Question

Question 2: Transcription, RNA Processing and Translation A particular gene codes for a mature mRNA transcript containing 1200 bases, which is translated into a protein containing 300 amino acids.

A. How long is the coding sequence in this mRNA and how many nucleotides are in the UTRs? For the purposes of this question we are ignoring the G’ cap and the polyA tail.

B. A mutant form of the gene created by one nucleotide being changed to another nucleotide also yields a 1200-base mature mRNA transcript but the protein made from the mutant gene only contains 280 amino acids and has somewhat modified enzymatic activity.

(i) What type of mutation most likely happened in this mutant?

(ii) Why might the activity of the protein be only “somewhat modified”? Explain.

C. Another mutant form of the gene yields a 1500-base mature mRNA transcript (so longer than it was before) and the protein made from the mutant gene contains 150 amino acids and has no enzymatic activity.

(i) Describe the mutation (and its outcome) that may have occurred that would lead to a longer mature mRNA transcript and a modified coding region.

(ii) Why might the activity of this protein be a complete loss of function? Explain.

Answer 1

A. It has 903 bases coding sequence and 297 bases UTR region.

B. I) point mutation in codon probably has changed it into stop codon earlier than actual termination sequence. For example transversion of C into G in UAC(tryptophan codon) to UAG(stop codon) will have same results.

II). As it has 280 AA and will be lacking the terminal 20 AA, active site may still be there in 280 AA globulin so it might happen that enzyme still work.

C. I). Increase in transcript length by 300 bases may have happened because of mutations at splicing site and therefore intron retention by the transcript. As this retained intron is coding for additional 150 amino acids, it is not in UTR region and be somewhere inside the coding region.

II). As this extra 150 AA chain will be somewhere inside the peptide chain(not at terminal), it will possibly change the entire conformation of peptide residues and organisations leading to complete loss of function.