How does base excision repair (BER) differ from nucleotide excision repair (NER)?

Base excision repair (BER) is a DNA repair mechanism that specifically targets and removes damaged bases. It involves the recognition of a single damaged base by a DNA glycosylase, which excises the base, creating an abasic site. Other enzymes, including an AP endonuclease, then cleave the DNA backbone, and DNA polymerase and ligase fill in and seal the gap. This process highlights that BER is focused on the repair of individual base modifications and operates primarily in response to less bulky, non-helix distorting lesions.

In contrast, nucleotide excision repair (NER) is a more complex mechanism that can recognize and excise bulky DNA adducts, such as those caused by UV light (e.g., thymine dimers) or chemical mutagens. NER removes a short single-strand segment of DNA that contains the damage, which typically consists of several nucleotides. This mechanism, therefore, is broader in scope compared to BER, as it addresses a variety of lesions that disrupt the DNA helix more significantly.

This distinction in the focus of damage recognition—single base repair with BER versus bulky lesion excision with NER—makes option C the correct choice, illustrating the specific role of BER