SIZE AND MORPHOLOGY OF BACTERIOPHAGES 47 



TABLE III 

 Sizes of Phage Particles Estimated from Particle Weights 



"■ Calculated for a measured or assumed density of 1.5 grams per cubic cm. 



feet. However, the purity of the best preparations is excellent, 

 and at least half the particles form plaques (Luria, Williams, and 

 Backus, 1951). Therefore the particle weight of T2 given by 

 Herriott and Balrow (1952) is overestimated by a factor less than 

 two, and the particle diameter by less than 26 per cent. 



9. Summary 



The well-studied phage particles are sperm-shaped structures 

 containing, in the head portion, mostly nucleic acid surrounded 

 by a protein membrane, often hexagonal in cross section, to 

 which the tail is attached. From some phages, the membrane 

 and tail can be isolated as a functionally intact "ghost" following 

 osmotic shock. Different phages range in size from approxi- 

 mately 0.1 micron in diameter to a poorly defined minimum of 

 perhaps 20 mju. They also show characteristic differences in 

 shape and structure. 



Sizes of these particles can be measured in three more or less 

 satisfactory ways. Electron microscopy is the most informative. 

 It probably underestimates size in general, because of unavoid- 



