Genes contain the instructions cells use to make RNA and, in many cases, proteins. But those instructions are not always arranged as one uninterrupted stretch of useful sequence. In eukaryotic organisms, many genes are divided into exons and introns, two types of sequence with different roles in gene expression.
The simplest distinction is this: exons are the sequences retained in mature RNA after RNA splicing, while introns are sequences removed from the RNA during splicing. For protein-coding genes, exons typically contain the information that contributes to the protein, but the relationship is more nuanced than simply “exons code and introns do not.”
Understanding that distinction requires looking at what happens to a gene after it is transcribed.
What are exons?
An exon is a segment of a gene that remains in the mature RNA after the initial RNA transcript has been processed.
When a protein-coding gene is expressed, the cell first produces a preliminary RNA molecule called pre-mRNA. This molecule contains both exons and introns. The cell then removes the introns and joins the exons together through a process called RNA splicing.
The resulting mature messenger RNA (mRNA) can then be used by ribosomes to make a protein.
In many protein-coding genes, the portions of exons that lie within the protein-coding sequence determine the amino acid sequence of the resulting protein. However, exons can also contain untranslated regions (UTRs) at the beginning and end of an mRNA. These regions remain in the mature mRNA but do not directly encode amino acids. Instead, they can influence processes such as mRNA stability, localization, and translation.
So, an exon is best defined by its fate during RNA processing, not simply by whether it encodes protein.
What are introns?
An intron is a segment of a gene that is transcribed into the initial RNA but removed during RNA splicing.
Introns therefore appear in the pre-mRNA but normally do not remain in the mature mRNA. The cell’s splicing machinery recognizes specific signals around introns and removes the intervening sequence, joining the surrounding exons.
Introns are not simply useless stretches of DNA. They can contain regulatory sequences and can influence how genes are expressed or how RNA is processed. Some introns can also contain sequences that give rise to functional noncoding RNAs.
Their most defining feature, however, is their removal from the RNA during the normal processing of the transcript.
Exons and introns compared
| Feature | Exons | Introns |
|---|---|---|
| Present in the initial RNA transcript? | Yes | Yes |
| Present in mature mRNA? | Usually yes | Usually no |
| Removed during RNA splicing? | No | Yes |
| Can contain protein-coding sequence? | Yes | Generally no in the conventional mature protein-coding transcript |
| Can contain functional regulatory or noncoding sequences? | Yes | Yes |
| Main role in conventional pre-mRNA processing | Retained and joined together | Removed and discarded from that transcript |
The distinction is therefore about RNA processing, not simply whether a DNA sequence is “important.”
How splicing separates exons from introns
The process becomes clearer when following a gene from DNA to protein.
First, transcription copies the gene’s DNA sequence into a pre-mRNA molecule. At this stage, the RNA includes both exons and introns.
Next, the cell performs RNA processing. During splicing, molecular machinery identifies introns and removes them. The remaining exons are connected to form a continuous mature RNA sequence.
For example, imagine a simplified gene arranged like this:
Exon 1 — Intron 1 — Exon 2 — Intron 2 — Exon 3
The initial RNA has the same basic organization:
Exon 1 — Intron 1 — Exon 2 — Intron 2 — Exon 3
After splicing, the mature RNA becomes:
Exon 1 — Exon 2 — Exon 3
If the RNA is a protein-coding mRNA, its coding portions can then be translated into a protein.
Splicing is carried out primarily by a large RNA-protein complex called the spliceosome, although some introns can be removed through other mechanisms.
Why do genes have introns?
Introns are a normal and widespread feature of eukaryotic genes, but scientists do not consider them to have one universal purpose.
One important consequence of having introns is that they allow alternative splicing. In this process, a cell can combine exons in different ways when producing mature RNA. A single gene can therefore give rise to multiple RNA molecules and, in many cases, different protein products.
Introns can also contain regulatory elements or sequences that produce functional RNAs. In addition, intron-containing gene structures can affect the regulation and processing of transcripts.
