7L.6.2 ### Part B - Point MutationsThe instructions for assembling polypeptides are contained in DNA. Consequently, mutations in DNA sometimes affect protein structure. A mutation will initially affect a protein's primary structure, but changes at higher levels of structure may follow.Point mutations are mutations that affect a single nucleotide pair in DNA. The figure below shows different types of point mutations and the effects they have on protein primary structure.#### Wild Type- **DNA Template Strand:** 3' - T A C T C A A A C C A G C A T - 5' - **mRNA:** 5' - A U G A G U U U G G U C G U A - 3' - **Protein (Amino End to Carboxyl End):** Met - Lys - Phe - Gly#### (a) Nucleotide-pair Substitution1. **Silent Mutation** (No effect on amino acid sequence) - **A instead of G** DNA: 3' - T A C T C A A C C A G C A T - 5' mRNA: 5' - A U G A G U U G G U C G U A - 3' Protein: Met - Lys - Phe - Gly2. **Missense Mutation** (Amino acid change) - **T instead of C** DNA: 3' - T A C T C T A A C C A G C A T - 5' mRNA: 5' - A U G A G A U U G G U C G U A - 3' Protein: Met - Lys - Phe - Ser3. **Nonsense Mutation** (Premature stop codon) - **A instead of T** DNA: 3' - T A C T C A A A C T A G C A T - 5' mRNA: 5' - A U G U U U G U A G C U A - 3' Protein: Met - Stop#### (b) Nucleotide-pair Insertion or Deletion1. **Frameshift Causing Immediate Nonsense** (1 nucleotide-pair insertion) - **Extra In this activity, you will examine three different mutations.1. **Instructions**: - Drag the white and purple labels to the white targets to indicate what each mutant mRNA codon codes for. (You may need to consult the [codon table for mRNA](#) for this.) - Drag the pink labels to the pink targets to indicate the type of mutation. - Drag the blue labels to the blue targets to indicate the effect on the polypeptide’s primary structure.2. **Labels**: - **Codon Labels**: - Stop - Val - Glu - **Mutation Type Labels**: - Substitution - Insertion or deletion - **Polypeptide Effect Labels**: - Silent - Missense - Nonsense - Frameshift—immediate nonsense - Frameshift—extensive missense3. **Table Explanation**: The table below describes the DNA sequence, mRNA transcript, corresponding amino acid, type of mutation, and effect on the polypeptide for a wild-type and three mutant sequences. - **Wild-type**: - **DNA**: 3' C T T 5' / 5' G A A 3' - **mRNA**: 5' G A A 3' - **Amino acid**: Glu - **Type of Mutation**: N/A - **Effect on Polypeptide**: Normal - **Mutant 1**: - **DNA**: 3' C A T 5' / 5' G T A 3' - **mRNA**: 5' G U A 3' - **Amino acid**: (Target for codon label) - **Type of Mutation**: (Target for mutation type) - **Effect on Polypeptide**: (Target for polypeptide effect) - **Mutant 2**: - **DNA**: 3' C T C 5' / 5' G A G 3' - **mRNA**: 5' G A G 3' - **Amino acid**: (Target for codon label) - **Type of Mutation**: (Target for mutation type) - **Effect on Polype

The genetic code is the set of rules used by living cells to translate information encoded within genetic material into proteins.A codon is a sequence of three DNA or RNA nucleotides that corresponds to a specific amino acid or stop signal during protein synthesis.A mutation may convert a codon that specifies an amino acid into another amino acid or a termination codon.Mutations can be of many types, such as substitution, deletion, insertion, and translocation.ANSWER: The point mutation in Mutant 1 leads to the replacement of a codon encoding the amino acid Glu to a codon encoding the amino acid Val. This mutation is therefore a missense mutation and leads to the substitution of the amino acid Glu by Val at the protein level.The point mutation in Mutant 2 leads to the replacement of a codon encoding the amino acid Glu to another codon encoding the amino acid Glu. This mutation is therefore a silent mutation and that leads to no change at the level of the protein and is therefore silent.The point mutation in Mutant 3 leads to the replacement of a codon encoding the amino acid Glu to a stop codon. This mutation is therefore a nonsense mutation. This mutation therefore leads to premature termination of polypeptide chain and hence deletion of the amino acid sequence that lies towards the C-terminus of the site of mutation. The completed table is shown below: