Question

A single mutation has occurred in the following DNA sequence.

5' ATG TTC CAG CCA 3' wild-type (normal) sequence

5' ATG TTC TAG CCA 3' mutant sequence

a) Identify and classify the mutation according to its molecular structure (i.e., insertion, deletion base substitution (transversion), or base substitution (transition)). Briefly explain why you selected this classification.

b) Identify and classify the mutation according to its functional effects (i.e., frameshift, missense, nonsense, or silent). Briefly explain why you selected this classification.

Answer 1

Answer:
(a)The mutation give here is base substitution. We can call this mutation as base substitution because, the base Cytosine (C) in wild DNA is replaced (substituted) with Thymine (T). Base substitutions are classified into two viz. Transition and transversion. Transition is referred to the substitution of a purine by some other purine OR the substitution of a pyrimidine by some other pyrimidine. Here, a pyrimidine (Cytosine) is replaced by another pyrimidine (Thymine), hence, it is in fact a transition type of base substitution.
(Note:- Deletion is a kind of mutation in which a base is removed without adding another. Insertion is the addition of a base without the removal of any. Transversion is the type of base substitution involving the substitution of a purine by a pyrimidine or a pyrimidine is substituted by a purine.)

(b) The given mutation is a missense mutation. This is because, the codon in mRNA produced from the mutated DNA segment will code for a different amino acid compared to the wild DNA.
The base C is replaced by the base T in DNA. This will cause the base G to be changed to A in the mRNA. To analyse the amino acid, we need to look for the triplet codon that has undergone change. The codon GUC in wild mRNA will be converted to AUC in the mRNA from the mutated DNA. This will cause a change in amino acid from Valine (GUC) to Isoleucine (AUC). Hence, the mutation is of missense type.
(Note:- Nonsense mutation is the one in which the mutation cause a change from an amino acid coding codon to a stop codon. Silent mutations are those in which changes in the base pairs don’t cause a change in the amino acid sequence, thus keeping the protein chain unaltered. Frame shift mutations are caused by either addition or removal of some number of bases which is not divisible by 3 (for example, removal or addition of 1,2,4,5,7 etc. Bases)