A polypeptide has the following amino acid sequence
Met-Ser-Pro-Arg-Leu-Glu-Gly
In a mutant the amino acid sequence of this polypeptide was found to be
Met-Ser-Ser-Arg-Leu-Glu-Gly
What is the type of mutation that occurred in the reading frame of its DNA?

Options

A. Silent mutation
B. Nonesense mutation
C. Mis-sense mutation
D. Frame-shift mutation

Answer

To determine the type of mutation that occurred in the reading frame of the DNA based on the given amino acid sequences, we can follow these steps:

### Step 1: Identify the Original and Mutant Amino Acid Sequences
- Original sequence: Met-Ser-Pro-Arg-Leu-Glu-Gly
- Mutant sequence: Met-Ser-Ser-Arg-Leu-Glu-Gly

### Step 2: Compare the Two Sequences
- The first two amino acids (Met and Ser) are the same in both sequences.
- The third amino acid differs: Pro in the original sequence is replaced by Ser in the mutant sequence.
- The remaining amino acids (Arg, Leu, Glu, Gly) are identical in both sequences.

### Step 3: Determine the Type of Mutation
- A **silent mutation** would not change the amino acid sequence at all, which is not the case here.
- A **nonsense mutation** introduces a premature stop codon, which is also not present in this case.
- A **frameshift mutation** results from insertions or deletions that shift the reading frame, altering all subsequent amino acids. Since only one amino acid has changed and the rest remain the same, this does not apply here.
- A **missense mutation** is when a single nucleotide change results in a codon that codes for a different amino acid. In this case, the proline (Pro) has been changed to serine (Ser).

### Conclusion
The type of mutation that occurred is a **missense mutation**.

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