22 



CELL HEREDITY 



60 m^ 



Head 



100 rriM 

 1000 A 



FIGURE 1.8. Structure of 72 bac- 

 teriophage (from Kozloflf, in: Zirkle, 

 A Symposium on Molecular Biology, 

 Chicago, Copyright (1959) by the 

 University of Chicago, p. 178). 



Tail fibers 



,20 mjuj 



streptomycin resistance are all lost together; and in transformation 

 studies these three genes exhibit linkage. Thus the parallel loss of 

 activity by linked genes and independent loss of activity by unlinked 

 genes, at characteristic melting out temperatures, are very fine evidence 

 that linked genes are on the same molecule and unlinked genes are on 

 different molecules of DNA. 



Phage DNA 



Another system in which the DNA composition of hereditary material 

 has been demonstrated is that of certain bacterial viruses, the T phages, 

 which attack the bacterium, Escherichia coli. These viruses, or phages, 

 have a complex structure composed primarily of protein and DNA (Fig. 

 1.8). All of the DNA is in the head of the phage, and the tip of the tail 

 contains a specific protein which attaches to particular receptor sites of 

 the bacterial wall. After infection, multiplication occurs in the bacterial 

 host cell, and a large number of progeny phages, genetically identical 

 with the infecting one, are produced. Phages may differ from one an- 

 other by a number of heritable properties which are transmitted to their 

 progeny. Thus the phage carries hereditary determinants. 



One may then ask: Can the hereditary determinants be identified with 

 a particular chemical constituent of the phage? This question was 

 answered by the following experiment. 



