Question

A DNA molecule has the sequence CAT CAT CAT. If a guanine base is added at the beginning of the sequence, which of the following would be the most appropriate option?
A) G UAT CAT CAT
B) GCA TCA TCA T
C) Frameshift mutation
D) B and C

Answer 1

Hint: On the DNA strand, the bases are read in triplet codons. Each set of three bases will code for an amino acid, a start codon, or a stop codon. Of the 64 codons, 61 code for amino acids, and 3 are stop codons.

Complete Answer:
- In option A, the Guanine has been inserted and left alone without adding it to any codon Nucleotides will be arranged in sets of three. Option A is incorrect.
- In option B, with the addition of guanine, the codons will be read in triplets from there. So the CAT codons are now changed. Option B is correct.
- A frame shift mutation is when the addition or deletion of 1 or 2 bases alters the reading frame of the existing sequence. In this instance, one base has been added, so the reading frame has shifted by one making it a frameshift mutation. As it is only one base, this is also a point mutation. Option C is also correct.
- As both option B and C give a correct premise, option D is the correct answer. This is a frameshift mutation which will now read GCA TCA TCA T

Hence the correct answer is option C.

Note: As there are 61 codons coding for 20 amino acids, there will be some amount of redundancy. That is, many of the amino acids are coded for by more than one triplet codon. If the number of nucleotides added or removed is 3, the rest of the reading frame continues to remain the same.