3. (8 points) You are studying a mutation of a signaling protein. The mutant pro

Question

3. (8 points) You are studying a mutation of a signaling protein. The mutant protein is completely nonfunctional and is shorter than wild type. The mutant mRNA, however, is significantly longer than wild-type mRNA. You find the mutation is a single base-pair substitution. This substitution is not in the coding sequence but is the second base pair in the first intron. Propose a molecular explanation for how a single base change in the intron results in a longer mRNA and a shorter non-functional protein product.

Explanation / Answer

Proposed Explanation: Exon and Intron junction are demarketed with specific sequences, mostly enriched by AG-GT sequences. All the exon intron junctions must not be these sequences, as there are as many as 9 cryptic site for exon and intron junctions reported. There are specific exon junction complexes (EJC) that is a specific machinery to act on exon intron junction. These have important roles in splicing and translation. Thus, if there is single nucleotide change at the 2nd base pair of the intron, it might have great effect to abolish the exon intron junction. This may even lead to loss of recognition of Exon junction complex (EJC) for that site to be spliced, which might result in making that into the proper exon sequence and finally into the m-RNA un-spliced, which is ready for translation. These events will convert all the amino acids in that intron into the protein, finally making that a non-functional protein.

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