460 ROLLIN D. HOTCHKISS 



frequency/^^ or to stages in synchronously dividing plant tissue/^" ''^^ and 

 radioautographic estimation of DNA-phosphorus incorporation/^^ have led 

 to similar conclusions (Chapter 19). These results suggest that new DNA 

 for the daughter nuclei is available before division begins. 



2. The Significant Role of DNA in Bacteriophage 



Growth of living matter involves, as we have seen, the duplication of 

 DNA-containing structures, and is in all probability genetically oriented by 

 these entities. One may expect that the growth of intracellular viruses will, 

 among other things, require the production of new specific DNA's within 

 a cell already organized for the production of the host's own DNA deter- 

 minants. In the infections of E. coli with the T bacteriophages, the study 

 of these relationships has to some degree confirmed and extended the con- 

 ceptions already presented in this chapter. 



a. Composition of the T Phages 



The so-called T coliphages lytic for the host bacteria E. coli are small, 

 hexagonal or rounded, tail-bearing objects principally made up of protein 

 (around 60%) and DNA (about 40%), together with small amounts of 

 lipid. Although these viruses were once thought to be nucleoproteins, it is 

 now considered that the DNA is mechanically, rather than chemically, 

 confined by the protein. 



Intact phage particles are agglutinated by antiserum which reacts with 

 the protein, but are not affected by deoxyribonuclease. After plasmolysis, 

 however, by rapidly diluting phage suspensions from high into low salt 

 concentrations,!^* the DNA of T2 , T4 , and Te partly passes into solution^^^ 

 and all of it can now be rendered soluble by deoxyribonuclease. ^^^ The 

 remaining sedimentable "ghosts" appear to be collapsed and ruptured 

 phage membranes'^* and are still precipitable by the antiphage serum.^^^ 

 If phages are labeled with isotopic phosphorus (P*-) or sulfur (S*^), the 

 latter is all found in the protein ghosts and the P*' is all found in the DNA 

 made hydrolyzable by the rupture of the protein membranes. '^^ 



Apparently, then, phage protein exists largely as a membrane surround- 



1^9 J. M. Price and A. K. Laird, Cancer Research 10, 650 (1950). 



>6o M. Ogur, R. O. Erickson, G. U. Rosen, K. B. Sax, and C. Holden, Exptl. Cell Re- 

 search 2, 73 (1951). 



'" A. H. Sparrow, M. J. Moses, and R. J. Dubow, Exptl. Cell Research Suppl. 2, 245 

 (1952). 



'" S. R. Pelc and A. Howard, Expil. Cell Research Suppl. 2, 269 (1952). 



1" T. F. Anderson, Botan. Rev. 15, 464 (1949). 



'" R. M. Herriott, J. Bacteriol. 61, 752 (1951). 



'" A. D. Hershey and M. Chase, /. Gen. Physiol. 36, 39 (1952). 



