IMAT 2018 Q39 [tRNA Anticodons]

Part of a polypeptide sequence consists of four amino acids -1-2-3-4-.

The tRNA anticodons for each amino acid are:

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Which one of the following is the correct sequence, in a 3′ to 5′ direction, for the bases on the template strand of the DNA?

A. GCAACATTGGGT
B. CGTTGTAAGCCA
C. CGUUGUAAGCCA
D. GCAACAUUCCCU
E. ATGGTGCCTTTC

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The answer here is choice A.

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

  • The nitrogenous bases that are always paired together in DNA are [Adenine & Thymine] and [Guanine & Cytosine].
  • In any RNA sequence, uracil is used instead of thymine. Thus, mRNA and tRNA contains uracil instead of thymine.

What’s the overview of the steps that I have to take to answer this question?

  1. What is the question asking from you? It is asking for the TEMPLATE strand of DNA.
  2. How will we do that if all that is given is the tRNA anticodon? Remember that tRNA anticodons are complementary to the mRNA! This is used in the process of translation.
  3. As you can recall, mRNA is produced through the process of transcription, where the template DNA is transcribed using RNA polymerase. Thus, to find out the template DNA, you will need to figure out the mRNA first!
  4. You will be able to deduce the mRNA sequence since the tRNA anticodons are complementary to mRNA.

Let us then figure out this question:

  • If you lay out the anticodons in 1 line, it will look like this: GCA-ACA-UUG-GGU
  • Keeping in mind the rules of base pairing, you can then figure out the mRNA sequence: CGU-UGU-AAC-CCA
  • Now that you have the mRNA sequence, you can now work out the complementary DNA sequence! (reminder: DNA uses thymine instead of uracil): GCA-ACA-TTG-GGT
  • If you notice, this DNA sequence is consistent with choice A, making it the correct answer.
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