Question

In studies of frameshift mutations, Crick, Barnett, Brenner, and Watts-Tobin found that either three nucleotide insertions or deletions restored the correct reading frame. (a) Assuming the code is a triplet, what effect would the addition or loss of six nucleotides have on the reading frame? (b) If the code were a sextuplet (consisting of six nucleotides), would the reading frame be restored by the addition or loss of three, six, or nine nucleotides?

Short answer

In a sextuplet code, is the reading frame restored by adding or losing 3, 6, or 9 nucleotides? Answer: In a triplet code, adding or losing six nucleotides will have no effect on the reading frame since they represent complete codons. In a sextuplet code, the reading frame will only be restored by adding or losing six nucleotides.

Step-by-step solution

Part (a) - Effect of adding or losing six nucleotides in a triplet code

In the case of a triplet code, each codon consists of three nucleotides. Now, let's analyze the addition or loss of six nucleotides. 1. Addition of six nucleotides: - In this scenario, we add six nucleotides, which represent two extra codons. This means that their effect on the reading frame would be neutral since they create complete codons and the original reading frame is maintained. 2. Loss of six nucleotides: - In this scenario, we remove six nucleotides. This means two complete codons are removed from the sequence. Again, their effect on the reading frame would be neutral since by removing two complete codons, the original reading frame remains unchanged. So, adding or losing six nucleotides will have no effect on the reading frame in a triplet code.

Part (b) - Restoration of the reading frame by adding or losing 3, 6, or 9 nucleotides in a sextuplet code

In the case of a sextuplet code, each codon consists of six nucleotides. Now, let's analyze the restoration of the reading frame by adding or losing three, six, or nine nucleotides. 1. Addition or loss of three nucleotides: - In this scenario, we are either adding or losing three nucleotides, which would disrupt the reading frame since each codon in the sextuplet code must contain six nucleotides. So, adding or losing three nucleotides will not restore the reading frame. 2. Addition or loss of six nucleotides: - In this scenario, we are either adding or losing six nucleotides, which represent a complete codon in the sextuplet code. By either adding or losing a complete codon, the reading frame is unaffected. Thus, the reading frame is restored by adding or losing six nucleotides. 3. Addition or loss of nine nucleotides: - In this scenario, we are either adding or losing nine nucleotides. However, this represents one and a half codons in the sextuplet code, which results in a frameshift mutation and disruption of the reading frame. So, adding or losing nine nucleotides will not restore the reading frame. Therefore, the reading frame in a sextuplet code will only be restored by the addition or loss of six nucleotides.