Which type of nucleophilic substitution reaction is most likely to occur when pyrimidine dimers in DNA are repaired, potentially leading to mutations that can cause cancer?

• A. Base excision repair
• B. $S_{\mathrm{N\; 1}}$
• C. $E_1$
• D. $S_{\mathrm{N\; 2}}$

Answer: (A)

Verified step by step guidance

1. Understand the context: Pyrimidine dimers are a type of DNA damage caused by UV light, leading to covalent bonding between adjacent pyrimidine bases (usually thymine) on a DNA strand.
2. Recognize the repair mechanism: Base excision repair (BER) is a cellular mechanism that repairs damaged DNA throughout the cell cycle. It is responsible for removing small, non-helix-distorting base lesions from the genome.
3. Identify the role of nucleophilic substitution: In the context of DNA repair, nucleophilic substitution reactions are not directly involved in the repair of pyrimidine dimers. Instead, the repair involves the removal of damaged bases and the insertion of correct ones.
4. Differentiate between SN1, SN2, and E1 reactions: SN1 and SN2 are types of nucleophilic substitution reactions, while E1 is an elimination reaction. SN1 involves a two-step mechanism with a carbocation intermediate, and SN2 involves a one-step mechanism with a direct displacement.
5. Conclude the relevance: Since the repair of pyrimidine dimers involves base excision repair rather than nucleophilic substitution, the options SN1, SN2, and E1 are not directly applicable to this specific DNA repair process.