4. A gene encodes a protein with the following amino acid sequence A nonsense mu

Question

4. A gene encodes a protein with the following amino acid sequence A nonsense mutation from a single-base-pair occurs in this gene, resulting in protein substitution suppressor mutation allows the with the amino acid An extragenic mutation and the gene to the sequence Met-Lys. original produce full protein. With amino acid present, the gene now produces protein withthe sequence: Met-Lys-Cys-Pro-Ala-Thr-Pro a) In the original mutation, what specific DNA sequence changes could have occurred? List the possible codons for serine that can be converted to stop codons by a single base substitution. Also indicate which stop codon(s) they can be converted to. pts) b) Describe the location and nature ofthe extragenic suppressor.2 pts) c) Which of the following changes you listed in part a is most likely, given the nature of the extragenic suppressor?

Explanation / Answer

Answer:

4. a. Original protein: Met-Lys-Ser-Pro-Ala-Thr-Pro

Mutated protein: Met-Lys-Stop

So, the codon encoding Ser got mutated to a stop codon.

Codons of serine: UCU, UCC, UCA, UCG

Stop codons: UAA, UGA, UAG

So, possible codons of serine which can be converted to a stop codon through a single base substitution are:

i. UCA: Changing the C either to A or G. (Stop codons: UAA and UGA)

ii. UCG: Changing the C to A. (Stop codons: UAG)

b. The nonsense mutant can potentially be suppressed by nonsense suppressor tRNA, in which a sense codon has been changed to be able to decode the nonsense triplet.

Since it is a tRNA, it is found in the cytoplasm.

c. In practice, only amber suppressors have been recovered.

So, UCG to UAG (amber) is the most likely change which occured in the gene.

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