Subunit structure of chromatin and the organization of eukaryotic highly repetitive DNA: recurrent periodicities and models for the evolutionary origins of repetitive DNA.

A restriction enzyme analysis of the repeat structure of mouse satellite, sheep satellite II, human highly repetitive fractions, calf satellite I, and a repetitive fraction of the rat indicates that those DNAs share repeat periodicites in common with one another and with the highly repetitive component α DNA of the African green monkey. The basic repeat periodicity of component α is 176 ± 4 nucleotide base-pairs: the repeat periodicities of the various highly repetitive fractions described here also seem based on this fundamental unit, but it is disguised by a superimposed, higher order repeat organization in each case. The higher orders of organization are based on integral multiples of the basic unit which may reflect the nucleosome spacing of constitutive heterochromatin. With the exception of component α DNA, which shows a repeat structure based on a monomer of 176 ± 4 nucleotide base-pairs, all of the highly repetitive DNAs examined showed a preference for even-numbered or geometric multiples of the basic unit in their higher order sequence organization. It is suggested that such organization is a relatively recent development in the hierarchical evolution of the sequences.Several models are discussed which may account for the higher order organization and expansion of these highly repetitive DNAs. Either a modified unequal crossover model (Smith, 1973) or a modified replicative loop model (Keyl, 1965a) seems consistent with many of the properties of highly repetitive DNAs. The models may have implications for the number, distribution and intranuclear rearrangements of transcribed sequences associated with such DNAs.