Question

Match LIST-I with LIST-II

Choose the correct answer from the options given below:

• A. A-I, B-II, C - III, D- IV
• B. A-I, B - III, C - II, D - IV
• C. A-I, B-II, C - IV, D - III
• D. A-III, B - IV, C - I, D - II

Hint:

Associate keywords: \textbf{Mismatch} \(\rightarrow\) Methylation (\textbf{Dam methylase}). \textbf{Base} Excision \(\rightarrow\) AP site (\textbf{AP endonuclease}). \textbf{Nucleotide} Excision \(\rightarrow\) Bulky lesion (\textbf{ABC excinuclease}). \textbf{Direct} Repair \(\rightarrow\) Light/UV (\textbf{Photolyase}).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This question requires matching specific DNA repair pathways in E. coli with the key enzymes or proteins that are characteristic of each pathway.
Step 2: Detailed Explanation:
Let's match each repair system with its key enzyme:


A. Mismatch repair (MMR): This system corrects errors made during DNA replication. To distinguish the new strand (with the error) from the old template strand, the E. coli MMR system relies on methylation. The Dam methylase enzyme methylates adenine bases in GATC sequences. Newly synthesized strands are temporarily unmethylated, allowing the repair machinery to identify and correct them. Thus, Dam methylase is a key component of this system's specificity. This matches I.

B. Base-excision repair (BER): This pathway removes a single damaged or incorrect base. After a DNA glycosylase removes the base, it leaves an apurinic/apyrimidinic (AP) site. An AP endonuclease then cuts the DNA backbone at this site to allow for repair. This matches III.

C. Nucleotide-excision repair (NER): This pathway removes bulky lesions. The key enzyme complex in E. coli is UvrABC, which is also known as ABC excinuclease. It recognizes the lesion and makes cuts on either side. This matches II.

D. Direct repair: This is the simplest form of repair where the damage is directly reversed. A classic example is the repair of pyrimidine dimers caused by UV light, which is directly reversed by DNA photolyases using the energy from visible light. This matches IV.

Step 3: Final Answer:
The correct matching is A-I, B-III, C-II, D-IV.