278 ORIGIN OF STRUCTURES AND FUNCTIONS 



infectivity. In a very recent experiment Fraenkel-Conrat^*^ 

 assembled active virus particles from the protein of one strain 

 of virus and the nucleic acid of another. A particularly 

 interesting complex virus of this kind is obtained from the 

 protein of common tobacco mosaic virus and the nucleic 

 acid of the Holmes ribgrass strain of virus. This combination 

 is notable for the fact that the protein of tobacco mosaic virus 

 differs from that of the ribgrass virus both in amino acid 

 composition and in serological properties. 



The complex virus thus obtained is inactivated by anti- 

 tobacco mosaic sera but not by anti-ribgrass. Furthermore, 

 plants infected with this complex, artificial virus form only 

 the nucleic acids and proteins characteristic of the ribgrass 

 virus. In particular, the protein contains methionine and 

 histidine which are characteristic of this virus. 



These results are in complete agreement ^vith those of 

 A. Gierer and G. Schramm.^** In contradistinction to the 

 findings of Fraenkel-Conrat and Williams which have already 

 been described, Gierer and Schramm showed that the nucleic 

 acid of a virus, completely freed from protein, when intro- 

 duced into a tobacco plant ^vould evoke the formation of the 

 characteristic nucleoprotein of the virus. 



This fact may be compared with earlier observations on 

 the infection of bacteria with the corresponding phages. In 

 their experiments A. D. Hershey and M. Chase^*® used 

 bacteriophage of group T which had been labelled with 

 phosphorus and sulphur. They showed that only the DNA 

 of the phage enters into the bacterium and evokes there the 

 formation of fresh virus while the protein stays outside the 

 cell of the host and therefore does not play a direct part in 

 the infection. 



From the examples which have been given it is quite 

 obvious that in living cells there is no ' multiplication ' of 

 protein molecules, nor do they arise as a result of straight- 

 forward autocatalysis. In cells there occurs a complicated 

 biosynthetic process whereby new protein is formed, the 

 carrying out of ^\hich requires the participation of a large 

 number of complicated energic, catalytic and structural 

 systems ^vhich we have remarked on in the living body. 

 Nucleic acids with their definite intramolecular structure 



