18 



M.\rRUE J. nESSMAN 



there is a net synthesis of i)oIynier. Arguments for a nanowcr definition 

 ha^■e an emotional rather than practical basis. 



The most sensitive measure of DNA synthesis imolvcs deoxyrihoiiu- 

 cleotitles hihelcd with radioactive isotopes of cai-hon or i)hos])liorus. 



Base-deoxyribose-P'--P3--P'2 



Any combinations of these labels may be present in the substrate, but 

 it should be borne in mind that the two terminal phosphorus atoms will 

 not become part of the DNA molecule. This technique is extremely 

 sensitive and has been used to measure veiy minute reactions. For 

 example, the incorporation of 1 /x^mole of deoxyribonucleotide with a 

 specific activity of 1 X 10"^ cpm j^er /^mole would give 100 ci)m above 

 background. By suitable wash procedures, the blank values can be kept 

 to a relatively low^ level. For routine assays, specific activities two 

 orders of magnitude lower (1 X 10'' cpm per /xmole) may be employed. 

 For situations in w^hich substantial amounts of DNA have been 

 synthesized; the acid-insoluble precipitate may be used for direct 

 measurements of deoxyribose, phosphorus, or ultraviolet extinction. 



2. Viscosity Measurements 



"The viscosity of dilute DNA solutions exceeds that of any other 

 widely studied natural or synthetic substance. . . ." (Doty, 1957). Pouyet 



Itp 



50 



450 



150 250 350 



Time in minutes 



Fig. 4. Measurcniont of DNA syntliesis by visco.sity. (From Lehman, 1959.) 



