What Are Introns? — Definition & Role in Transcription

What Is an Intron?

By now, you might know that our DNA contains genes, and that genes are used to string together amino acids to make proteins which help us survive. But it's not really that simple. When we take a closer look at a gene, we can find two different types of sequences: exons and introns.

I'm going to talk about introns here, but feel free to check out our lessons on exons too.

Let's get started.

Introns are also called the non-coding regions of a gene because these sequences don't actually code for a protein. They're found between sequences called exons, which are parts of the DNA that do code for proteins.

For a protein to be made, a gene has to be transcribed into a messenger RNA (mRNA) molecule, and then the mRNA can be translated into a protein.

Before translation though, the mRNA needs to be edited a little. When transcription is done, we have an immature mRNA strand, also called pre-mRNA, which has both introns and exons. Each intron is spliced out of the pre-mRNA, leaving behind only the exons. This creates a mature mRNA which can go on to be translated into a protein.

So why do these non-coding regions even exist if they just get cut out anyway?

Well, we aren't entirely sure yet, but research suggests that intron sequences are responsible for alternative splicing. This basically means that one gene can be transcribed into more than one unique mRNA molecule. And that means that a single gene can potentially code for more than one protein.

(Video) What Are Introns and Exons?

by A Tale of Two Genes

A strand of DNA contains many genes. Genes are the parts of DNA that contain the genetic information used by our cells to make proteins. Within a gene, there are segments called introns and segments called exons.

An intron is a part of the gene sequence that is not used to carry genetic information. It does not code for a protein.

Parts of a Gene: Intron

Within DNA we find segments called genes, which code for proteins. Within protein-coding genes are segments called introns and exons. A scientist named Walter Gilbert first hypothesized that these different regions might exist in DNA.

Intron sequences are segments of DNA that are not used in the production of a protein. A gene on a section of DNA contains more base pairs than is necessary to produce the desired protein. These intron sections do not code for the protein, so they are not needed.

The extra base pairs in the gene have to be removed so that the protein can function properly. The removal of these base pairs is called RNA splicing. It is performed by an enzyme called a spliceosome that severs the RNA strand, removes the intron sequences, and puts the RNA molecule back together. Before the spliceosome modifies the RNA, it is called pre-mRNA. After splicing introns and exons, it is just called mRNA.

Non-coding regions play an important role in alternative splicing, a process which removes different combinations of gene segments to create different mRNA strands using just one DNA sequence. In other words, introns allow one gene to code for more than one protein. You can see locations of possible splice sites in the image below.

Note that there are a few different types of introns, but all you need to know for now is that they don't directly code for a protein.