Born Under The Sun: UV Light And The Origin Of Life

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Born Under The Sun:

UV Light And The Origin Of Life

by BioMed Central (http://www.biomedcentral.com/)London - Jun 02, 2003Early evolution of life as we know it may have depended on DNA's ability toabsorb UV light. This insight into the early moments of life on Earth comesfrom research published today in the journal BMC Evolutionary Biology.The research fills in one of the major gaps in our understanding about theorigins of life: how single molecules were able to join together to createthe self-replicating long chain molecules of RNA, the precursors of DNA. It"sheds new light on the earliest steps of evolution," write ArmenMulkidjanian and his colleagues from Osnabrück University, Germany andNational Institutes of Health, USA.With no ozone layer the primordial Earth was a hostile place. This wasespecially true for long-chain molecules that would be broken up by UVradiation, which was at 100 times today's level. Most existing theoriesabout how life evolved involve hiding the first life forms away from thelight. Instead, Mulkidjanian and his colleagues have investigated the ideathat high levels of UV light hitting the primordial earth were vital toRNA's survival.The researchers used computer-modelling technology to assess the ability ofRNA to form from its constituent parts, sugar phosphates and nitrogenousbases, with and without high levels of UV light.They found that the ability of nitrogenous bases to absorb and disperse UVradiation could protect the backbone of primordial RNA from breaks. Underhigh levels of UV, RNA molecules were more stable than other large moleculesand the small molecules that join together to create the RNA.This gave RNA molecules a selective advantage, so that their levels thenincreased through the simulated process of natural selection. Moreover, partof the energy from the absorbed UV light could have driven the elongation ofRNA chains."The suggested mechanism turns the high UV levels on primordial Earth from aperceived obstacle to the origin of life into the selective factor that, infact, might have driven the whole process", write the authors."It seems quite unlikely that the extremely effective UV-quenching by allmajor nitrogenous bases is just incidental. We can assume that these baseswere selected to perform the UV-protecting function before they becameinvolved in the maintenance and transfer of genetic information."In this (primordial) world the nitrogenous bases served just as protectingunits. Accordingly these units were replaceable and variable. Exactly thisvariability could have paved the way to the variability of the futuregenomes".Three of the four nitrogenous bases that protected RNA from UV on primordialEarth are the same as those that make up the genetic code of DNA.Ironically, the ability of DNA to absorb UV light is now responsible formany skin cancer deaths.When the bases of DNA absorb UV light they often suffer structural damage,although the DNA backbone remains intact. If this damage occurs within agene it can lead to the alteration of that gene, which may cause cancer.