A splice, in biology, is the process of cutting out non-coding segments from a raw RNA copy of a gene and stitching the remaining useful segments together to form a finished message that cells can read to build proteins. This editing step happens inside nearly every cell in your body and is essential for turning genetic instructions into functional proteins. The concept also appears outside biology (splicing rope, film, or electrical wire all follow the same logic of joining two ends together), but the term carries its deepest significance in genetics, where it helps explain how roughly 25,000 human genes can produce over 90,000 different proteins.
How RNA Splicing Works
When a gene is first copied from DNA into RNA, the resulting transcript is a rough draft. It contains segments called exons, which carry the actual protein-building instructions, interspersed with segments called introns, which do not code for protein. Before the cell can use this RNA to make anything, it needs to cut out the introns and join the exons together in order. The finished product is called messenger RNA (mRNA), and it’s what ribosomes read to assemble a protein.
The cutting and joining is performed by a large molecular machine called the spliceosome, built from five smaller units (U1, U2, U4, U5, and U6) along with dozens of helper proteins. These units assemble on the raw RNA in a specific order, first recognizing the boundaries where introns begin and end, then snipping the intron out in a loop-shaped structure called a lariat, and finally fusing the two neighboring exons together. The whole operation relies on two precise chemical reactions that happen back to back. In the first, one end of the intron is cut and looped back on itself. In the second, the two exons are joined and the looped intron is released and eventually broken down.
Why One Gene Can Make Many Proteins
If every gene were spliced the same way every time, each gene would produce exactly one protein. But cells routinely mix and match which exons they keep and which they skip, a process called alternative splicing. A muscle cell might include exon 5 of a particular gene while a brain cell skips it entirely, producing two different versions of the same protein with different properties. Up to 95% of human multi-exon genes undergo alternative splicing, which is why the old idea of “one gene, one protein” no longer holds.
This flexibility is a major source of biological complexity. Alternative splicing influences almost every aspect of protein function: how proteins bind to other molecules, where they end up inside a cell, and how they catalyze chemical reactions. It plays roles in embryonic development, immune system function, and the specialization of complex tissues like the brain. In a real sense, alternative splicing is one of the reasons a relatively modest number of genes can build and maintain an organism as complex as a human being.
What Happens When Splicing Goes Wrong
Because splicing depends on the spliceosome recognizing exact sequences at intron-exon boundaries, even a single DNA mutation at one of these boundary sites can cause an intron to be left in or an exon to be accidentally removed. The resulting protein may be shortened, misfolded, or completely nonfunctional.
Spinal muscular atrophy (SMA) is one well-known example. In about 95% of cases, patients are missing a critical exon in a gene responsible for motor neuron survival. Without the correct protein, motor neurons degenerate, leading to progressive muscle weakness. Familial dysautonomia offers another illustration: a single mutation causes one exon to be skipped predominantly in nerve cells, disrupting the autonomic nervous system while leaving other tissues less affected. This tissue-specific pattern highlights how the same mutation can have different splicing consequences depending on which helper proteins are available in a given cell type.
Medicines That Fix Splicing Errors
One of the more promising areas in modern medicine involves using short synthetic molecules called splice-switching oligonucleotides to correct or redirect faulty splicing. These molecules are designed to bind to specific spots on a raw RNA transcript, blocking signals that cause the spliceosome to skip an exon or include one it shouldn’t. By masking the wrong signal, the treatment coaxes the cell’s own machinery into producing a functional protein.
The SMA drug nusinersen (sold as Spinraza) works this way. Patients with SMA have a backup gene that could produce the needed protein but normally skips a key exon. Nusinersen blocks the signal responsible for that skipping, so the backup gene starts producing a working version of the protein. A similar strategy is being used to treat Duchenne muscular dystrophy, where skipping a mutated exon can restore a partially functional protein and slow disease progression.
Splicing in the Lab
Outside the cell, scientists also splice DNA deliberately using recombinant DNA technology. This involves cutting DNA at precise locations with molecular scissors called restriction enzymes, then using another enzyme to glue the pieces back together in a new arrangement. This is the foundation of genetic engineering: you can splice a human gene into a bacterial cell to produce insulin, or combine DNA from different species to study gene function. The logic mirrors what happens naturally during RNA splicing (cut out what you don’t want, join what you do), but the tools and context are entirely different.
Why Introns Exist at All
If introns just get removed, it’s fair to wonder why they’re there in the first place. Scientists still debate this, but several compelling ideas have emerged. Introns may act as a mutational buffer, absorbing random DNA damage that might otherwise hit protein-coding regions. They also appear to improve the efficiency of natural selection by reducing interference between nearby genes. And perhaps most intriguingly, the non-coding space within introns may serve as a reservoir for the evolution of entirely new genes, with short sequences inside introns occasionally gaining function over evolutionary time. Most spliceosomal introns appear to have been gained after the split between prokaryotes (bacteria) and eukaryotes (plants, animals, fungi), which helps explain why bacteria generally lack this system while complex organisms rely on it heavily.
Richard J. Roberts and Phillip A. Sharp received the 1993 Nobel Prize in Physiology or Medicine for discovering that genes are “split” into exons and introns, a finding that upended the assumption that genes were continuous stretches of code and opened the door to understanding splicing as a fundamental feature of complex life.