how does evolution explain DNA? answer in 200 words
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Summary
The transition from the RNA to the DNA world was a major event in the history of life. The invention of DNA required the appearance of enzymatic activities for both syntheses of DNA precursors, retro-transcription of RNA templates and replication of single and double-stranded DNA molecules. Recent data from comparative genomics, structural biology and traditional biochemistry have revealed that several of these enzymatic activities have been invented independently more than once, indicating that the transition from RNA to DNA genomes was more complex than previously thought. The distribution of the different protein families corresponding to these activities in the three domains of life (Archaea, Eukarya, and Bacteria) is puzzling. In many cases, Archaea and Eukarya contain the same version of these proteins, whereas Bacteria contain another version. However, in other cases, such as thymidylate synthases or type II DNA topoisomerases, the phylogenetic distributions of these proteins do not follow this simple pattern. Several hypotheses have been proposed to explain these observations, including the independent invention of DNA and DNA replication proteins, ancient gene transfer and gene loss, and/or nonorthologous replacement. We review all of them here, with more emphasis on recent proposals suggesting that viruses have played a major role in the origin and evolution of the DNA replication proteins and possibly of DNA itself.
Introduction
All cellular organisms have double-stranded DNA genomes. The origin of DNA and DNA replication mechanisms is thus a critical question for our understanding of early life evolution. For some time, it was believed by some molecular biologist that life originated with the appearance of the first DNA molecule!1 Watson and Crick even suggested that DNA was possibly replicated without proteins, wondering “whether a special enzyme would be required to carry out the polymerization or whether the existing single helical chain could act effectively as an enzyme”.2 Such extreme conception was in line with the idea that DNA was the aperiodic crystal predicted by Schroedinger in his influential book “What's life”.3 Times have changed, and several decades of experimental work have convinced us that DNA synthesis and replication actually require a plethora of proteins.4 We are reasonably sure now that DNA and DNA replication mechanisms appeared late in early life history, and that DNA originated from RNA in an RNA/protein world. The origin and evolution of DNA replication mechanisms thus occurred at a critical period of life evolution that encompasses the late RNA world and the emergence of the Last Universal Cellular Ancestor (LUCA) to the present three domains of life (Eukarya, Bacteria and Archaea.).5-7 It is an exciting time to learn through comparative genomics and molecular biology about the details of modern mechanisms for precursor DNA synthesis and DNA replication, in order to trace their histories.