A fragment of a wild-type polypeptide is sequenced for seven amino acids. The sa

Question

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants. Identify the mutations by matching the number to the correct type of mutation:

Wild-type N . . . Thr-His-Ser-Gly-Leu-Lys-Ala . . . C polypeptide

       Mutant 1 N . . . Thr-His-Ser-Val-Leu-Lys-Ala . . . C

       Mutant 2 N. . . Thr-His-Ser-C

       Mutant 3 N . . . Thr-Thr-Leu-Asp-C

       Mutant 4 N . . . Thr-Gln-Leu-Trp-Ile-Glu-Gly . . . C

Answer Choices:

Missense

Nonsense

Deletion

Insertion

(This is biochemistry

Missense

Nonsense

Deletion

Insertion

(This is biochemistry

Explanation / Answer

In mutant 1, 4th amino acid, glycine became valine. This is a missense mutation, in which a single nucleotide change results in change of a single amino acid in the polypeptide chain.

Mutant 2: - The amino acid chains stops after translating three amino acid. This always happens due to incorporation of a stop codon. And such a mutation is known as non sense mutation.

Mutant 3 and 4 are frame shift mutations. Frame shift mutations occur either due to insertion or deletion of a nucleotide in the growing polypeptide chain.

The original chain was Thr-His-Ser-Gly-Leu-Lys-Ala

Its mRNA sequence would be AC_- CAC-UCU-GGA-UU(A/G)-AA

Mutant 3 is Thr-Thr-Leu-Asp; AC_- ACU-CUG-GAU-U(A/G)A ; means a deletion (bolded) has been there at the fourth position of mRNA chain.

Mutant 4 in an insertion mutation. Insertion of A or G occurs at the 6th position of the mRNA chain.

The sequence now becomes AC_CA (A/G)CUCUGGAUU(A/G)AA..... in mutant 4.

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