122 CHARLE.s A. THOMAS, JR. 



any significant systematic error in the light-scattering molecuhir weight 

 detennination. Sinsheinier concluded that there was a single molecule 

 of DNA in each \-irus jjarticlc. A vai'iety of chemical and phy.-ical 

 experiments indicates that this DNA molecule does not have the chai- 

 actoristic secondary structure of DNA's from other phages. A large 

 fraction of the amino groups is free for reaction with foinialdehydc, 

 and the extinction coellicient does not show a characteristic sharp in- 

 crease with temperature that is characteristic of DNA's having the 

 duplex structure. It would appear that the molecule consists of a single 

 polynucleotide chain. This molecule is resistant to the single-chain 

 specific exonuclease from Escherichia coll (Lehman, 1960) anrl there- 

 fore has no free 3' end — a finding which suggests a circular form (Fiers 

 and Sinsheimer. 1962). 



2. The Antonulioijraphii of T2 and T4 DNA 



Turning now to the T phage, the first information that one would 

 like is the number and size of the DNA molecules making up the phage 

 genome. The molecular weight of the DNA in the even phages is so 

 large that the standard methods of molecular weight determination 

 cannot be trusted; however, autoradiography has proved useful. When 

 P^--labeled DNA molecules are mixed with a sensitive photographic 

 emulsion, it is possible to detect single molecules of DNA because they 

 give rise to a cluster of tracks in the develoi)cd emulsion. Each track 

 corresponds to the path of a single /?-ray emitted by a decaying P"*- 

 atom. The first experiments of this kind on T2 DNA resulted in two 

 populations of stars, the first arising from DNA molecules comprising 

 36-40% of the phosphorus of the intact phage particles, and the re- 

 mainder in molecules one-fifth this size or smaller (Levinthal, 1956; 

 Levinthal and Thomas, 1957; Thomas, 1959). The validity of this 

 distribution became questionable as a result of the work of Davison, 

 who demonstrated the extreme sensitivity of phage DNA molecules to 

 fragmentation by hydrodynamic shear (Davison, 1959). This work sug- 

 gested the possibility that larger molecules might have existed, and that 

 they were brok(>n during the dilution and mixing with the melted 

 emulsion. 



Further doubt was cast on the autoradiographic results by the work 

 of Hershey and Burgi (1960). By employing a chromatographic colunm 

 which was able to fractionate large DNA molecules primarily with 

 respect to molecular weight (Mandell and Hershey, I960), these authors 

 were able to show that unbroken molecules rechromatographed uniformly 

 overlapping the original material, even though the sample selected for 

 rechromatography was taken from the edge of the elution profile. In 



