Bouyant density

It is possible to establish heterogeneous populations of mitochondria experimentally in the same cell. In human-mouse somatic cell hybrids, the mitochondria from the two parental types can be distinguished by differences in bouyant density of their DNA s (Attardi and Attardi, 1972) or differences in their nucleotide sequences (Coon et al., 1973). In human-mouse hybrid cells, Attardi and Attardi (1972) have failed to find persistence of human mitochondria, but Coon et al. (1973) reported... 

As judged from the bouyant density the G -(- C content of mt DNA from many mammalian species is rather similar. Densities between 1.698 and 1.704 gm/ml have been reported for the mt DNA isolated from many mammalian species with. some variations with different animals and with different techniques used (Table II). Together with T values of 85-88°C (in SSC) these data suggest a mole percent for C -j- G of 40-45%. In some cases the base composition has been confirmed by direct measurements of bases in hydrolysates of mt DNA. 

The base composition of mitochondrial and nuclear DNA differs only slightly, but in some cases the differences are large enough that the two types of DNA can be separated by their bouyant densities. (Bouyant density is related to the guanosine cytosine content of the DNA.) However, this is not the case for mammalian mitochondrial DNA, the buoyant density of which resembles that of nuclear DNA (nDNA) because of the small difference in base composition. 

The density of one molecule of DNA rich in guanine-cytosine pairs is greater than that of a DNA rich in adenine-thymine pairs. Thus, if a DNA suspension composed of a mixture of two different DNA s—one rich in AT pairs and another rich in CG pairs—is centrifuged in gradient concentrations of cesium chloride, the two types of DNA band separately at the end of the centrifugation. The position of the band depends on the bouyant density of the DNA molecule. The bouyant density of the DNA molecule was shown to be directly proportional to the amount of CG pairs present. The determination of the bouyant density of DNA has thus provided a convenient method for establishing quickly the base composition of small amounts of DNA. 

Validation of these calculations will require several years of precision measurement on an experimental area. Higher thermal loading densities may be employed to accelerate bouyant effects and allow more rapid validation of the code calculations. 

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