III. BACTERIOPHAGE DNA AND BACTERIAL DXA 125 



shaped molecule is broken once by shear, its maximum extension would 

 now be twofold greater than in the closed form. This would mean that 

 it would be susceptible to further breakage. The hydrodynamics of this 

 breakage process has been examined by Levinthal and Davison flQei). 



The structure of To DXA is probably equivalent to that of T2 since 

 the critical stirring speeds that are required to break the T5 molecule are 

 the same as those required to break a T2 DNA fragment of 84 X 10^ 

 daltons (Hershey et al, 1962). In the case of T5, P^-, and T7 DXA, the 

 sedimentation coefficients are the same as that found for T2 fragments 

 of the same DXA content, if we assume a single molecule per phage 

 particle. However, the sedimentation behavior of these large molecules 

 is not understood, and it would be risky to infer molecular structure 

 from sedimentation coefficients at this time. 



The fact that the smallest and largest known bacteriophages have 

 been shown to contain a single molecule of DX'A leads one to the 

 obvious hypothesis that all bacteriophage contain a single molecule of 

 DXW. and that this unitary structure is a fundamental feature of the 

 function and construction of virus particles. 



3. The Mass per Unit Length — the Xumber of Polynucleotide 

 Chains in the Molecule 



While the autoradiographic results indicate that there is a single 

 linear molecule of DXA liberated from the phage particle, no conclu- 

 sions can be drawn about the number of polynucleotide chains making 

 up the molecule. Therefore, it would be appropriate to review the evi- 

 dence that the T2 DX"A molecule is a linear duplex of the Watson- 

 Crick type. The X-ray fiber photographs of DX^A extracted from T2 and 

 T7 phage show the same characteristic pattern as do DXA's from a wide 

 variety of other sources, except that T2 DX^\ is unable to form the 

 A form as the relative humidity is reduced; a fact which may be related 

 to the presence of glucose on the hydroxymethylcytosine residues (Ham- 

 ilton et al., 1959). Thus it would seem safe to conclude that at least a 

 certain fraction of these phage DX^A's have the structure derived for 

 the B form of DXA (Langridge et al, 1957, 1960a,b). The equatorial 

 spacing corresponding to the side-to-side packing of the molecules in 

 the fiber increases in a smooth fashion from a minimum value of 20 A 

 at 50% relative humidity to more than 30 A as the relative humidity 

 approaches 100% (Xorth and Rich, 1961). This is in accord with 

 expectation for the packing of helical duplex molecules of diameter 20 A 

 or less and makes unlikely the suggestion that there is a stable associa- 

 tion between two duplex molecules, unless one made veiy special 

 requirements on the packing and on the manner in which hypothetical 



