A DNA sequence undergoes three subsequent point mutations which result in subsequent changes in transcription of mRNA as shown below:
Original mRNA : AUG CAU CUC ACG GAU UAG
Point mutation 1: AUG CAU CUU ACG GUA UAG
Point mutation 2 : AUG CAU GCU UAC - GGU AUA
Point mutation 3 : AUG CAU GCU UAA GGU AUA
select the correct sequence of point mutations that occurred in the DNA.

Options

A. Missense mutation `to` Frame shift mutation `to` Nonsense mutation
B. Silent mutation `to` Nonsense mutation `to` Frameshift mutation
C. Silent mutations `to` Frame shift mutation `to` Nonsense mutation
D. Missense mutation `to` Frameshift mutation `to` Silent mutation

Answer

To determine the correct sequence of point mutations that occurred in the DNA, we will analyze the original mRNA sequence and the subsequent mutated sequences step by step.

### Step 1: Identify the Original mRNA Sequence
The original mRNA sequence is:
- **Original mRNA**: AUG CAU CUC ACG GAU UAG

### Step 2: Analyze the First Point Mutation
The first point mutation results in the following mRNA sequence:
- **Point Mutation 1**: AUG CAU CUU ACG GUA UAG

**Change**:
- The codon CUC (which codes for leucine) has changed to CUU (which also codes for leucine).

**Type of Mutation**:
- This is a **silent mutation** because the amino acid encoded remains the same.

### Step 3: Analyze the Second Point Mutation
The second point mutation results in the following mRNA sequence:
- **Point Mutation 2**: AUG CAU GCU UAC GGU AUA

**Change**:
- A base 'G' has been added between CAU and CUU, changing the sequence to CAU GCU.

**Type of Mutation**:
- This is a **frameshift mutation** because the addition of a nucleotide alters the reading frame of the genetic code.

### Step 4: Analyze the Third Point Mutation
The third point mutation results in the following mRNA sequence:
- **Point Mutation 3**: AUG CAU GCU UAA GGU AUA

**Change**:
- The codon UAC (which codes for tyrosine) has changed to UAA, which is a stop codon.

**Type of Mutation**:
- This is a **nonsense mutation** because it introduces a premature stop codon, terminating polypeptide synthesis.

### Conclusion: Sequence of Point Mutations
Based on the analysis:
1. **Silent Mutation** (CUC to CUU)
2. **Frameshift Mutation** (addition of G)
3. **Nonsense Mutation** (UAC to UAA)

Thus, the correct sequence of point mutations that occurred in the DNA is:
- **Silent Mutation** → **Frameshift Mutation** → **Nonsense Mutation**