Transcription/translation — MCQs

Question 1: Expression of an mRNA encoding for a soluble form of the Fas protein prevents a cell from undergoing programmed cell death. However, after inclusion of a certain exon, this same Fas pre-mRNA eventually leads to the translation of a protein that is membrane bound, subsequently promoting the cell to undergo apoptosis. Which of the following best explains this finding?

• A. Histone deacetylation
• B. DNA missense mutation
• C. Alternative splicing (Correct Answer)
• D. Base excision repair
• E. Post-translational modifications

Explanation: ***Alternative splicing*** - The scenario describes a single **pre-mRNA** producing two different protein forms (soluble vs. membrane-bound Fas) with distinct functions, depending on the inclusion or exclusion of a specific **exon**. This is the hallmark of alternative splicing. - **Alternative splicing** allows a single gene to encode multiple protein isoforms, leading to diverse cellular functions and regulation. *Histone deacetylation* - **Histone deacetylation** is a mechanism of gene regulation that typically represses gene expression by making DNA less accessible for transcription, not by altering the protein product of an already transcribed gene. - It affects whether a gene is turned "on" or "off," but doesn't explain how the same pre-mRNA produces different protein versions. *DNA missense mutation* - A **DNA missense mutation** would alter a single base pair in the DNA, potentially changing one amino acid in the resulting protein. - While it can lead to functional changes in a protein, it would not explain the complete inclusion or exclusion of an entire exon, which profoundly changes the protein's overall structure and membrane association in this manner. *Base excision repair* - **Base excision repair** is a DNA repair pathway that corrects small, non-bulky DNA lesions, such as damaged or modified bases. - This process is involved in maintaining genomic integrity and does not explain the differential processing of an mRNA transcript to produce two distinct protein isoforms. *Post-translational modifications* - **Post-translational modifications** (PTMs) occur after protein translation and involve chemical changes to the protein (e.g., phosphorylation, glycosylation). - While PTMs can alter protein function or localization, they do not explain how an entire exon's inclusion or exclusion leads to fundamentally different protein structures (soluble vs. membrane-bound).