Repetitive DNA (Satellite DNA)

If DNA is subjected to heat it becomes denatured, i.e. the two strands of double helical DNA separate. Under certain conditions the two strands reassociate to form active double helical DNA (renaturing). This has been achieved experimentally in the bacterium Haemophilus. Many vertebrate DNAs reassonciate, especially if broken into small pieces. This observation gave rise to the hypothesis that certain short sequences of bases are repeated hundreds of time in DNA. Such DNA has been called repetitive DNA or satellite DNA.

Repetitive DNA consists of short identical genes which are repeated in tandem several hundred or thousand times. Such DNA is found in the region of the chromosome adjacent to the centromere. In many cases the base compositions of the repeating sequences are unlike that of the rest of the DNA. It is therefore easy to separate repetitive DNA by ultracentrifugation.

All eukaryotes, except perhaps yeast, contain repetitive DNA. In Drosophila about 25% of the DNA is of repetitive type. In contrast to eukaryotes, the DNA of prokaryotes does not contain repeated base sequences. This DNA can replicate but cannot transcribe RNA for protein synthesis. This is probably because the short sequences lack promoter sites on which RNA chains can be initiated by RNA polymerase. Repetitive DNA is therefore inert and is partly dispensable.