Final answer:
In base excision repair, uracil glycosylase corrects DNA damage by replacing U-G with C-G, thereby fixing errors caused by cytosine deamination. The correct answer is D. U-G; C-G
Step-by-step explanation:
In base excision repair, the uracil glycosylase enzyme corrects DNA damage by biased replacement of U-G to C-G. Base excision repair (BER) is a cellular mechanism that repairs DNA damage to a single nucleotide caused by oxidative stress, alkylation, hydrolysis, or deamination. Deamination of cytosine results in the formation of uracil, which is not normally present in DNA.
The uracil glycosylase enzyme recognizes the uracil in DNA and removes it, creating an abasic site. Subsequent actions by other repair enzymes fill in the gap with the correct nucleotide, which is cytosine, thereby restoring the original C-G base pairing. By contrast, nucleotide excision repair (NER) deals with bulkier, helix-distorting lesions such as thymine dimers caused by UV radiation. The NER process involves a different set of enzymes that cut out a segment of DNA containing the thymine dimer and fill in the gap with newly synthesized DNA, followed by ligation to restore DNA integrity.