Mutation is the sudden change that results in change of nucleotide sequence. The insertion or deletion of nucleotide into the gene causes mutation. Thus, the sequence of produced protein varies.
In point mutation, a single nucleotide change occurs due to insertion or deletion in the nucleic acid. The nonsense mutation is a type of point mutation where insertion or deletion of nucleotide in the nucleic acid sequence results in premature stop codon.
Introns are the non-coding nucleotide sequences that are removed by a process called RNA splicing (ribonucleic acid). Exons are the coding nucleotide sequences that are joined to form a gene after the process of RNA splicing (ribonucleic acid).
The nonsense mutation in an exon results in premature stop codon. Thus, the gene expression is severely affected.
Hence, the correct option is a.
A point mutation in the exon alters the structure of produced protein. Thus, the gene expression is severely affected. Hence, the option (b) is not correct.
The mutation in the splice donor site leaves behind the fragment of intron in the nucleic acid. This enhances the expression of genes. Hence, the option (c) is not correct.
A point mutation in the middle of an intron has no effect on gene expression. This is because intron is a non-coding sequence. Hence, the option (d) is not correct.