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: Understanding the Concept:
Nucleotide Excision Repair (NER) is a DNA repair mechanism that removes bulky, helix-distorting lesions such as pyrimidine dimers caused by UV light. The question asks for the chronological sequence of events in this process.
Step 2: Detailed Explanation:
The steps of NER occur in a specific logical order:


Step C: Damage Recognition and Unwinding. First, the repair machinery must recognize the distortion in the DNA helix. Following recognition, a helicase unwinds the DNA around the lesion, creating a "bubble" to provide access for other enzymes.

Step B: Excision. An excinuclease (a type of endonuclease) makes two cuts in the damaged strand, one on the 5' side and one on the 3' side of the lesion. This excises a short single-stranded DNA segment containing the damage.

Step A: Synthesis. A DNA polymerase uses the intact, complementary strand as a template to synthesize new DNA, filling the gap created by the excision.

Step D: Ligation. Finally, the enzyme DNA ligase seals the nick in the phosphodiester backbone, joining the newly synthesized DNA to the pre-existing strand and completing the repair.

Step 3: Final Answer:
The correct sequence of steps is C \(\rightarrow\) B \(\rightarrow\) A \(\rightarrow\) D.