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Question

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Describe the possible effects of each of these mutations on a gene’s protein. Which of these mutations in a gene would likely have the most severe effect on its protein? Explain.

A. missense

B. point mutation in an intron

C. a deletion of three bases

D. a translocation

E. a nonsense mutation at the end of the gene               (12 marks)

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Explanation / Answer

Mutation and its impact on gene protein:

Proteins are consisting of amino acids, and mutation mainly affect the synthesis of protein either by enhancing or decreasing the process of protein synthesis or by changing the amino acid sequence of resultant protein. When a mutation alters a protein (by altering amino acid) that plays crucial role in its function may lead to non-functional protein.

A gene mutation mainly means a change (permanent) in DNA sequence (nucleotide sequence) which alters a gene in such a way that it is different from normal gene. Mutation can be of different types and due to different reasons. In general mutation mainly occurs as:

As in present question impact of mutation on gene protein is asked for so:

Classify the above 5 process into major and minor changers:

Minor mutation or changes include:

Major mutation or changes include

This mutation causes changes in an amino acid to another amino acid, hence may or may not affect the protein function. It is also a type of point mutation.

For Eg., Substitution of a base from a nucleotide, GGA codes for glycine but with missense mutation GGA will be changed to AGA (by altering first guanine base with adenosine base) that will code for arginine. Hence the protein which contains glycine as one of amino acid in the initial sequence will be altered by substitution of a base and replaced with arginine. This may render resultant protein non-functional. Hence could be harmful.

2. Point mutation in an intron:

The intron (also called non coding regions) is a nucleotide coding sequence within gene which is removed by RNA splicing during final processing of RNA. However the introns don’t code for any protein so they should not have any impact on protein incase of point mutation.

However the intronic mutations can affect gene expression levels (as Exons code for proteins). For Eg., point mutations (single nucleotide polymorphisms) in introns can introduce novel splice sites, activate novel promoters (which may direct sense or antisense transcription causing alterations in mRNA, miRNA or lncRNA expression), or introduce/eliminate enhancer activity.

So mutation (Insertions/deletions) in intronic sites may carry drastic effect up to some extent.

3. Deletion of three bases:

General, the functional consequences of any point mutation (deletion) depends on its location and on whether it disrupts a particular target site for protein or RNA binding.

Deletions of bases (3 bases) have consequences on polypeptide sequence that extend far beyond the site of the mutation because the sequence of mRNA is “read” by the translational apparatus in groups of three base pairs (codons), the deletion of single base of DNA will cause a change of reading frame starting from the location of the deletion and extending through to the carboxyl terminus of the protein. However in case of 3 bases the chances of frame-shift may or may not arise.

These mutations may or may not cause the entire amino acid sequence translationally downstream of the mutant site to bear no relation to the original amino acid sequence. Thus, frame-shift mutations typically exhibit complete loss of normal protein structure and function. However with three base deletion these chances are minimized hence harmful impact may be less or none. But if it is done on the stop codon this may cause continous elongation of protein hence may have sever impact.

4. Translocation:

Mainly involves chromosomal mutation in which chromosome segments and the genes change position (by breaking and rejoining). If occurs within a chromosome it is intra-chromosomal or if occurs between chromosomes it is called inter-chromosomal. If there is any loss or gain of segment then there are harmful effects (like cancer etc.) or else it is neutral.

5. Non sense mutation at the end of gene

A non sense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or also called a nonsense codon {amber(UAG), ochre (UAA) and opal (UGA)} in the transcribed mRNA, often nonfunctional protein product.

So a nonsense mutation causes changes in an amino acid to a STOP codon, resulting in premature termination of translation.

However as in present case it is occurring at the end of gene hence it will have no derogative effect or no harmful impact on protein.

So from above analysis I found that all procee A-E have some effect of protein however the adversity can by given the following order:

D>C>A>B>E

D being most harmful and E being least.

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