Smellie, 1955 ) , and modern auto- 

 radiography, as we shall see later, has given strong support to these 

 observations by clearly demonstrating RNA synthesis in the nucleus. 



Besides being active in protein synthesis, RNA is capable of car- 

 rying the necessary genetic information. Many plant and animal 

 viruses are ribonucleoproteins, not containing any DNA. It has been 

 possible to isolate RNA from some of them (Fraenkel-Conrat, 1956; 

 Gierer and Schramm, 1956) and to infect the hosts with pure RNA. 

 The infected cells produced new complete viruses, proving that RNA 

 and not the protein moiety was necessary for virus reproduction. In 

 tobacco mosaic virus, Fraenkel-Conrat (1956) went a step further 

 and reconstituted a virus from its RNA and protein component parts. 

 RNA could be thus combined with a protein of another strain, dif- 

 fering in several properties. Such a "hybrid" virus was perfectly 



