DNA sequences and chromosome structure

In this review evidence for the possible relationship between higher order chromosome structure and the distribution of tandem and dispersed repeated sequences in DNA has been examined. Evidence from studies of chromosome diminution in lower eukaryotes suggests that simple sequence DNAs may have a germ-line function and, in mammals, changes in simple sequence methylation, associated with changes in chromosome condensation, support the idea that the centromeric and telomeric heterochromatin may have a function in germ-line cells. For the major families of dispersed repeats, the weight of available evidence suggests that if they do play a role in chromosome organization, it is not an important one. Cytosine methylation is suggested as a candidate for a role in organizing the chromatin. Long-range patterns of methylation in vertebrates, the relationship of this to gene structure, and the association of changes in methylation with gene activity accord with current evidence linking specific nuclear structures to defined points in coding regions.