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Difficulty: MediumGene Mutations and Chromosomal Aberrations

Which of the following mutagenic agents primarily induces gene mutations by causing adjacent thymine bases on a single DNA strand to covalently bond and form pyrimidine dimers?

  1. Ultraviolet radiationAnswer
  2. B
    Colchicine chemical solution
  3. C
    Ionizing X-ray radiation
  4. D
    Nitrous acid chemical mutagen

Answer

Ultraviolet radiation is the mutagenic agent that causes adjacent thymine bases on a DNA strand to form pyrimidine dimers.
Ultraviolet radiation is absorbed strongly by pyrimidines, promoting covalent linkage between adjacent thymine bases on the same DNA strand. This forms thymine dimers that block normal DNA replication and transcription.

Step-by-Step Solution

1
Identify the primary mechanism of action described in the stem.
The mechanism is the formation of intrastrand thymine (pyrimidine) dimers due to absorption of radiant energy.
Different physical and chemical mutagens operate through distinct biochemical pathways on nucleic acids.
2
Evaluate physical versus chemical mutagens.
Non-ionizing ultraviolet (UV) light specifically excites pyrimidine rings (thymine and cytosine), leading to covalent dimer formation.
UV light lacks the energy to ionize atoms or break double strands, but has sufficient energy to alter nitrogenous base bonding.

Key Concept

Physical mutagens and the molecular mechanism of UV-induced pyrimidine dimerization
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Topics

Gene Mutations and Chromosomal Aberrations

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