33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 195 



nature of this DNA is found in the radioautographic experiments of Levin- 

 thai and Thomas. 5355 



By growing virus on cells grown in a medium containing a high specific activity 

 of P 32 , virus particles are prepared which contain as many as 100-200 P 32 atoms per 

 particle. This P 32 is almost entirely in DNA molecules. When such particles are em- 

 bedded in a nuclear emulsion, each beta particle arising from the P 32 disintegrations 

 will give rise to a developable track in the emulsion, originating at the phage particle. 

 The cluster of tracks originating from a particle after allowing radioactive decay to 

 take place for one or two half-lives is called a "star." 



The average number of tracks per star is thus a measure of the average P 32 content 

 per particle. If, before embedding in the emulsion, the virus particle is disrupted by 

 any of several means, 56 the tracks will arise from the separated DNA macromolecules 

 and the number of tracks arising from each molecule will be a measure of the P 32 

 content and hence (assuming uniformity of labeling) of the size of the macromole- 

 cule. 



When this experiment is performed, the DNA is found to consist of 

 one large component comprising 36 % of the DNA of a single particle (mo- 

 lecular weight 45 X 10 6 ) and smaller components each containing 10% or 

 less of the DNA of a single viral particle. Quantitative observations indicate 

 that one 36% piece arises per particle and that the size of this component 

 is discrete. The number of smaller DNA components arising per particle 

 is estimated to be between 10 and 20. 



The large component or "star" has been shown by partition cell centrif- 

 ugation 57 to have a sedimentation coefficient of 41 ± 5 S. This result is 

 in good agreement with the sedimentation constant to be expected for a 

 DNA molecule of this size from the empirical relationship between sedi- 

 mentation rate and molecular weight of DNA described by Doty et al. bs 



Brown and Martin have fractionated T2 DNA on columns of histone 59 

 and have reported that the stars are to be found in their fraction A 60 (vide 

 infra) . 



When such heavily labeled phages are used to initiate an infection in 

 unlabeled medium and the DNA of the progeny phage is examined by the 

 autoradiographic technique, stars approximately one-half the size of the 



53 C. Levinthal, Proc. Natl. Acad. Sci. U. S. 42, 394 (1956). 



54 C. Levinthal and C. A. Thomas, Jr., Biochim. et Biophys. Acta 23, 453 (1957). 



55 C. Levinthal and C. A. Thomas, Jr., in "The Chemical Basis of Heredity" (W. 

 D. McElroy and B. Glass, eds.), p. 737. Johns Hopkins, Baltimore, 1957. 



56 C. A. Thomas, Jr., J. Gen. Physiol. 42, 503 (1959). 



57 C. A. Thomas, Jr., and J. H. Knight, Proc. Natl. Acad. Sci. U. S. 45, 332 (1959). 

 ™ P. Doty, B. B. McGill, and S. A. Rice, Proc. Natl. Acad. Sci. U. S. 44, 432 (1958). 



59 G. L. Brown and A. V. Martin, Nature 176, 971 (1956). 



60 G. L. Brown, in "The Chemical Basis of Heredity" (W. D. McElroy and B. Glass, 

 eds.), p. 743. Johns Hopkins, Baltimore, 1957. 



