Pyrimidine Moieties in Animals, Plants, and Bacteria 



95 



and shown to differ in density (11). The density differences could 

 reflect differences in the base compositions of tlie strands. Tliere 

 are alternative explanations, however. For example, one strand 

 might contain more glucose residues attached to hydroxymethyl- 

 cytosine than tlie other strand. 



Equilibrium Sedimentation of DNA in CsCl Density Gradients 



Equilibrium sedimentation of DNA in CsCl density gradients 

 provides an elegant method for characterizing DNA with respect 

 to average nucleotide composition and heterogeneity of composi- 

 tion (46). Following the centrifugation of a DNA solution for 

 twenty-four hours in a CsCl density gradient, the DNA tends to 

 form a band at a position in the cell corresponding to its effective 

 buoyant density. A photograph, taken with ultraviolet light, of the 

 banding of bacterial and mouse DNA is shown in Figure 6, and 

 a microphotodensitometer tracing of the photograph is presented 

 in Figure 7. The white (ultraviolet light absorbing areas) in the 



24 Hours, 25°C (CsCI-p= 1.7208) 



24 Hours, 25°C (Cs CI- p= 1.7165) 



Fig. 7. Microdensitometer tracing of photograph of the banding of mouse DNA 

 and Streptomyces DNA after density gradient centrifugation. See Figure 6. 



