Editing Ultraviolet (section)

==Evolutionary significance==
The evolution of early reproductive [[proteins]] and [[enzymes]] is attributed in modern models of [[evolutionary theory]] to ultraviolet radiation. UVB causes [[thymine]] base pairs next to each other in genetic sequences to bond together into [[thymine dimers]], a disruption in the strand that reproductive enzymes cannot copy. This leads to [[frameshifting]] during genetic replication and [[protein synthesis]], usually killing the cell. Before formation of the UV-blocking ozone layer, when early [[prokaryote]]s approached the surface of the ocean, they almost invariably died out. The few that survived had developed enzymes that monitored the genetic material and removed [[thymine dimer]]s by [[nucleotide excision repair]] enzymes. Many enzymes and proteins involved in modern [[mitosis]] and [[meiosis]] are similar to repair enzymes, and are believed to be evolved modifications of the enzymes originally used to overcome DNA damages caused by UV.<ref>{{Cite book |author1=Margulis, Lynn |author1-link=Lynn Margulis |author2=Sagan, Dorion |author2-link=Lynn Margulis |name-list-style=amp |title=Origins of Sex: Three Billion Years of Genetic Recombination |version=1 |publisher=Yale University Press |year=1986 |url=https://books.google.com/books?id=3hDVTEk3ioIC&q=origins+of+sex:+three&pg=PP1 |isbn=978-0-300-04619-9 |format=book |access-date=22 November 2020 |archive-date=29 May 2024 |archive-url=https://web.archive.org/web/20240529134756/https://books.google.com/books?id=3hDVTEk3ioIC&q=origins+of+sex:+three&pg=PP1#v=snippet&q=origins%20of%20sex%3A%20three&f=false |url-status=live }}</ref> 

Elevated levels of ultraviolet radiation, in particular UV-B, have also been speculated as a cause of mass extinctions in the fossil record.<ref>{{Cite journal |last=Cockell |first=Charles S. |date=Spring 1999 |title=Crises and extinction in the fossil record—a role for ultraviolet radiation? |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/crises-and-extinction-in-the-fossil-recorda-role-for-ultraviolet-radiation/00A85C94DE1673AB767ABDB9F40D93AC |journal=[[Paleobiology (journal)|Paleobiology]] |language=en |volume=25 |issue=2 |pages=212–225 |doi=10.1017/S0094837300026518 |bibcode=1999Pbio...25..212C |issn=0094-8373 |access-date=12 November 2024 |via=Cambridge Core}}</ref>