Difference between Exons and Introns: What You Need to Know
To those of us who are not exactly well versed on the genetics side of things, hearing the words “intron” and “exon” may conjure up images of some type of robots. Rightfully so—the science community can get pretty creative when naming parts of the body, cell, or nucleus. And if you are not a geneticist, hearing those terms for the first time may not always ring the correct bell. (And that is quite alright!)
Okay, so by now you have probably justifiably assumed that these terms do not refer to robots of any kind, but to certain parts of the cells that are related to genetics. If you have not assumed that yet, now is the time to do so.
Before we get into what these robots—I mean introns and exons—are, we have to take a few steps back in biology, as always. We have to zoom out first before we zoom in, otherwise, things would be difficult to make sense of. So, before we begin talking about introns and exons, let’s first talk about the elegant two-step process of protein formation. In other words, let’s talk about transcription and translation. The dictionary definition of the word transcription is the process of creating a written or printed version of something. In molecular biology, it is the first step that happens during gene expression. It refers to the process of a part of one DNA strand being copied into a complementary strand of RNA. This takes place with the help of the enzyme RNA polymerase and it all goes down in the nucleus. The RNA that is the transcript of the DNA strand is called messenger RNA (mRNA) that is then transported out of the nucleus to the ribosomes, which is where the process of protein creation takes place.
So, now that the DNA within the nucleus has been copied, or transcribed, it has to be translated or made into proteins. That happens outside of the nucleus on ribosomes. More specifically, each ribosome consists of two parts that come together to form one whole and as such provide a location for translation to take place and a protein to be produced. Now, the protein is also called a polypeptide as it is made up of several (poly) peptides. Each of the peptides in the polypeptide is based on the RNA code. The peptides are also called amino acids and so the protein is just a long string of amino acids that fold into final structures. The choice of amino acid that is to be added to the elongating protein chain depends on the mRNA sequence.
So, now that we have covered the concepts of how genes turn into proteins or the processes of transcription and translation, let’s back up and talk about where exons and introns fit into the whole sequence of events. (Pun intended.)
Both introns and exons are the genetic material. But the essential difference between the two is that one is made into protein and the other is not. DNA molecules are made up of small molecules that are called nucleic acids. An exon is a sequence of nucleic acids that are represented in the mRNA molecule. An intron, on the other hand, is a sequence of nucleotides within the gene but are removed from the sequence before a final mRNA molecule is made. In other words, that step during which a DNA molecule is transcribed, whereby an mRNA strand is made, and before the mRNA strand leaves the nucleotide, all intronic sequences are removed. So, the mRNA strand leaves the nucleus in a final sequence that is only made up of exons. This means that transcription involves a process called splicing, whereby introns are removed from the DNA and exons are fused together before the mature mRNA molecule is made.In other words, exons code for proteins, while introns are not involved in the process of protein-coding. In yet other words, exons can be termed as coding areas while introns are often referred to as—you guessed it—non-coding regions of a gene.
Also, it is important to mention that the processes of transcription and translation differ in prokaryotes and eukaryotes. Prokaryotes are typically unicellular organisms that do not have a nucleus that is surrounded by a membrane and several other cellular components that have membranes. Prokaryotes are typically archaea and bacteria. Eukaryotes, on the other hand, have a nucleus that is surrounded by a membrane as well as all other cellular compartments that have a membrane as well. By virtue of having these compartments that are sectioned off by membranes, eukaryotic cells are capable of carrying out complex reactions that prokaryotes cannot. So, in terms of introns and exons, the former is typically not found in prokaryotes while they are present in most eukaryotes.
Furthermore, introns do not tend to preserve their sequence over time, meaning that they are less conserved. Exons, on the other hand, are conserved. This means that their sequence is not subject to change over time between species.
In terms of where the words come from, the word ‘intron’ comes from the term ‘intragenic sequence’ that refers to a region within a gene. The word ‘exon’ comes from the term ‘expressed region.’ This was termed by Walter Gilbert, an American biochemist.
Now that you know the difference between introns and exons, you may rightfully wonder about the grand purpose of introns. If they do not code for proteins, what is their point? While researchers have not reached a solid consensus yet, the informal idea is that they are remnants of genetic sequences that were once spacers between the coding sequences. Evolution may have shuffled all the information around and created new sequences that make new proteins that acquired new functions. Another theory that has been proposed is that introns contain information that instructs which genes will be expressed early and which will not. This theory, however, has not been fully explored and its plausibility is uncertain.