12(1 CHAKLKS A. IHOMAS, .IK. 



linkages hctwoon molecules changed with changing humidity. The jjack- 

 ing re(iuii-ements of helical duplexes have been examined carefully hy 

 Mai-vin ct <;/.. (19()ll. .\ii analy.><is of the l;itticc spaciiigs and (he iiia.-^.^^ 

 per unit length of the rodlike i)article.s ha,s been made by Luzzati 

 (t (il. (19G1). Unfortunately, the preparations were iii.ammalian and 

 chicken DNA, not b.acteriophage DNA; ne\-cilh('lcss. their icsult> arc 

 proliably jiertinent. 'i'hey conclude that the particles in solution haxc 

 the mass per unit length that one would expect for a Watson-C "lick 

 duplex, and that this is so under a \"aiiety of conditions of conceiitialion 

 and ionic strength. Taking these results together implies that the mas-s 

 per unit length for T2 DNA would be approximately 210 d dtons/A. 

 For the T2 molecule this would correspond to a sti'ucture (V2 n. in contour 

 length. 



This conclusion gains strong sui)poi't fi-om the direct electron micro- 

 scopic detei-mination of the thickness of the phage DNA molecule by 

 Beer and Zobel (1961). These authors stained the DNA with uranyl 

 acetate, deposited the molecules on a cai'bon grid witli polystyrene 

 latex spheres, and then shadowed with platinum. The molecuU^s were 

 seen to run through the shadow of the s])heres. In this region the 

 uranyl-stained molecule can be seen and its thickness measured. Outsirle 

 the shadow of the sphere the height of the molecule can be measured 

 from the shadow length and known shadow angle. This gives two inde- 

 pendent measurements on the same molecule. Both give about 20 ± 5 A 

 for the diameter of the molecule. Since the entire molecule can be seen 

 in overlapping pictures (Beer, 1961), one would like to know whether 

 all of the molecule is in the form of a 20 A fiber. The vicissitudes of 

 the substrate prevent a confident answ(M' to this question; the best one 

 can say is that there are no obiuous si)ecial features along the limited 

 number of molecules so far seen. 



Another way to settle the (juestion of the mass per unit length of the 

 DNA molecule is to measure its total length, and to comb'ne this number 

 with the molecular weight determination. 



A direct observation on the total length of T2 DNA molecules has 

 been made by Beer (1961, 1962), who has perfected techniques which 

 allow the molecule to be picked up and stretched out on the electron 

 microscope grid. Using these procedures he has been able to measure 

 30 different molecules from samples of varying fi-actional length. When 

 the observed length is divided by the fractional length of the chromato- 

 graphic fraction, the calculated total lengths range from 30 to 90// with 

 an average of 49//,. Similar studies on T3 (Bendet et al., 1962) and T7 

 (Davi.son and Freifelder, 1961; Beer, 1962) have i)een made. 



Another approach to this problem was nuide by Cairns (19621, who 



