IMAT 2015 Q39 [Mutated DNA Sequence]

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A normal sequence of triplets in a section of DNA is given by: ATCGAACGG

The same section of DNA has been changed by mutation, and is given by: ATCTTGCGG

Some of the triplets below represent the tRNA triplets which code for amino acids.

Key:
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Using the tRNA triplets, which amino acid sequence given below could be formed from the mutated DNA sequence?

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The answer to this question is C.

Before we start working on this answer, let us be reminded of a few concepts:

  • In RNA, uracil is used instead of thymine.
  • Base pairing is always A with T, G with C.
  • tRNA or transfer RNA, is the RNA tasked carry an amino acid to the ribosome and to read the mRNA sequence in the process of translation. tRNA triplets or anti-codon are complementary to the given mRNA.

With those reminders in mind, let us start systematically working on this question:

  • What’s being asked of you here? You are tasked to find the amino acid sequence given the MUTATED DNA sequence: ATC-TTG-CGG
  • Based on the reminders above, during translation tRNAs read mRNAs and NOT the DNA. Thus, it is your job now to discern the mRNA first, given the mutated DNA sequence above.
  • The mRNA sequence would now be: UAG-AAC-GCC (Keep in mind the rules of base pairing)
  • What’s next? Now that you have the mRNA sequence, you can now use this for translation, since tRNA is complementary to mRNA. Thus given the mRNA, you will now need to find the triplets that match up with them.
  • It will be easier for you to write down the set of triplets that will match up with the mRNA (Keep in mind tRNA is still RNA, so uracil will still be used in place of thymine.)
  • The set of tRNA triplets that match with the mRNA: AUC-UUG-CGG
  • You can now find the corresponding shapes with the table given to you, which will show that choice C is the correct amino acid sequence, given the mutated DNA sequence.
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What about 5’, 3’?
I think first sequence of DNA which is given is lined from 5’ to 3’.
So when it comes to RNA section the direction should be changed.
The sequence on mRNA from 5’ to 3’ should be CCG/CAA/GAU