Genetic code exercise

Question

an mRNA segment encodes the peptide Met- Thr - Phe - Ile - Trp. Proflavin causes the removal of a single base from the gene encoding this mRNA. The new peptide translated from the modified mRNA is Met-Pro-Ser-Tyr-Gly.

a) in which coding the base removal occurred

b) what is the order of normal mRNA bases?

c) what would be the amino acid sequence if a G were inserted after the first 3 bases of the mutated mRNA?

Answer 1

The codon table is helpful for this, so you can find likely mRNA sequences.

We have Met-Thr-Phe-Ile-Trp. I'll make "X" as multiple possible bases.

Which could be: 5' AUG-ACX-UUX-AUX-UGG 3'

The new peptide is Met-Pro-Ser-Tyr-Gly, which could be:

5' AUG-CCX-UCX-UAX-GGX 3'

The differences start at the second codon, so I think the delete base is somewhere in there. I also don't see an A in position 4, so I'll check if base 4 is the missing base. We would then have:

5' AUG-CXU-UXA-UXU-GGX 3'

And it fits! So base 4 was removed.

Now we can use the mutant peptide to fill in the X's, just by seeing what overlaps.

5' AUG-ACC-UUC-AUA-UGG 3'

Now if G were inserted after the first 3 bases of the mutant mRNA, we'd get:

5' AUG-GCC-UUC-AUA-UGG 3'

And the resulting peptide is: Met-Ala-Phe-Ile-Trp