<span>A photon of UV light causes mutations in the double-stranded DNA via a photochemical reaction. The product of such photochemical reaction in DNA is a cyclobutane pyrimidine dimer. Dimers may be repaired by mechanisms of photoreactivation or nucleotide excision repair (NER), but if the repair doesn’t occur dimers are mutagenic.
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UV light is absorbed by a double bond in pyrimidine bases (thymine and cytosine in DNA). The bond is opened and UV-modified base can now react with neighbouring molecules. If neighbouring molecule is another pyrimidine base, the UV-modified base forms direct covalent bonds with it. A cyclic ring (cyclobutane), not normally found in DNA, is formed (links the two pyrimidines). Next step is forming a single bond between two carbon atoms on the rings.The product of this reaction is a 6-4 molecule pyrimidine-pyrimidone which is more mutagenic than cyclobutane pyrimidine dimers.</span>
Elements 1 through 92 (except for elements 43 and 61) occur naturally on Earth, although some are only present in extremely small quantities. The elements following uranium on the periodic table are only produced artificially, and are known as the transuranium or transuranic elements.
Absorption it regulates the pH of the filtrate by exchanging hydrogen ions in the interstitium for bicarbonate ions in the filtrate; it is also responsible for secreting organic acids, such as creatinine and other bases, into the filtrate.
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The answer is D. Hope this helps!
