I. REPLICATIOX OF DNA IN CELL-FREE SYSTEMS 



19 



(1952) obtained a valiR- of 50 deciliters (dl)/gin for the inti-in.sic viscos- 

 ity using a Couettc viscometer, and several other measurements of 

 various DNA samples put the range between 40 to 60. This property is 

 veiy useful for kinetic measurements of DNA synthesis since the whole 

 reaction may be carried out in a viscometer. Figure 4 demonstrates the 

 increase in specific viscosity of a reaction mixture primed with calf 

 thymus DNA. This technique is applicable to systems in which the newly 

 synthesized DNA represents a substantial increase over the initial DNA 

 content of the reaction mixture. 



3. Measurements of Hypochromicity 



Kunitz (1950) observed an increase in the ultraviolet extinction of 

 solutions of DNA during enzymatic digestion with deoxyribonuclease 

 and utilized this ''hyperchromic effect" as a means to assay this enzyme. 

 This hyperchromic effect is due to the fact that the ultraviolet extinction 

 of a nucleic acid preparation is less than the sum of the extinctions of its 

 individual nucleotides. The theoretical basis for this effect is under in- 



£(P) 



220 



240 



m^ 



260 



280 



Fig. 5. Absorption spectra of an equimolar mixture (phosijhorus basis) of poly- 

 adenylic acid and polyuridylic acid. The upper curve refers to the mononucleotides 

 obtained by alkaline hydrolysis of the mixture. The middle curve is that for the 

 separately measured polymer spectra. The lower cur\'e is the measured cur\"e for 

 the mixture of polymers. (From Warner, 1957.) 



