Q29 - Reverse-Engineering a Peptide-Shortening Mutation
The genetic code can be used to reverse engineer the genetic information coding for peptides of a desired amino acid sequence. Your task is to design possible nucleotide substitution mutations that change the DNA coding for the peptide comprising the M-A-R-C-E-L-L-A amino acid sequence into one coding for M-A-R-C-E-L.
Q29.1. Design a nucleotide substitution mutation (no insertions/deletions) that changes the DNA coding for peptide M-A-R-C-E-L-L-A into one coding for M-A-R-C-E-L (i.e. removes the last two residues). What is the minimum possible number of base substitutions needed to achieve this?
1, 2, or 3 (all accepted by the official key): The 7th codon (coding for the first L to be removed) can be changed into a stop codon by substituting anywhere from 1 to 3 of its bases, depending on which codon is actually used for that Leucine in the original sequence, some Leu codons are only 1 substitution away from a stop, others need 2 or 3, so the official key accepts any of these depending on the specific codon reasoning shown.
Q29.2. Fill in codon 7 with the nucleotides that code for the new, shortened M-A-R-C-E-L peptide, using the minimum possible number of substitutions.
A stop codon reached from the original codon 7 by the minimum-edit substitution(s), e.g. changing it to TAA, TAG, or TGA, whichever is fewest substitutions away from the original Leucine codon actually printed on the exam page.: Converting codon 7 into any of the three stop codons (TAA/TAG/TGA) terminates translation immediately after codon 6 (Leu), producing the shortened M-A-R-C-E-L peptide, the exact substitution depends on which Leucine codon the original sequence used.
Q29.3. Circle the nucleotide base(s) that had to be changed to turn the original peptide into the new one.
Whichever base(s) within codon 7 differ between the original Leucine codon and the chosen stop codon.: Only the bases that actually differ between the original codon and the new stop codon need circling, exactly the base(s) counted in the Q29.1/Q29.2 substitution.
Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key