Satellite DNA

Abstract

In recent years it has been discovered that the greater proportion of the DNA of higher organisms (eukaryotes) is composed of repetitive nucleotide sequences. This finding has revolutionized ideas concerning the organization of genetic material and stimulated much research and discussion on the functions of repetitive DNA. In general its function is not yet understood although DNA of known function has been shown to be repetitive in a number of instances. These mainly involve genes whose terminal products are RNA rather than proteins. Examples are the genes for 28S, 18S, and 5S ribosomal RNA. The genes for histones are also reiterated. The few examples of genes controlling proteins synthesized by differentiated cells which have so far been examined indicate that they are represented once, or at most a few times per diploid genome. Satellite DNA forms a distinct class of repetitive DNA. It is the most highly repeated DNA in the genome and it exhibits the property of separation from the rest of the DNA in dense salt gradients in the ultracentrifuge. The discovery and subsequent study of satellite DNA have done much to advance ideas about the molecular structure of chromosomes but so far little to explain the significance of this DNA itself. The strongest correlates are between satellite DNA, speciation, and constitutive heterochromatin; particularly that near the centromere. It is therefore probable that some causal connection will eventually be shown to exist between them. Already there are observations which demonstrate a reciprocal relationship between degree of evolutional specialization and amount of satellite DNA and attention has been drawn to possible evolutional advantages connected with the acquisition of centromeric heterochromatin. Documentation of the phenomenology of constitutive heterochromatin is voluminous and testifies convincingly regarding its role(s) in controlling a broad range of cellular and developmental activities. The involvement of satellite DNA is probably structural and changes in satellite sequences presumably bear witness to prolific alterations in constitutive heterochromatin in evolution. Such changes have stabilized within species such that each individual of a given species, regardless of global habitat, as far as is known exhibits the same spectrum of satellite DNA components. The conclusion from this must be that alterations in satellite DNA are not permissible except in the context of evolution and speciation. This conclusion suggests that satellite DNA is involved in mechanisms, resident in constitutive heterochromatin, which are essential to the balance of gene expression which is recognized as a species. Further studies of satellite DNA should reveal the nature of the processes involved.