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In the coding sequence of a gene, a cytosine at position 237 is replaced by a thymine. The original codon was GCG coding for alanine. Which of the following is the resulting codon and the type of mutation?

easy MCQ 1 mark Class 12 · Biology · Molecular Basis of Inheritance
  1. GUG, missense mutation
  2. GCG, silent mutation
  3. GUG, nonsense mutation
  4. GAG, missense mutation
Answer
GUG, missense mutation

The correct answer is GUG, missense mutation — Changing the amino acid is a missense mutation.

Step-by-step solution

  1. Identify the original codon: GCG, which codes for alanine.
  2. Replace the cytosine (C) with thymine (T) at the second position of the codon.
  3. The new codon becomes GUG.
  4. GUG codes for valine, a different amino acid than alanine.
  5. Changing the amino acid is a missense mutation.
Common mistake: Confusing codon change with amino acid change

Concept tested: Point mutation classification

Published · How this solution was prepared and checked

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