33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 227 



e. Phage Heterozygotes 



Among the progeny of a phage cross are a small number (a few per 

 cent) of particles which appear to contain, in a small region of the genetic 

 map, the genomes of both parents. 167 " Such particles are known as "het- 

 erozygotes." Upon further culture such heterozygotes do not usually prop- 

 agate as such but give rise to particles of both genotypes. 



Recently evidence has been obtained that chemical mutagens such as 

 nitrous acid 158 (I. Tessman, personal communication, 1959) and 5-bromo- 

 uracil (D. Pratt and G. Stent, personal communication, 1959) produce con- 

 siderable numbers of heterozygotes. Since these mutagens are expected, as 

 an initial act, to cause the formation of a mismatched base pair in a DXA 

 molecule, this evidence strongly suggests that in naturally occurring het- 

 erozygotes, in a small portion of the DNA, one strand of the double helix 

 represents one parental genome while the other strand is partially mis- 

 matched and represents the other parental genome. 



This idea is in accord with the evidence from the minute bacteriophages 

 (vide infra) that a single DXA strand is adequate to convey genetic in- 

 formation. 



5. Radiobiological Studies of the Process 

 of Phage Infection 



Varied attempts have been made to obtain some insight into the proc- 

 ess of bacteriophage infection by studies of the changes, during infection, 

 of the sensitivity of the phage-bacterium complex to inactivation, either 

 by ultraviolet radiation, 40 ' 8889168 or by X- or 7-radiation, 90 ' 169 or by 

 decay of P 32 incorporated within the parental phage DXA.. 6 17 ° 



These studies have indicated that the sensitivity of the phage-bacterium 

 complex to any of these modes of inactivation is, immediately after in- 

 fection, effectively the same as the sensitivity of the free phage. Since, 

 upon infection the principal phage material introduced into the bacterium 

 is the phage DXA, this result implies that the major action of these agents 

 when applied to the free phage is upon the DXA. 



With phage T4, there is, during the first 4 or 5 minutes of infection, no change in 

 the sensitivity of the complex to inactivation by ultraviolet radiation, X-radiation, 

 or P 32 decay. 40 Following this period, there is a decline in sensitivity until about 10 

 to 15 minutes after infection. At this time, by which a pool of some 40 to 50 phage 

 DXA complements has been made, the complexes are completely insensitive to the 



167a A. D. Hershey and M. Chase, Cold Spring Harbor Symposia Quant. Biol. 16, 471 

 (1951). 



168 N. Symonds, Virology 3, 485 (1957). 



169 R. Latarjet, J. Gen. Physiol. 31, 529 (1948). 



170 G. S. Stent, J. Gen. Physiol. 38, 853 (1955). 



