IMAT 2014 Q31 [Substitution Mutation]

The diagram shows a sequence of bases on a DNA template:

CAC GTT CGC ATA GAC

Which one of the following shows the sequence of bases on the complementary mRNA strand with one substitution mutation?

A. CUC GTT CGC UTT GUC
B. GTG CAA GGG TAT CTG
C. GUG CAA CCG UAU CUG
D. GUG CAA GCG UAU GUC
E. GTG CAA GCG TAT CTG

The answer to this question is choice C.

Before we answer this question, it would be useful to be reminded of a few concepts regarding base pairing and substitution mutation:

  • The nitrogenous bases that are always paired together in DNA are [Adenine & Thymine] and [Guanine & Cytosine].
  • When DNA is being transcribed by RNA polymerase, uracil is used instead of thymine.
  • Substitution mutation is when a single nucleotide in a DNA sequence is changed/replaced by a different nucleotide.

Let us then tackle this question in a step by step manner:

  1. First identify the template sequence given in this question, which is CAC GTT CGC ATA GAC
  2. Knowing your knowledge of base pairing, write out the mRNA that can be transcribed from this template which is: GUG CAA GCG UAU CUG
  3. Now that you have the correct mRNA sequence, along with your knowledge of the definition of base pairing (especially in transcription), you will need to compare that to each of the choices, and discern which of them has ONLY ONE nucleotide that does NOT match up with the mRNA sequence you wrote.
  4. You will then arrive at the answer choice C: GUG CAA CCG UAU CUG, since the CGC triplet on the DNA template strand should be transcribed to GCG and NOT CCG.

For visualization purposes, here are the 2 mRNA sequences close together:

  • The CORRECT mRNA sequence: GUG CAA GCG UAU CUG
  • The mRNA with substitution: GUG CAA CCG UAU CUG