Question

Choose the correct order of steps involved in nucleotide excision repair of damaged bases as their occur.
A. A fresh burst of DNA synthesis
B. Cuts are made on both the 3' side and the 5' side of the damaged area
C. The DNA is unwound producing a "bubble".
D. A DNA ligase covalent binds the fresh piece into the backbone.
Choose the correct answer from the options given below:

• A. A, B, C, D
• B. B, C, A, D
• C. C, B, A, D
• D. A, B, D, C

Hint:

Think of NER as "Find, Cut, Fill, Seal." \textbf{Find} and open up the damage (unwind, C). \textbf{Cut} out the bad section (excision, B). \textbf{Fill} in the gap with new DNA (synthesis, A). \textbf{Seal} the final connection (ligation, D).

Answer 1

The Correct Option is C

Step 1: Concept Overview:
Nucleotide Excision Repair (NER) corrects DNA damage, like pyrimidine dimers from UV light, that distort the helix. This question requires ordering the NER steps chronologically.
Step 2: Step-by-Step Breakdown:
NER proceeds in a defined sequence:


Step C: Recognition and Unwinding. The repair system identifies the DNA distortion. Then, a helicase unwinds the DNA around the damage, forming a "bubble" for enzyme access.

Step B: Excision. An excinuclease makes two cuts, flanking the lesion on both the 5' and 3' sides, removing a short, damaged single-stranded DNA segment.

Step A: Synthesis. DNA polymerase uses the undamaged strand as a template to synthesize replacement DNA, filling the gap.

Step D: Ligation. DNA ligase seals the phosphodiester backbone, connecting the new DNA to the existing strand, finalizing the repair.

Step 3: Solution:
The correct order is C \(\rightarrow\) B \(\rightarrow\) A \(\rightarrow\) D.