High statistics block entropy measures of DNA sequences.

We have used an improved block-entropy measure in order to gain some further insights into the short-range correlations present in whole chromosomes of S. cerevisiae, viruses and organelles and very large genomic regions of E. coli. Although DNA sequences are largely inhomogeneous and word frequencies are unevenly distributed, the comparison of entire chromosomes and large genomic regions show a "bulk" composition homogeneity. This property suggests that biases in selection, directional mutational pressure and recombination processes act in homogenizing the base composition of the DNA molecules within a genome but their mode of action, relative impact and direction may vary in different organisms. The most interesting results appear to be the differences between the SW (C,G/A,T) and RY (A,G/C,T) two-letter alphabet entropies. Deviations from randomness in E. coli and S. cerevisiae sequences particularly concern SW dinucleotide frequencies and RY tetranucleotide frequencies.