Question

Irradiation by UV light causes:

• A. Dimerization of purines
• B. Dimerization of pyrimidines
• C. Dimerization of both purines and pyrimidines
• D. Dimerization of either purines or pyrimidines

Hint:

UV -> Thymine Dimers.
X-raysGamma rays -> DNA strand breaks (Ionizing radiation).
Thymine and Cytosine are the pyrimidines involved.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Ultraviolet (UV) radiation is a known mutagen.
The question asks for the specific biochemical lesion that UV light induces in the DNA structure.

Step 2: Detailed Explanation:

• Nature of UV Damage: Non-ionizing radiation like UV light (specifically UV-B and UV-C) is absorbed by the nitrogenous bases of DNA.

• Pyrimidine Dimers: The most common damage occurs when two adjacent pyrimidine bases (Thymine-Thymine or Cytosine-Thymine) on the same DNA strand undergo a photochemical reaction.
This forms covalent bonds between the bases, creating a "pyrimidine dimer" (commonly a thymine dimer).

• Consequences: These dimers create a "kink" or "bulge" in the DNA helix.
This distortion stalls DNA polymerase during replication and RNA polymerase during transcription, leading to mutations or cell death if unrepaired.

• Repair Mechanisms: Cells use photoreactivation (using light energy and photolyase) or Nucleotide Excision Repair (NER) to fix these dimers.

• Purines: Purines (Adenine and Guanine) are much more resistant to dimerization by UV light compared to pyrimidines.

Step 3: Final Answer:
UV light primarily causes the dimerization of pyrimidines.