4. (4 pts) Nonsense mutations are mutations that introduce a STOP codon in place

Question

4. (4 pts) Nonsense mutations are mutations that introduce a STOP codon in place of a sense codon, and result in premature termination of translation. These mutations are not as rare as you might think. For example there are 8 different sense codons that can be changed to the stop codon UAG by a single point mutation (base change) Using the codon table in the textbook, list these 8 sense codons (and their corresponding amino acid) Consider the following introduction of a nonsense mutation into the provided wild type sequence. Transcribe both the wild type and mutant sequence into mRNA and then translate each into its respective polypeptide. I WANT TO SEE POLARITY FOR ALL MOLECULES! Nonsense , mutation 5,-ATGccGGTCTAGCTT-3" template--> 3 ,-TACGGCCAGGTCGAA-5 , 3 '-TACGGCCAGATCGAA-5'_template What is a nonsense suppressor mutation? Describe how a mutation in a tRNA for serine with the anticodon sequence 3'-AGC-5' could correct for the above described nonsense mutation. How would this affect the final translated protein in relation to the normal protein? *Be specific as to the exact mutational change in the protein and speculate as to whether you think this might be a significant change in the function of the protein e.g. consider the biochemical properties of the amino acids.

Explanation / Answer

The 8 sense codons that can be changed to the stop codon UAG by a single point mutation are (bold shows the point mutation):

AAG-lysine

CAG-Glutamine

GAG-Glutamic acid

UUG-Leucine

UCG-Serine

UGG-Tryptophan

UAC-Tyrosine

UAU-Tyrosine

3' TACGGCCAGGTCGAA 5' Wild template   

mRNA transcript-- 5' AUGCCGGUCCAGCUU3'

amino acid sequence ---methionine proline valine glutamine leucine

  3' TACGGCCAGATCGAA 5' Mutated template

mRNA transcript-- 5' AUGCCGGUCUAGCUU3'

amino acid sequence ---methionine proline valine

A non sense supressor mutation is a mutation that can inhibit the effect of nonsense mutation.

If the tRNA for serine gets mutated from 3'AGC 5' to 3' AUC 5' then the new amino acid sequence will be

methionine proline valine serine leucine

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