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

Options

A. Missense mutation - Frame shift mutation - Nonsence mutation
B. Silent mutation - Nonsense mutation - Frame shift mutation
C. Silent mutation - Frame shift mutation -Nonsense mutation
D. Missense mutation - Frame shift mutation - Silent mutation

Answer

To determine the correct sequence of point mutations that occurred in the DNA leading to the changes in the mRNA, we will analyze each point mutation step by step.

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

This sequence translates to the following amino acids:
- AUG (Methionine) - Start codon
- CAU (Histidine)
- CUC (Leucine)
- ACG (Threonine)
- GAU (Aspartic acid)
- UAG (Stop codon)

### Step 2: Analyze Point Mutation 1
The first point mutation changes the mRNA from:
- AUG CAU CUC ACG GAU UAG
to
- AUG CAU CUU ACG GAU UAG

Here, CUC (Leucine) is changed to CUU (Leucine). Since both codons code for the same amino acid (Leucine), this mutation is a **silent mutation**.

### Step 3: Analyze Point Mutation 2
The second point mutation changes the mRNA from:
- AUG CAU CUU ACG GAU UAG
to
- AUG CAG GCU UAC GGU AUA

In this case, CAU (Histidine) is changed to CAG (Glutamine), and CUC (Leucine) is changed to GCU (Alanine). This change alters the amino acid sequence, indicating that this mutation is a **missense mutation**.

### Step 4: Analyze Point Mutation 3
The third point mutation changes the mRNA from:
- AUG CAG GCU UAC GGU AUA
to
- AUG CAU GCU UAA GGU AUA

Here, UAC (Tyrosine) is changed to UAA (Stop codon). This change results in the premature termination of the polypeptide chain, which is characteristic of a **nonsense mutation**.

### Summary of Mutations
1. **Silent Mutation**: CUC to CUU (no change in amino acid)
2. **Missense Mutation**: CAU to CAG (Histidine to Glutamine) and CUC to GCU (Leucine to Alanine)
3. **Nonsense Mutation**: UAC to UAA (Tyrosine to Stop codon)

### Correct Sequence of Point Mutations
The correct sequence of point mutations that occurred in the DNA is:
1. Silent Mutation
2. Missense Mutation
3. Nonsense Mutation

### Final Answer
The correct answer is option C: Silent mutation, Missense mutation, Nonsense mutation.

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