The previously discovered linear relation between the base composition of DNA, expressed in terms of percentage of guanine plus cytosine bases, and the denaturation temperature,
m, has been further investigated. By means of measurements on 41 samples of known base composition the previously observed relation has been confirmed. It can be summarized thus : for a solvent containing 0·2
m = 69·3 + 0·41 (G-C) where
m is in degrees Centigrade and G-C refers to the mole percentage of guanine plus cytosine. The deviations of experimental points from this relation are no more than that expected from the uncertainties of base analysis and the variations of a half degree in the reproducibility of determining the
m. Consequently it appears that the measurement of the
m is a satisfactory means of determining base composition in DNA. The
m values are most simply measured by following the absorbance at 260 m
μ as a function of temperature of the DNA solution and noting the midpoint of the hyperchromic rise. Only 10 to 50
μg of DNA are required.
A number of other DNA samples of unknown base composition have been examined in this manner and their base compositions recorded.