Long-range correlations in prokaryotic chromosomes

One of the fascinating properties of the DNA sequences of prokaryotic and eukaryotic chromosomes is that they possess long-range order. Computational methods like spectral analysis, mutual information and DNA random walks have been used to probe long-range order via-long range correlations. This work attempts to show the advantage of using the Information Theoretic measure of mutual information for this purpose. A number Mu is found which indicates the existence of long-range order. Mu is the ratio between the value of mutual information function between two nucleotides of a DNA sequence separated by a large distance of 100 kilobases to the value expected from a randomized sequence of the same DNA. It is found that in spite of the constant shuffling of nucleotides due to insertion, deletion, inversion and recombination that occur during evolution, the chromosomal structure of prokaryotes is not always mosaic. While all archaeal chromosomes show mosaic structure and lack long-range order, a sizable fraction of the bacterial chromosomes do possess long-range order. A statistical multivariate analysis has been done to find which of the physical variables like genome size or GC% affects the organization of the chromosome or correlates with the long-range order. The existence of long-range order in bacterial chromosomes could be directly correlated to the degree of gene strand bias shown by it. Firmicutes which have low GC content also have pronounced strand bias and show long-range correlations. It is observed that the occurrence of long-range order in bacteria is independent of genome size, but depends on its GC content and gene strand bias.