Genome Evolution: Overview

Our understanding of the structure and evolution of genomes (the total genetic constitution of an organism) is presently undergoing a revolution on a par with any other in the history of science. We now have the ability to sequence entire genomes and this is happening at a rapidly increasing pace. Further, molecular studies have provided a fairly good understanding of the deep phylogeny of life (how the major groups of life are related to each other), providing the framework for examining the evolution of genomes across all of life. It is now established that rather than the old prokaryotic–eukaryotic division (bacteria versus cells with nuclei) there are instead three very deep, fundamental branches of life, the Bacteria (or eubacteria), the Archaea (or archaebacteria), and the Eucarya (or eukaryotes). Prokaryotes thus consist of two radically different groups (the eubacteria and archaebacteria) that are as distinct from each other as either is from the eukaryotes. While we will still use the term prokaryote when referring to features shared by both eubacteria and archaebacteria, the reality is that the archaebacteria are (slightly) more closely related to eukaryotes than they are to eubacteria. Excluding those of viruses, genomes consist of one or more double-stranded molecules of DNA. Depending on the particular strain, the genomes of viruses use all four potential types of nucleic acids. Some use double-stranded DNA, others use single-stranded DNA, still others either single-stranded or double-stranded RNA.