Genome inhomogeneity is determined mainly by WW and SS dinucleotides

According to the hypothesis of the modular structure of DNA,genomes consist of modules of various nature which may differin statistical characteristics. Statistical analysis helps inrevealing the differences in statistical characteristics andpredicting the modular structure. In this connection the questionabout the contribution of each word of length l (l-tuple) tothe inhomogeneity of genetic text arises. The notion of stationary(i.e. relatively evenly distributed over a genome) versus non-stationaryl-tuples has been introduced previously. In this paper, thedinucleotide distributions for all long sequences from GenBankwere analyzed and it was shown that non-stationary dinucleotidesare closely associated with polyW and polyS tracts (W denotes‘weak’ nucleotides A or T, while S stands for the‘strong’ nucleotides G or C). Thus, genome inhomogeneityis shown to be determined mainly by AA, TT, GG, CC, AT, TA,GC and CG dinucleotides. It has been demonstrated that neither‘codon usage’ nor the ‘isochore model’can account for this phenomenon.