Question

A frameshift mutation caused by the insertion of a nucleotide in the mRNA with the sequence 5' CAT CAT CAT CAT CAT CAT CAT CAT CAT produces an inactive protein. A second frameshift, caused by the deletion of a second nucleotide at some position downstream of the nucleotide insertion, restores the protein function. If the restored protein product contains four amino acids residues that differ from the wild-type protein, what are the maximum and minimum number of nucleotides between the two point mutations?

Answer 1

Answer:

As per the question, the original sequence of the mRNA which codes for a wild type of a protein is;

5’ CAT CAT CAT CAT CAT CAT CAT CAT CAT 3’

These 9 set of nucleotide base pairs forms a codon which codes for a particular amino acid; 9 in number.

A frameshift mutation is the one which shifts the reading frame of the codons thereby altering the proteins formed. Given in the question, to the above sequence of mRNA, there takes place a frameshift mutation wherein the addition of a base pair takes place at any particular given point due to which the frame of the proteins formed gets altered and as a result of which a non functional inactive protein is formed.

Further again, when there takes place a second frameshift mutation to the already mutated nucleotide sequence; in which the deletion of a particular nucleotide occurs downstream to the point of inserted nucleotide such that a functional protein is formed thus restoring the protein product. This time the protein formed was with 4 amino acids as against the previous wild type of protein which had 9 amino acids.

Summarizing:

wild type protein: 9 amino acids

Restored protein: with 4 amino acids

Therefore, the difference in the amino acids between the two types of proteins is 5

Since 3 nucleotides together form a codon which in turn codes for a particular amino acid; the difference in the nucleotides (or the total number of the nucleotides present between the two point mutations) would be 5*3=15.

(A combination of 3 nucleotides together forms a codon. And a codon codes for an amino acids. Proteins are the chains of the amino acids joined together by the peptide bonds)

Alteratively; this can also be represented by the total number of the nucleotides coding for the wild type of protein having 9 amino acid =9*3=27.

With first point mutation in place, an inactive protein was formed

with second point mutation, a protein was formed with 4 amino acids. Therefore the total number of nucleotides=4*3=12

The difference of the nucleotides between the two different types of proteins formed=27-12=15