The human genome contains approximately 20,000 protein-coding genes, yet human cells produce an estimated 80,000–100,000 distinct proteins. What is the primary mechanism responsible for this discrepancy?
APost-translational modifications such as phosphorylation and glycosylation create distinct protein variants.
BAlternative splicing generates different mRNA isoforms from the same gene by including or excluding different combinations of exons.
CGene duplication has created extra gene copies not yet fully cataloged by the genome project.
DTranscriptional errors randomly produce variant proteins at low frequency.
Alternative splicing is the primary driver of proteomic diversity beyond the gene count. A gene with 10 alternatively spliced exons can theoretically generate over 1,000 distinct mRNA isoforms, each encoding a protein with different functional domains or regulatory properties. Post-translational modifications also contribute to protein diversity, but alternative splicing is responsible for the largest categorical expansion from genes to protein isoforms. This is why the human proteome vastly exceeds the genome in complexity.
Question 2 Multiple Choice
A researcher introduces a point mutation changing the conserved GU dinucleotide at the 5' splice site of an intron to GC. What is the most likely consequence for gene expression?
AThe intron is skipped normally because the spliceosome uses the 3' splice site as its primary recognition signal.
BSplicing is disrupted — the spliceosome fails to recognize the mutant 5' splice site, likely causing intron retention in the mature mRNA or activation of a nearby cryptic splice site.
CThe mutation is corrected by RNA editing enzymes before splicing occurs.
DTranscription of the gene stops because the promoter recognizes the downstream mutation.
The 5' splice site GU dinucleotide is recognized by U1 snRNP through base-pairing with U1 snRNA. This recognition is essential for initiating spliceosome assembly. Mutating GU to GC disrupts this base-pairing, preventing U1 snRNP from binding. Without proper 5' splice site recognition, the spliceosome cannot assemble correctly: the intron may be retained in the mRNA (intron retention), or the spliceosome may use a nearby 'cryptic' splice site with a weak GU-containing sequence. Either outcome produces an aberrant mRNA that often encodes a nonfunctional protein. This class of mutation accounts for a significant fraction of disease-causing variants.
Question 3 True / False
In the first transesterification reaction of RNA splicing, the 2'-OH group of the branch point adenosine attacks the phosphodiester bond at the 5' splice site, forming a lariat intermediate with a 2'-5' phosphodiester bond linking the intron's 5' end to the branch point.
TTrue
FFalse
Answer: True
This is the defining chemical step of step 1 of splicing. The branch point adenosine (located 20–50 nucleotides upstream of the 3' splice site) has a free 2'-OH group — unusual because most phosphodiester bonds in RNA link 3' to 5'. This 2'-OH acts as a nucleophile, attacking the phosphodiester bond at the 5' splice site. The result is: (1) the upstream exon is released with a free 3'-OH, and (2) the intron's 5' end is joined to the branch point via a 2'-5' linkage, creating the characteristic lariat structure. Step 2 then completes splicing by ligating the two exons.
Question 4 True / False
RNA splicing requires ATP hydrolysis to provide the energy needed to break the phosphodiester bonds at the 5' and 3' splice sites.
TTrue
FFalse
Answer: False
Splicing proceeds by transesterification — a bond-exchange reaction in which one phosphodiester bond is broken while another is simultaneously formed. Because the number of phosphodiester bonds is conserved across each reaction step, the reaction is energetically neutral and requires no external energy input (like ATP hydrolysis) to drive the chemistry itself. ATP is consumed during spliceosome assembly and remodeling (by RNA helicases like Prp28, Brr2), but not for the catalytic transesterification reactions. This makes splicing thermodynamically favorable without requiring energy investment in each catalytic cycle.
Question 5 Short Answer
Why does a single nucleotide mutation at a splice site represent such a serious molecular threat to protein function, even though the mutation occurs in a non-coding intronic sequence rather than in the protein-coding exon?
Think about your answer, then reveal below.
Model answer: Splice site sequences (especially the conserved GU at the 5' splice site and AG at the 3' splice site) are recognition signals for the spliceosome. Even a single nucleotide change can prevent spliceosome assembly at that site, causing intron retention in the mRNA. When an intron remains in the mRNA, it introduces non-coding sequence into what the ribosome reads as coding sequence. This almost certainly causes a frameshift — the ribosome reads the intron's nucleotides as codons, scrambling the downstream protein sequence and typically encountering a premature stop codon. Even if the intron happens to be in-frame, it encodes a foreign amino acid sequence that disrupts the protein's structure. The result is a nonfunctional protein from a mutation in sequence the original gene would have discarded.
This is why splicing mutations (splice site mutations, branch point mutations, or mutations in exonic splicing enhancers) account for an estimated 15–50% of disease-causing mutations — comparable to missense mutations in the coding sequence itself. Splicing fidelity is as critical to gene expression as transcriptional accuracy.