Question

Choose the correct answer from the alternatives given :
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.

• A. Missense mutation $\to$ Frameshift mutation $\to$ Nonsense mutation
• B. Silent mutation $\to$ Nonsense mutation $\to$ Frameshift mutation
• C. Silent mutation $\to$ 4 Frameshift mutation $\to$ Nonsense mutation
• D. Missense mutation $\to$ Frameshift mutation $\to$ Silent mutation

Answer 1

The correct option is C Silent mutation $\to$ 4 Frameshift mutation $\to$ Nonsense mutation

In mutation 1, the codon CUC changes to CUU. As both the codons code for the same amino acid i.e., leucine, thus, this mutation is silent mutation and does not affect individual having it.

In mutation 2, a base G is inserted between codons CAU and CUU and changes the frame of reading of genetic code and changes the entire polypeptide coded by it. It is a frameshift mutation.

In mutation 3, codon UAC is changed into a termination codon UAA which stops polypeptide synthesis, hence such type of mutation is called nonsense mutation.

So, the correct answer is 'Silent mutation → 4 Frameshift mutation → Nonsense mutation'.