Question

A single mutation has occurred in the following DNA sequence.

5' ATG TTG GCC CAT 3' wild-type (normal) sequence

5' ATG TTG CCC CAT 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. (1.75 marks)

  

(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. (1.75 marks)

You will require the following genetic code to answer this question.

  

Answer 1

a) The type of mutation that has occured is substitution and to be specific it is transversion. insertion is when there is an extra base added instead of the usual no of bases. deletion occurs when a base is missing from its usual location. Substitution is when there is a different base instead of another. it can be of 2 types- Transition- where a purine gets substituted for another purine and a pyrimidine for a pyrimidine only. But a transversion occurs when a purine substitutes for a pyrimidine and the vice-versa. Since in the given example- A guanine (purine) has been replaced by a cytosine(pyrimidine) it is called as transversion.

b) Based on functional effects mutations can be classified as

frameshift- as the name says here there is an addition or deletion of a base which causes change in the reading frame of the DNA which means the coding region of the DNA has been changed causing a change in the RNA sequence formed and subsequently change in the protein formed.

missense- it is a mutation where a single base changes and therefore the codon now codes for a different amino acid than before.

nonsense- here also there is a base substitution but now instead of coding for a different amino acid it codes for one of the 3 stop codons because of which the mRNA is terminated prematurely and the protein becomes non functional.

Silent- Here too there is a change in a single base, but the codon now still codes for the same amino acid.this is possible because some amino acids are coded by multiple codons.Hence though there is a mutation and change in base it does not cause any effect in the final protein coded.

IN our example- the codon affected is GCC and it becomes CCC. GCC codes for Alanine while CCC codes for Proline. Therefore it is clear that the amino acid coded has been changed than before that will surely affect the protein in terms of its structure and function.Therefore it can be said that the mutation is Missense.