Final answer:
A non-synonymous mutation in an intron is not possible because introns do not code for proteins and such mutations will not change the amino acid sequence of a protein. Non-synonymous mutations are relevant to exons, which code for proteins. Indel mutations can occur in both introns and exons, with frameshifts having significant effects if in exons.
Step-by-step explanation:
In biology, the question asks which of the following scenarios is not possible:
- A non-synonymous mutation in an intron
- A non-synonymous mutation in an exon
- An indel mutation in an intron
- An indel mutation in an exon
The scenario that is not possible is a non-synonymous mutation in an intron. Non-synonymous mutations change the amino acid sequence of proteins and are therefore significant in exons, which code for proteins. Introns, however, do not directly code for proteins. While introns can influence gene expression and alternative splicing, mutations within introns generally would not be non-synonymous as they do not change the coding sequence of the expressed protein.
However, if a mutation within an intron affects the spliceosome recognition sequence, this may lead to abnormal splicing and potentially result in protein changes - but this is not what is commonly understood by 'non-synonymous'. In contrast, indel mutations, which can lead to frameshifts, can occur in both introns and exons. If they occur in exons, they can profoundly affect the coding sequence of a protein by shifting the reading frame.