TANDEMLY REPEATED PENTANUCLEOTIDES IN DNA-SEQUENCES OF EUKARYOTES

Genetic sequence data banks were scanned in order to retrieve tandemly repeated pentanucleotides (pnts). It was found that among 102 (=(1024 - 4)/2/5) possible distinct pnts roughly each fourth is involved in t andem repeats. It is shown that tandemly repeated pnts are composed of frequently occuring di- and trinucleotides and that those pnts which occur frequently in the form of mono- or di-pnts form also tandem repe ats either in the form of satellites or in the form of shorter tandem repeats. Human satellite III is taken as a specific example. It is sho wn that the first guanine within GG-AAT pnt exhibits the highest mutab ility. Sequential distribution of base changes gives evidence that the mutations do not occur at random positions but in a correlated fashio n so that long stretches of original pnts remain intact. It is found t hat pnts related to the satellite III are present in introns and flank ing regions of some structural genes, but are not preserved between or thologous genes of related species. The results corroborate the most p lausible mechanism of their evolution - rapid amplification followed b y successive divergence of repeat units by various mutational processe s.