The genomes of eukaryotic organisms are regularly composed of three different classes of DNA sequences: Single copy, middle or moderately repetitive and highly repetitive DNA sequences, with copy numbers per genome of 1 to 10, 10 to 1000, and 1000 to 1000000 respectively. The highly repetitive sequences can be separated easily from the rest of the genomic DNA by density centrifugation if it happens that they deviate in base composition from the genome average. This deviation causes a higher (GC-rich) or lower (AT-rich) buoyant density and gives rise to the formation of discrete “satellite bands” of DNA in usual CsCl gradients at different densities than the “main band” DNA. Yet, conspicious departure in base composition is not observed in all families of repetitive sequences so that the repetitive DNA remains hidden in the bulk of the main band DNA as a so called “cryptic satellite”.
KeywordsMitotic Chromosome Heterochromatic Region Satellite Band Obscura Group Basic Repetition Unit
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