Question

Consider an mRNA strand that has a sequence of base pairs of GUU CUA ACU. Suppose a substitution mutation occurred in the DNA, resulting in the last base in the first codon of the mRNA to be replaced by Cytosine. Using the codon chart below, a) identify which amino acid will now be formed, b) identify whether or not there will be phenotypic change, and a bried explanation.

Answer 1

To analyze the given situation, let's break down the steps related to the substitution mutation and how it affects the mRNA and the resulting amino acid.

1. Original mRNA Sequence: The provided mRNA sequence is GUU CUA ACU.

   The codons are as follows:

   • First codon: GUU
   • Second codon: CUA
   • Third codon: ACU

2. Mutation Description: A substitution mutation occurs, replacing the last base in the first codon (GUU) with Cytosine, transforming it into GUC.

3. New mRNA Sequence after the mutation:

   • First codon: GUC (instead of GUU)
   • Second codon: CUA
   • Third codon: ACU

4. Identify the Amino Acids:

   • Using the codon chart:
     • GUU codes for Valine (Val)
     • GUC also codes for Valine (Val)
     • CUA codes for Leucine (Leu)
     • ACU codes for Threonine (Thr)

5. Result of the Mutation:

   • The first codon changed from GUU (Val) to GUC (Val). Therefore, there is no change in the first amino acid produced.
   • The other codons remain the same (CUA and ACU).

Answers: a) Amino Acid Formed: Valine (Val) is still formed after the mutation.

b) Phenotypic Change: There will be no phenotypic change. Explanation: Since both GUU and GUC code for the same amino acid (Valine), this type of mutation is classified as a silent mutation. Silent mutations do not affect the polypeptide chain's final structure, leaving the resulting protein and its function unchanged.