Quasireplicas and universal lengths of microbial genomes

Statistical analysis of distributions of occurrence frequencies of short words in 108 microbial complete genomes reveals the existence of a set of universal "root-sequence lengths" shared by all microbial genomes. These lengths and their universality give powerful clues to the way microbial genomes are grown. We show that the observed genomic properties are explained by a model for genome growth in which primitive genomes grew mainly by maximally stochastic duplications of short segments from an initial length of about 200 nucleotides (nt) to a length of about one million nt typical of microbial genomes. The relevance of the result of this study to the nature of simultaneous random growth and information acquisition by genomes, to the so-called RNA world in which life evolved before the rise of proteins and enzymes and to several other topics are discussed.