In DNA these bases form hydrogen bonds with their Purine S complementary:
DimerousUnder the action of the rays UV, the bases nitrogenized like the Cytosine and the Guanine can rather form connections with the adjacent bases than with their anti-complementary bases. So at the time of the replication, the sequence of the bit of newly formed DNA can be modified. This modification of the nucleotidic sequence will affect in cascade the sequence of ARN-m produces at the time of the transcription and even the primary sequence of protein produced at the conclusion of the translation. Although the ultraviolet rays can induce changes in the DNA, those systematically do not induce a difference on the level of the sequence of a protein. For that, the change must affect an area of the DNA corresponding to a gene (only some pourcents in the genomes eucaryotes), in this area, the change must take place in a exon, from the degeneration of the genetic code, a change taking place on the third basis of a codon is likely much less to induce a difference on the level of the amino-acid coded by the transferred triplet, finally, the cell has mechanisms of correction of the errors which can eliminate incorrect associations. As it is seen, and fortunately, the appearance of a change and its possible harmful effects are not systematic, but prudence remains of setting vis-a-vis the U.V.
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