Molecular biology · BEGINNER LESSON
Splice site
A splice site is a short RNA sequence at an intron–exon boundary that the spliceosome recognizes when it cuts out introns and joins exons in pre-mRNA. In eukaryotes, the main splice sites are the 5′ splice site at the intron start and the 3′ splice site at the intron end.
What it is and what is recognized
Pre-mRNA is the first RNA copy made from a gene. Before it can become mature RNA, the cell removes introns, which are RNA segments that do not stay in the final message, and ligates exons, which are the kept segments. A splice site is one of the boundary signals that marks where this editing happens. The main signals described in the sources are the 5′ splice site, the branch point sequence, the polypyrimidine tract, and the 3′ splice site. The 5′ splice site is usually at the exon–intron junction near a GU sequence, while the 3′ splice site is usually near an AG sequence at the intron–exon junction. Recognition of these sites is part of precise gene expression and can affect which RNA and protein isoforms are produced from one gene. [0,1]
[1] Genome-wide functional annotation and interpretation of splicing variants: toward RNA-targeted therapies [2] pubmed.ncbi.nlm.nih.gov · 42799960Splice-site variants in neurology: from molecular mechanisms ...
How splice sites are used
Splice sites are read by the spliceosome, a large ribonucleoprotein complex made of small nuclear ribonucleoproteins (snRNPs; pronounceable as "snurps") and other proteins. U1 snRNP helps recognize the 5′ splice site, while U2-related factors help recognize the branch point sequence and polypyrimidine tract. The sources also describe an exon definition model, where the two splice sites flanking an exon are recognized together as a functional unit. This cooperative recognition helps define exon–intron boundaries and supports accurate exon use. [0]
Why variation matters
A canonical splice site is the usual, annotated site used by the cell. If a variant weakens that site or creates a new similar sequence, the spliceosome may use a nearby cryptic site instead. That can cause exon skipping, alternative splice-site usage, or other abnormal RNA products. The sources also note that many splicing decisions are context-dependent, meaning the outcome depends on surrounding sequence and regulatory proteins, and that some cryptic sites are used only at low frequency or are quickly degraded. [0,2]
[1] Genome-wide functional annotation and interpretation of splicing variants: toward RNA-targeted therapies [3] Pervasive noise in human pre-mRNA splice site selection - PMC
Limitations and evidence
Splice site use is not explained by a single rule. Prediction tools can disagree, RNA-based tests depend on the tissue and developmental stage being examined, and some splicing errors are rare or rapidly destroyed, making them hard to detect. The sources also note that splicing noise and stochastic, or partly random, spliceosome binding can produce low-fidelity site use. These points mean that a sequence change near a splice site does not always have a simple yes-or-no effect. [1,2]
[2] pubmed.ncbi.nlm.nih.gov · 42799960Splice-site variants in neurology: from molecular mechanisms ... [3] Pervasive noise in human pre-mRNA splice site selection - PMC
SEE THE IDEA
How a splice site is used
The initial RNA copy that still contains introns and exons.
Boundary signals at the 5′ and 3′ ends of introns, plus nearby helper sequences.
The RNA-protein machine that recognizes the signals and performs splicing.
The edited RNA after introns are removed and exons are joined.
Pre-mRNA contains splice sites; the spliceosome reads those sites to remove introns and join exons, producing mature RNA.
Splice sites are the boundary cues that let the spliceosome choose where RNA editing should occur. [0,2]
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.