It is therefore misleading to describe introns simply as biological “junk.” At the same time, not every intronic sequence has a known or demonstrated function. The function of a particular intron depends on its sequence and genomic context.
What is alternative splicing?
Alternative splicing occurs when the same pre-mRNA can be processed in different ways, producing different combinations of exons in the mature RNA.
Suppose a gene contains:
Exon 1 — Exon 2 — Exon 3 — Exon 4
One mature RNA might retain all four exons:
1 — 2 — 3 — 4
Another might skip exon 3:
1 — 2 — 4
If the resulting RNA is translated, the proteins produced from these transcripts can differ.
This gives cells a way to generate multiple RNA or protein products from a single gene. Alternative splicing is especially important in multicellular organisms, where different cell types and tissues may use different RNA isoforms.
Importantly, alternative splicing does not mean that introns become exons. Rather, it means that different exon combinations can be retained in different mature RNA molecules.
Do exons always code for proteins?
No.
The term exon describes a sequence that is retained in a mature RNA product. Some exons contain protein-coding sequence, while others may consist partly or entirely of noncoding regions.
For a protein-coding mRNA, the coding sequence is the portion that is translated into a protein. Exons can also include untranslated regions, which remain part of the mature mRNA but are not translated.
This is why the statements “exons are coding” and “introns are noncoding” are useful as rough introductions but are not precise definitions.
A more accurate distinction is:
Exons are retained in the mature RNA; introns are removed during RNA splicing.
Are introns found in DNA or RNA?
They can be discussed in both contexts.
An intron is fundamentally a segment of a gene’s DNA sequence. When that gene is transcribed, the corresponding sequence appears in the initial RNA transcript. It is subsequently removed during RNA processing.
The same logic applies to exons: an exon is a region of the gene that is represented in the initial transcript and retained in the mature RNA.
Thus, saying that an intron is “removed” does not mean the corresponding DNA is cut out of the chromosome. Splicing changes the RNA, not the underlying genomic DNA.
Where do exons and introns fit into gene expression?
Exons and introns are easiest to understand as part of the pathway from DNA to a functional product:
DNA → pre-mRNA → spliced mature RNA → protein
For a typical protein-coding gene, transcription creates pre-mRNA containing both exons and introns. RNA processing then removes introns and joins exons. Additional processing steps, such as addition of a 5′ cap and a poly(A) tail, help produce a mature mRNA. The mature mRNA can then be exported from the nucleus and translated by ribosomes.
Not every gene follows this exact path. Some genes produce functional noncoding RNAs rather than proteins, and RNA processing can vary substantially among different types of genes.
Why the distinction matters
The exon-intron structure of a gene is important because changes to either region can affect gene expression.
A mutation within a protein-coding exon can alter the amino acid sequence of a protein. But mutations affecting splicing signals can also cause problems, even when they occur outside the protein-coding portion of an exon or within an intron.
If a mutation causes an exon to be incorrectly removed, causes an intron to be retained, or changes which splice sites are used, the resulting RNA may produce an abnormal protein or fail to produce a functional protein at all.
This is one reason genetic analysis cannot always treat introns as irrelevant background sequence. The boundaries and regulatory signals that control RNA processing are biologically important.
Exons vs. introns: the key difference
The central difference between exons and introns is their fate during RNA processing.
Exons are retained in the mature RNA, while introns are removed during splicing. In protein-coding genes, exons often contain the sequences that determine the protein’s amino acid sequence, along with untranslated regions that have other roles. Introns are removed from the mature transcript but can still contribute to gene regulation and RNA biology.
The distinction becomes especially important when considering alternative splicing, genetic mutations, and the way a single gene can produce multiple RNA products. Rather than thinking of exons as “useful” DNA and introns as “useless” DNA, it is more accurate to view both as parts of a gene’s architecture that can influence how genetic information is processed and used.
