The human genome contains approximately 20,000 protein-coding genes, yet the human proteome contains far more than 20,000 distinct proteins. Which RNA processing mechanism best explains this discrepancy?
AThe 5' cap adds molecular variants to each mRNA, producing different protein start sites
BAlternative splicing can include or exclude different exons from the same pre-mRNA, producing multiple distinct protein isoforms from a single gene
CPoly-A tail length variation changes mRNA stability and therefore the relative abundance of each protein
DRNA editing changes specific nucleotides after transcription, generating extensive sequence diversity
Alternative splicing is the dominant mechanism. Over 90% of human multi-exon genes undergo alternative splicing, and combinatorial inclusion or exclusion of exons can generate dozens or hundreds of distinct mRNA variants from one gene. The Drosophila Dscam gene can theoretically produce over 38,000 variants from a single locus. While RNA editing (option 3) does contribute some diversity, it is far less widespread and impactful than alternative splicing in explaining the proteome-to-genome complexity ratio.
Question 2 Multiple Choice
What would most likely happen to a eukaryotic mRNA if its 5' cap were removed immediately after transcription?
ATranslation would be faster because ribosomes could access the start codon more easily without the cap in the way
BThe mRNA would be rapidly degraded by 5'→3' exonucleases and translation initiation would fail
CSplicing could not occur because the spliceosome requires the cap to identify the correct pre-mRNA
DThe poly-A tail would compensate, and the mRNA would function normally
The 5' cap serves two critical functions: protecting the mRNA's 5' end from exonuclease degradation, and serving as the recognition signal for the ribosomal initiation complex (via the cap-binding protein eIF4E). Without the cap, the mRNA is vulnerable to rapid 5'→3' degradation and ribosomes cannot efficiently initiate translation. The poly-A tail (option 3) protects the 3' end but cannot compensate for loss of 5' protection or the ribosome-binding function that the cap provides.
Question 3 True / False
The same pre-mRNA can give rise to proteins with different functions in different cell types through alternative splicing.
TTrue
FFalse
Answer: True
Alternative splicing allows cell-type-specific regulation of which exons are included in mature mRNA. SR proteins that promote exon inclusion and hnRNPs that cause exon skipping are themselves differentially expressed across tissues, creating a tissue-specific splicing code. A gene might produce a membrane-bound isoform in neurons (by including an exon encoding a transmembrane domain) and a soluble isoform in liver cells (by excluding that exon) — two functionally distinct proteins from one gene.
Question 4 True / False
Prokaryotic genes contain introns that are removed by the spliceosome, just like eukaryotic genes.
TTrue
FFalse
Answer: False
Prokaryotic genes generally lack introns — their protein-coding sequences are continuous. This is why prokaryotes do not require an RNA processing pipeline and can begin translating an mRNA while it is still being transcribed. Spliceosome-mediated splicing is exclusively eukaryotic and evolved alongside the intron-containing genome organization of eukaryotes. (Rare self-splicing introns exist in some prokaryotes and eukaryotic organelles, but these are fundamentally different from spliceosomal splicing.)
Question 5 Short Answer
How does alternative splicing allow the human proteome to be far more complex than the ~20,000 protein-coding genes in the genome, and what machinery controls which isoforms are produced in different cells?
Think about your answer, then reveal below.
Model answer: Alternative splicing produces multiple distinct mRNA variants from a single gene by including or excluding different combinations of exons. A gene with several exons can generate many functionally distinct proteins through combinatorial exon selection. Over 90% of human multi-exon genes are alternatively spliced. The choice of which splice sites to use is regulated by splicing factors: SR proteins bind enhancer sequences to promote exon inclusion, while hnRNPs bind silencer sequences to cause exon skipping. Because these regulators are differentially expressed across cell types and developmental stages, different tissues produce different isoform profiles from the same gene.
This makes the spliceosome a major layer of gene regulation. Mutations that disrupt splice sites or splicing regulatory sequences can cause disease by shifting the isoform balance — several cancers involve mutations in spliceosome components or splicing regulators, underscoring that splicing is not a mere housekeeping step but a critical control point in gene expression.