No, mature mRNA does not contain introns. Before a messenger RNA molecule leaves the nucleus to be translated into protein, sections called introns are cut out, leaving only the protein-coding segments (exons) stitched together. This removal process, called splicing, is one of three major modifications that transform the initial RNA copy into its final, functional form.
What Happens Between Pre-mRNA and Mature mRNA
When a gene is first copied from DNA, the result is a rough draft called pre-mRNA. This transcript is a near-complete mirror of the gene, including both exons (the parts that code for protein) and introns (the non-coding stretches between them). Before this molecule can be used, it goes through three processing steps: a protective cap is added to the front end, introns are spliced out, and a long tail of repeating units (the poly-A tail) is attached to the back end.
The finished product, mature mRNA, looks quite different from the original. It’s shorter, intron-free, and bookended by untranslated regions at the front and back that help regulate how the molecule behaves in the cell. Scientists first realized this in 1977, when Richard Roberts and Phil Sharp independently showed that mRNA molecules in eukaryotic cells are significantly shorter than the genes they come from. Before that discovery, researchers assumed mRNA was a direct, unedited copy of DNA, which is true in bacteria but not in plants, animals, or fungi.
How the Cell Removes Introns
Intron removal is handled by a large molecular machine called the spliceosome, built from small RNA molecules and dozens of proteins. The process works in two back-to-back chemical reactions. First, a specific site within the intron loops back and attacks the junction at the start of the intron, cutting it free from the upstream exon and forming a lasso-shaped structure called a lariat. Second, the newly freed upstream exon attacks the junction at the end of the intron, snipping the intron out entirely and joining the two exons together in one smooth sequence.
The spliceosome doesn’t just show up and cut. It assembles in stages, with different components recognizing the precise boundaries where each intron begins and ends. Early in the process, small RNA molecules physically pair with the intron’s start site, while other factors lock onto the branch point deeper within the intron. Only after these recognition steps are verified does the cutting begin. This precision matters: a single misplaced cut would shift the reading frame of the entire message downstream, producing a garbled or nonfunctional protein.
Quality Control Before Export
The cell doesn’t just trust that splicing went well. Before a mature mRNA is allowed to leave the nucleus, it passes through a quality control system. Special “guard” proteins interact with the spliceosome during and after splicing. If an mRNA has been correctly processed, these guard proteins recruit the export machinery that ferries the molecule through nuclear pores into the cytoplasm.
If splicing failed and introns remain, the guard proteins instead flag the transcript for destruction. A complex marks the faulty RNA with a short tail that signals the nuclear degradation machinery to chew it up. As a final checkpoint, gatekeeper proteins stationed at the nuclear pore itself can physically retain transcripts that haven’t gathered enough export signals. The result is a multi-layered system that strongly favors the release of only properly spliced, intron-free mRNA. Spliced transcripts are also exported faster from the nucleus than unspliced ones, adding another layer of preference for correctly processed molecules.
The Exception: Intron Retention
While the textbook answer is that mature mRNA lacks introns, biology is rarely that clean. In a process called intron retention, certain introns are deliberately left in the final transcript. This is a form of alternative splicing, where the cell produces different versions of an mRNA from the same gene by choosing which segments to keep or remove.
A recent example involves the gene that produces tau, a protein important in brain function and implicated in Alzheimer’s disease. Researchers found mature RNA molecules in human brain tissue that still contained specific introns (intron 3 and intron 12). Rather than being errors, these retained introns produced truncated versions of the tau protein with distinct properties. The resulting “W-tau” isoforms were actually more soluble and less prone to clumping than standard tau, while still performing normal tau functions. These intron-retaining transcripts were confirmed in both human cell lines and brain samples using sensitive detection methods.
So while retained introns can be a sign of malfunction, they can also be a deliberate regulatory strategy. The cell uses intron retention to expand the range of proteins a single gene can produce.
When Splicing Goes Wrong
At least 15 percent of all human genetic diseases involve splicing errors. Mutations at intron-exon boundaries can prevent the spliceosome from recognizing where to cut, leading to introns being incorrectly included or exons being accidentally skipped. Research has also shown that problems with the protective cap added to the front of pre-mRNA can impair splicing across the entire transcript, not just the first intron. In experiments where capping was disrupted, all tested introns showed retention, regardless of their position in the molecule.
The cell has a backup system for catching some of these mistakes. A surveillance mechanism called nonsense-mediated decay detects transcripts where splicing errors have introduced premature stop signals. Introns actually play a role in this detection: proteins deposited at exon-exon junctions during splicing serve as markers. If one of these markers sits too far downstream of a stop signal, the cell recognizes the transcript as defective and destroys it.
Why Introns Exist at All
If introns are just going to be removed, why does the cell bother with them? The energy cost of copying and precisely excising introns is significant, yet they’ve persisted across hundreds of millions of years of evolution, which implies they provide real advantages.
The most important benefit is protein diversity. Alternative splicing, where different combinations of exons are included or excluded, allows a single gene to produce multiple protein variants. Introns are the spacers that make this flexibility possible. Beyond that, introns boost gene expression. Experiments have shown that genes containing introns can produce up to 400 times more protein than identical genes with their introns removed. Introns also contain sequence elements needed for correct transcription initiation and termination, and intron-bearing transcripts are preferentially shuttled to the cytoplasm where proteins are made.
Prokaryotes (bacteria and similar organisms) took a different evolutionary path. Their mRNA is translated directly without splicing, and their genomes are largely intron-free. The prevailing explanation is that prokaryotes’ large population sizes created strong selective pressure to streamline their genomes, while eukaryotes retained and co-opted introns for regulatory purposes.