CHEMISTRY AND VIRAL GROWTH 



the conservation of materials during multiplication, which in 

 turn leads back to questions about structure and function. 



THE NUMBER OF LINEAR MOLECULES 



The number of structures injected into the bacterium is not 

 known beyond the fact that one phage particle contains about 

 2 X 10~^° 7 of DNA, which corresponds to about 20 molecules 

 in the isolated form (19). Presumably the number could be 

 determined either by physical or genetic methods. The follow- 

 ing genetic experiment illustrates a principle of wide application 

 to questions of this kind. 



T2 contains at least three regions of genetic material that 

 assort independently during genetic recombination (24). If a 

 bacterial cell is infected with two phage particles genetically 

 marked in one of these regions, and if the infections are spaced 

 by an interval of 2 minutes or more, the second particle will 

 generally not contribute its genetic marker to the progeny (15). 

 At least some of the DNA of the second particle enters the cell, 

 because about half of the DNA of the superinfecting particles is 

 quickly split into acid-soluble materials (38). The mechanism 

 of exclusion is otherwise unknown, but one can ask whether the 

 unit of exclusion is the genetically intact phage particle, or 

 something smaller. To answer this, one infects bacteria in 

 succession with two phage particles marked in two of the inde- 

 pendently assorting regions of genetic material, and examines 

 yields from single bacteria to see whether one of the two markers 

 can contribute while the other is excluded. A single test of this 

 kind (26) showed that the two markers were always excluded 

 together. For what it is worth, this result suggests that the 

 genetic unit called vegetative phage (or its precursor) is also a 

 structural unit. 



PHYSICAL STRUCTURE 



The composition of vegetative phage is difficult to investi- 

 gate, first because there is no unambiguous way to identify 

 interesting structures if they could be isolated ; second because 



