Question

For this sequence of mRNA nucleotides GGUGCUCCUAUU, what happens if a nucleotide is inserted in the fourth position?

Answer 1

An insertion of a nucleotide between U and G at the forth location in the chain would result in a mutation that will change the final amino acid sequence.

Explanation:

If we assume that the start codon occurred earlier in the sequence the mRNA sequence listed would result in the following amino acids being produced: Gly-Ala-Pro-Lle
However, after the insertion every amino acid after glycine could be different.

Answer 2

There will be frameshift mutation.

Explanation:

Sequence of codons GGU-GCU-CCU-AUU will be translated in
a series of amino acids connected by peptide bonds, as follows:-

Glycine-Alanine-Proline-Isoleucine.

When one nucleotide is added at fourth location, the reading frame will change down the line. This is because, codons are arranged in a comma-less fashion and every three bases represent one amino acid.

The fourth newly inserted nucleotide could be any of the four (A, U, G, C) available: let us represent by the letter N and after insertion the sequence of codons will be read like this:-

GGU- N GC-UCC-UAU-Uetc.

(N could be any of the four RNA bases; thus second amino acid in the chain will now become either - AGC serine/UGC cysteine/GGC glycine/CGC-arginine. Third amino acid will become serine while fourth in the series will become tyrosine and so on.)

Please note, after the glycine, sequence of amino acids will definitely change as due to insertion of a base, reading frame shifts and successive codons change meaning.

1. When such frameshift mutation occurs in a gene, the protein it codes become structurally abnormal: as sequence of amino acids it codes change for ever beyond the point of mutation. Such a protein becomes functionally nonviable and normal physiology of a cell
   will be altered in its presence.

2. Frameshift mutation may also cause premature chain termination of polypeptide that the gene codes for, a serious defect in cell. This may happen due to shift in reading frame which could lead to appearance of a 'stop' codon in the mutated sequence. Thus the polypeptide will not fully grow during translation.

3. On the other hand such mutation also erases the normal stop codon and a readthrough translation frame may arise. In any case, translated protein loses its structural and functional attributes.

4. Deletion of base from reading frame will also result in similar frameshift mutation.

Consequence of frameshift mutation can be understood from the following example :