Q6 - A Two-Polypeptide Gene with an Intron
You are studying a small protein-coding locus, the sequence of which is shown below (5’ transcriptional START to 3’ transcriptional STOP). This part is responsible for coding of two different polypeptides. There is a short intron sequence, shown in bold. A C:G base pair discussed in Q6.5 is underlined.
5'-CTACGTACTATGTATTCCGATCTATACTCGATCTAGTCGCATTCCGATAAGATCGTAC-3'
3'-GATGCATGATACATAAGGCTAGATATGAGCTAGATCAGCGTAAGGCTATTCTAGCATG-5'(intron, bold in the original: GATCTATA / CTAGATAT - the underlined base pair for Q6.5 is the C:G pair immediately after the intron.)
Q6.1. The 58 bp locus shown codes for two different polypeptides and contains a short (8 bp) intron. How many nucleotides long would the final processed mRNA be (not including the 5' cap and 3' polyA tail)?
Q6.2. Which strand is used as the template in transcription for the shorter polypeptide, the strand shown reading 5'→3', or the strand shown reading 3'→5' (both read left to right as printed)?
Q6.3. What is the ratio of the numbers of amino acids in the longer/shorter polypeptide (including the amino acid built in because of the START codon)? Give your answer to the nearest second decimal place.
Q6.4. What is the third amino acid of the shorter polypeptide, if the initiator Met is counted as the first? Give the one-letter amino acid code.
L (Leucine): The shorter polypeptide's first three codons translate to Met-Arg-Leu, so its third residue is Leucine (L).
Q6.5. The C:G base pair underlined in the sequence was changed to an A:T base pair, creating a mutant form of this gene. What kind of mutation would this be for the shorter polypeptide?
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