Question

Mutation and replication errors may impair the D. Repair of D can be achieved by which of the following methods?
A. Base- excision repair
B. Nucleotide- excision repair
C. Mismatch repair
D. Single break repair

• A. A and B Only
• B. A, B and C Only
• C. C and D Only
• D. A and D Only

Hint:

Standard D Repair Systems:
1. BER = Small non-bulky damaged bases (Uracil, Deamition).
2. NER = Bulky lesions (UV Thymine Dimers).
3. MMR = Post-replication mismatched base pairs.
4. DSBR (NHEJ HR) = Double strand break repair.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify established biological pathways utilized by cells to repair mutated, modified, or mismatched D bases.

Step 2: Key Formula or Approach:
Major D repair mechanisms include:
• Base-Excision Repair (BER): Removes non-bulky damaged single bases (e.g., uracil, 8-oxoG).

• Nucleotide-Excision Repair (NER): Removes bulky helix-distorting lesions (e.g., UV-induced pyrimidine dimers).

• Mismatch Repair (MMR): Corrects replication slippage and mismatched base pairs escape proofreading.

• Double-Strand Break Repair (DSBR): Repairs double-strand breaks via HR or NHEJ.

Step 3: Detailed Explation:

• A. Base-Excision Repair (BER): Initiated by D glycosylases that excise damaged individual bases, followed by AP endonuclease cleaving the phosphodiester backbone to allow repair synthesis by D polymerase. Thus, A is a primary repair pathway.

• B. Nucleotide-Excision Repair (NER): Excises an oligonucleotide fragment containing bulky D lesions (e.g., thymine dimers) using dual endonucleases, followed by gap filling. Thus, B is a primary repair pathway.

• C. Mismatch Repair (MMR): MutS/MutL system detects mismatched base pairs immediately after replication and corrects the newly synthesized strand. Thus, C is a primary repair pathway.

• D. Single break repair: Non-standard nomenclature; single-strand breaks are handled by specialized single-strand break repair (SSBR) or BER enzymes, while major pathways are BER, NER, MMR, and DSBR. Thus, D is excluded from the standard trio.

Step 4: Fil Answer:
Base-excision repair, nucleotide-excision repair, and mismatch repair (A, B, and C) are standard D repair mechanisms.