200 ROBERT L. SINSHEIMER 



ing infection and the components are reused in the synthesis of the DNA 

 of many phages. Ultraviolet-changed parts of this DNA fraction may simply 

 be discarded after degradation. 



However, it should be pointed out that this bipartite character has al- 

 ways been observed in P 32 -labeled (often heavily labeled) and, in Hershey's 

 experiments, ultraviolet irradiated particles, and some thought must be 

 given to the possibility that it is an artifact induced by the conditions, 

 including the heavy irradiation during synthesis, necessary to prepare the 

 labeled particles. 



That the physical evidence of a bipartite structure, the stars, may be 

 just an artifact is indicated by the following evidence. While the sedimen- 

 tation rate of the stars has been measured to be 41 S, in good agreement 

 with expectation for a DNA of that size, no evidence can be observed of a 

 large 41 S component (which should be a 36% component) upon ultracen- 

 trifugal analysis of normal T-even phage DNA. 66 When banded in the 

 cesium chloride density gradient, T4 DNA provides a band of almost pre- 

 cisely Gaussian distribution of ultraviolet absorption 67 and a calculated 

 DNA molecular weight of 14 X 10 6 . A mixture of DNA of two components, 

 one of molecular weight 45 X 10 6 comprising 36 % of the total, and a second 

 component of DNA of molecular weight less than 12 X 10 6 , should have 

 resulted in a clearly non-Gaussian band of ultraviolet absorption. Appro- 

 priate controls have indicated that the stars are not disrupted by banding 

 in cesium chloride. 56 



These results, together with the previously presented data, comprise 

 at present a paradox. The paradox may be resolved if it is assumed that 

 the stars are an artifact of growth in highly radioactive medium and do 

 not represent a component normally present in T-even phage particles. 

 Such an artifact would also account for the P 32 -transfer-suicide experiments. 

 The experiments with P 32 -ultraviolet irradiated phage would, however, then 

 seem to require an ad hoc explanation. 



Alternatively, the paradox might be resolved if the negative evidence — 

 the failure to obtain any centrifugal indication of a very large DNA mole- 

 cule — is in error because some stage in the execution of the centrifugal 

 analysis results in the disruption of such large molecules. 



b. Composition 



The nucleic acids of the T-even phages are unusual in composition; 

 while they contain the customary nucleotides, deoxyadenylic acid (dAMP), 

 deoxyguanylic acid (dGMP), and thymidylic acid (TMP), they do not con- 



66 H. K. Schachman, J. Cellular Comp. Physiol. 49 Suppl. 1, 71 (1957). 



67 M. Meselson, F. W. Stahl, and J. Vinograd, Proc. Natl. Acad. Sri. U. S. 43, 581 

 (1957). 



