Strong adenine clustering in nucleotide sequences

We analyze the known long natural nucleotide sequences, looking for possible homonucleotide clustering. We find strong evidence for adenine clustering starting at the A₃ level. Single, isolated As and to a lesser extent AAs occur less frequently than expected from the base composition of the given sequence alone. A₃, A₄ and higher A—runs occur much more frequently than. expected. The effect is quite universal and occurs with equal strength in prokaryotes and eukaryotes. Analogues thymine clustering is observed in a single genome only. Cytosines and/or guanines mildly display the reverse trend.