Cells had to evolve mechanisms to repair damaged DNA

Cells had to evolve mechanisms to repair damaged DNA -...

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Cells had to evolve mechanisms to repair damaged DNA: Classes of repair mechanisms : 1) direct reversal of damage reaction 2) removal of damaged bases and replacement with newly synthesized DNA Also, mechanisms to cope with damage if it cannot be repaired. Direct reversal : Only a few types of damage are repaired in this way although it is probably the most energy efficient. Especially the formation of pyrimidine dimers , which is the major type of damage induced by UV light . Pyrimidine dimers are formed between adjacent pyrimidines (particularly thymines) on the same strand of DNA by the formation of a cyclobutane ring resulting from saturation of the double bonds in their ring structure (Fig. 5.20 A). Pyrimidine dimers distort the double helical structure of DNA and block transcription or replication past the damaged site. Recognition of distortions in the
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Unformatted text preview: double helix is the major way that DNA damage is generally recognized in the cell. One mechanism of repair (there are several others) is through direct reversal of the dimerization reaction. The process is called photoreactivation because the energy to break the cyclobutane ring is derived from visible light. Therefore, in this kind of repair mechanism the original pyrimidine bases are restored and remain in the DNA. The repair of pyrimidine dimers by photoreactivation is common to many prokaryotes and eukaryotes ( E. coli , yeast , and several species of plants and animals). However, photoreactivation is not universal . Many species (including humans) lack this kind of repair mechanism. But humans have other kinds of repair mechanisms that directly reverse certain damages....
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