Alternative splicing — single gene result in different proteins.
Alternative or differential splicing is RNA splicing that produces multiple proteins from single gene during gene expression.
Different Mechanisms Of Alternative Splicing:
1. Skipping of Exons:
Exons are coding sequences that are joined with each other by covalent bonding and produces mature mRNA. Transcription of mRNAs starts and ends with an exon and introns are present between the exons. In gene splicing, exon(s) are excluded or included from the final gene transcript and it leads to short or long mRNA variants respectively.
2. Retention of Introns:
Introns are non-coding sequences and are removed by RNA splicing. Therefore introns are not expressed in the final mRNA product. If introns are not removed then final mRNA product contain extra sequence that leads to production of wrong protein during translation.
Other mechanism of alternative splicing are alternative donor or acceptor site that change the 5′ or 3′ splice site.
An example of a single gene that can encode two completely different proteins is Calcitonin.
This is because of alternative RNA processing that leads to the different protein products.
In calcitonin 3′ cleavage site is used for alternative splicing and leads to different proteins.
A) The gene CGRP (Calcitonin Gene Related Peptide): Cleavage occurs after the sixth exon then it results in calcitonin-gene-related peptide in the brain.
B) Hormone calcitonin: Cleavage and polyadenylation occur in the thyroid gland, after the fourth exon results in hormone calcitonin.