Question

The RNA triplet UAG acts as a stop codon which terminates the synthesis of a polypeptide. Below is given a strand of DNA which codes for four amino acids.
In which position will an insertion of a thymine nucleotide result in the termination of translation?
DNA strand:
3′ T C C A C C C G A T G C 5′

• A. 3′ TCCACCCGATGC 5'
• B. 3′ TCCACCCGATGC 5′
• C. 3′ TCCACCCGATGC 5′
• D. 3′ TCCACCCGATGC 5′

Hint:

Frameshift mutations due to base insertion can create premature stop codons like UAG, ending translation early.

Answer 1

The Correct Option is B

The given DNA strand is 3′ to 5′, and the mRNA will be complementary in 5′ to 3′ direction. Without insertion, the original codons would be:

• DNA: 3′ TCC ACC CGA TGC 5′
• mRNA: 5′ AGG UGG GCU ACG 3′ — coding for four amino acids.

Inserting a thymine (T) at position (B) would result in a frameshift mutation. The sequence becomes:

• DNA: 3′ TCC ACC TCG ATG C 5′
• mRNA: 5′ AGG UGG AGC UAC G 3′ — which includes the stop codon UAG, terminating translation prematurely.