ANALYZING GENOMES WITH CUMULATIVE SKEW DIAGRAMS

A novel method of cumulative diagrams shows that the nucleotide compos ition of a microbial chromosome changes at two points separated by abo ut a half of its length. These points coincide with sites of replicati on origin and terminus for all bacteria where such sites are known. Th e leading strand is found to contain more guanine than cytosine residu es. This fact is used to predict origin and terminus locations in othe r bacterial and archaeal genomes, Local changes, visible as diagram di stortions, may represent recent genome rearrangements, as demonstrated for two strains of Escherichia coli, Analysis of the diagrams of vira l and mitochondrial genomes suggests a link between the base compositi on bias and the time spent by DNA in a single stranded state during re plication.