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If photolyase activity exists in organisms as distantly related as plants and insects, it is important to determine whether the activity is universal.The discovery of the photolyase is particularly significant in that the biological effects of photoreactivation have previously been ascribed to the alleviation of the toxic effects of cyclobutane dimers alone.Because no mechanism exists for the demethylation of this protein, this enzyme is permanently inactivated by the reaction and is sometimes termed a suicide DNA methyltransferase.MGMT has been identified in and cloned from bacteria. No evidence for its existence in plants has been established.It is difficult, if not impossible, to provide definitive proof that an enzymatic activity does not exist in a particular organism.The presence of MGMT in yeast was in doubt until the gene was cloned and sequenced. In fact, the existence of a bona fide photolyase in placental mammals is still a matter of some debate. These excision repair pathways fall into two major categories: base excision repair and nucleotide excision repair.The resulting abasic sites are then <a href="https://www.ncbi.nlm.nih.gov/pubmed/?term=19944072"></a> recognized by an apurinicapyrimidinic. The nicked DNA is then restored to its original sequence through the combined actions of exonucleases, a repair polymerase, and DNA ligase.Recent evidence has suggested that the repair polymerase itself, polb, possesses the ability to excise BRITT the deoxyribose phosphate residue that   is generated by the combined actions of DNA glycosylases and class II AP endonucleases. Because this lesion is directly mutagenic, all living things probably produce a uracil glycosylase.The crystal structure of uracil glycosylase from nonplant sources has recently been solved and suggests that the protein actually binds to a uracil base that has swiveled out to the exterior of the double helix. Although a gene corresponding to this protein has not yet been identified in plants, the activity has been purified from several plant sources. There is some evidence that this activity is downregulated by as much as fold in fully differentiated cells. The biological effects of an alkA mutation can be suppressed by the artificial overexpression of the tagA gene, which suggests that these additional substrates do not play an important role in the lethality induced by methylating agents.The tag and alkA genes share no significant homology.Although the mammalian genes have a high degree of homology with one another, the overall transkingdom homology is fairly weak.These nicks occur at a specific distance both and of the lesion, which is then excised as an oligonucleotide through the action of a helicase.The excision repair complex will, with varying efficiencies, cleave almost any abnormality in DNA structurefrom very small, nondistorting lesions to very bulky adducts. It is not likely that the cell produces a specific repair protein for every possible lesion, and nucleotide excision repair may exist, in part, to cope with the unexpected.As discussed above, placental mammals are generally thought to lack photolyase, and in mammalian cells NER is apparently the sole pathway for the repair of bulky adducts. It should be kept in mind, however, that most mammalian repair studies, for obvious reasons, are performed in tissue culture rather than in actual skin.

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