24 



THE BIOSYNTHESIS OF PROTEINS 



the DNA is introduced into a bacterium (Litman and Ephrussi-Taylor, 

 1959). 



Experiments by Hershey on the genetic material of bacteriophage are 

 almost as convincing. Bacteriophages are complex organisms made of 

 packed DNA enclosed in an elaborate protein structure. Hershey and 



Fig. 13. Schematic representation of the experiment of Hershey and 

 Chase (1952) (Reproduced from Kozloff, 1959). 



Chase (1952) grew bacteriophage in a medium containing either 32PO4 or 

 35SO4; the collected phage contained ^^p in DNA exclusively or ^^S in 

 protein constituents only. When non-labelled bacteria were infected with 

 labelled phages, it was found that ^^p (therefore DNA) enters the bacteria 

 and is retained in the progeny whereas most of the ^^S stays outside (Her- 

 shey, 1955). Phage particles may be compared to small syringes which inject 

 the DNA they contain into a bacterium. The injected DNA confers to the 

 infected bacteria the capacity of making the phage proteins and several 

 enzymes involved in the synthesis of phage DNA precursors (Flaks and 

 Cohen, 1959; Flaks et ah, 1959; Romberg et al., 1959; Somerville et ah, 

 1959; Earner and Cohen, 1959; Bessman, 1959; Keck et al, 1960). 



Phage DNA multiplies within the bacterium, it is later enwrapped into 

 its protein container. When two related phages infect the same bacterium, 

 that is when two phages inject their DNA into the same bacterium, re- 

 combination of hereditary characters may be observed. Phage DNA con- 

 tains all the information required for orienting the cell metabolism towards 

 the synthesis of a set of specific foreign proteins. It is clear that phage DNA 

 has all the features of the genetic material. 



There is also good reason to believe that during bacterial conjugation 

 it is essentially a DNA fibre which passes into the recipient bacterium, for 

 decay of ^^p^ which breaks the DNA molecule, causes breakages in the 

 chain of genetic markers (Jacob and Wollman, 1958). 



