268 NUCLEIC ACIDS AND GROWTH 3 



(c) Genetic activity of DXA 



Since the problem of the genetic functions of DNA Kes somewhat outside the 

 scope of the present book, a short summary only of the main evidence will be 

 given here; the whole subject has recently been excellently reviewed by Hotchkiss 



(1955a)- 



Of special importance in the problem of the genetic role of DNA are the experi- 

 ments of Hershey and Chase (1952) on the T coli phages, because they deal with 

 one very essential growth process : the duplication of DNA-containing structures, 

 so characteristic of mitosis. Recent work on the structure and chemical composi- 

 tion of the T phages, shows them to consist chiefly of a protein membrane, re- 

 presenting about 60% of the virus, surrounding a central core of DNA (40%). 

 On infection, the phage particles become adsorbed by their tails on the sensitive 

 bacteria. 



It has been found by Herriott (1951a) that the membranes can be emptied of 

 their DNA core when they suffer osmotic shock: these protein "ghosts" can still 

 infect the cells, but no phage is reproduced in the absence of DNA, suggesting 

 that DNA is essential for phage formation. 



In Hershey and Chase's (1952) elegant experiments, the DNA is specifically 

 labelled with -^^P, while the protein ghosts are labelled with -^-^S. The result, after 

 infection, is that the ■*'? labelled DNA of the phage enters into the cells, while 

 the protein (marked with ■''-''S) remains attached on the outside surface; most of 

 it can afterwards be removed from the cell membrane by subjecting the infected 

 bacteria to the action of a blender. The complete phage thus appears to function as 

 a syringe which injects the genetically important material, DNA, into the cell. 

 Phage reproduction, which involves synthesis of new DNA and new protein mem- 

 branes, is thus a consequence of the initial introduction of phage DNA into the 

 cell. 



While these experiments strongly support the view that in phage reproduction 

 DNA plays an extremely important role, they do not prove that DNA is equiva- 

 lent to genes. This last assumption is demonstrated by the study of bacterial trans- 

 formations : the famous experiments of Avery et al. ( 1 944) have shown that DNA 

 extracted from an encapsulated (smooth) strain of pneumococcus will transform 

 unencapsulated (rough) bacteria into encapsulated cells. Since the transformed 

 cells will multiply indefinitely with a capsule, the property of forming a capsule 

 has been genetically transmitted. 



The DNA extracted under these conditions behaves thus very much like a gene 

 in inducing a heritable change and in being capable of self-duplication. As shown 

 by Avery et al. (1944) and by Hotchkiss (1949), the biological activity of the 

 pneumococcal DNA is lost after digestion with deoxyribonuclease, while ribonu- 

 clease and proteolytic enzymes have no effect. Although it is impossible com- 

 pletely to exclude the possibility that proteins contaminate DNA, it has been 

 shown that the protein content of the purified factor is less than 0.02%. 



Since Avery et al.'s (1944) pioneer work, many other bacterial transformations 

 have been discovered and studied in detail : in all cases, the transforming activity 

 has been found to be present in the purified DNA fraction, while the enzyme 

 deoxyribonuclease always inactivated the transforming principle. 



