96 Information Storage and Neural Control 



center part of the photograph correspond to Streptomyces virido- 

 chromogenes DNA and to two mouse DNA bands, respectively. 

 The effective mean buoyant density of the Streptomyces virido- 

 chromogenes DNA band is 1.729 gcm~^ The corresponding values 

 for the principal and minor mouse DNA bands are 1.701 and 

 1.690 gcm~^ (Table II and Table VI). The mean densities of the 

 bands can be measured with an accuracy of ±0.001 gcm~^ 



It has been shown by Rolfe and Meselson (54) and by Schild- 

 kraut et al. (60) that the mean effective buoyant densities of double 

 stranded DNA bands vary linearly with the molar per cent (G+C) 

 content of the DNA. For example, Streptomyces viridochromogenes 

 DNA (density = 1.729 gcm~^) contains 73 molar per cent (G+C), 

 Escherichia coli DNA (density = 1.710 gcm"^) contains 51 molar 

 per cent (G+C), and mouse DNA (density = 1.701 gcm~^) con- 

 tains 42 molar per cent (G+C). Thus, if the density of DNA is 

 measured by equilibrium sedimentation in CsCl, the molar per 

 cent (G + C) can be calculated. TABLES II to VIII show the 

 densities of DNA preparations from various sources and the 

 agreement between molar per cent (G + C) as calculated from 

 the densities of the bands and as determined directly by chemical 

 analyses. 



The standard deviations of the DNA bands expressed in density 

 units (a ) depend upon at least two factors: 1) the molecular 

 size of the DNA, and 2) the heterogeneity of DNA composition 

 within the sample. This follows from the following considerations. 

 The centrifugal field tends to drive the DNA into a region where 

 the sum of the forces acting on a given molecule is zero. This 

 concentrating tendency is opposed by Brownian motion, with the 

 result that at equilibrium, the macromolecules are distributed with 

 respect to concentration in a band of width inversely related to 

 their molecular weight (46). 



When a DNA population consists of molecules which differ 

 considerably in density and in molar per cent (G+C), discrete 

 and nonoverlapping DNA bands may be formed. For example, 

 two discrete DNA bands are formed with mouse DNA (Figs. 6, 7) 

 and there is no overlapping between the mouse DNA (p = 1.701 

 gcm"^) and the Streptomyces (p = 1.729 gcm"^) DNA bands. On 

 the other hand, if a DNA preparation consists of a heterogeneous 



