Difference Between Introns and Exons in Gene Structure

Any nucleotides sequence within a gene that is removed by RNA  splicing of the final product during maturation is known as an intron. The final mature RNA produced by that gene is encoded by exons.

So let's discuss intron and exon in detail.

A gene containing coding regions unknown exons are interrupted by non-coding regions known as introns. The DNA region between exons and introns is encoded by exons proteins. In the coding region, only the eukaryotes contain introns.

In eukaryotes, both exons and introns are transcribed into the mRNA primary transcript.

Exons are termed nucleic acid sequences represented in the RNA molecule. Introns can be defined as the nucleotide sequences that are found in the genes that are removed by the process of RNA splicing. We can also say that exons are coding areas, whereas introns are non-coding areas. Both Roberts and Phillip Sharp discovered introns and exons respectively. The introns change their sequences frequently with time, whereas the exon sequences are highly conserved.

What are Introns?

• Introns play the role of intervening sequences between two exons found in eukaryotes. They do not directly code for proteins. They are removed before the mRNA forms proteins. Therefore, these introns undergo the process of splicing.

• Introns are the non-coding parts of the nucleotides and aren't highly conserved. Therefore, it's essential to get rid of introns to stop the formation of incorrect proteins.

• The word intron means ‘In the Nucleus’. Thus, the Universal feature in introns is to remove the splicing within the nucleus by RNA. The sequence of nucleotides found in RNA and DNA that interrupts the sequence of the gene is known as an intron. Introns are found in the mRNA,  primary transcript, and intergenic regions of the gene. Hence, mature RNA lacks introns on the other hand  RNA splicing mechanism lacks in prokaryotes.

What are Exons?

• Exons are the coding sequences that code for the amino acid sequence of the protein. The exons are transcribed into mature mRNA after post-transcriptional modification. These are highly conserved sequences, i.e., they are not changing frequently with time.

• Splicing is the process of removing introns. The production of different combinations of amino acids is promoted by alternative splicing by combining different combinations of exons together. Therefore the Amino acid sequence of the polypeptide exons is responsible.

Function of Introns

While introns were initially – and to an extent still are – considered ‘junk DNA’, it's been shown that introns likely play a crucial role in regulation and organic phenomenon. 

As introns cause a rise in gene length, this increases the likelihood of crossover and recombination between sister chromosomes. This increases genetic variation and may end in new gene variants through duplications, deletions, and exon shuffling. Introns also allow for alternative splicing. This allows one gene to encode multiple proteins because the exons are often assembled in multiple ways.

The RNA polymerase makes a copy of the whole gene during transcription, both introns, and exons, into the initial mRNA transcript referred to as pre-mRNA or heterogeneous nuclear RNA (hrRNA). As introns aren't transcribed, they need to then be removed before translation can occur. The excision of introns and therefore the connection of exons into a mature mRNA molecule occurs within the nucleus and is understood as splicing.

Introns contain a variety of sequences that are involved in splicing including spliceosome recognition sites. These sites help the spliceosome to identify the boundary between the introns and exons. Nucleolar ribonucleoproteins (snRNPs) are recognised by small sites themselves. There are a variety of snRNPs involved in mRNA splicing which combine to create a spliceosome. The splicing takes place in three steps.

Structure and Function of Exons

In protein-coding genes, the exons include both the protein-coding sequence and therefore the 5′- and 3′-untranslated regions (UTR). Often the primary exon includes both the 5′-UTR and therefore the first a part of the coding sequence, but exons containing only regions of 5′-UTR or (more rarely) 3′-UTR occur in some genes, i.e. the UTRs may contain introns. Some non-coding RNA transcripts even have exons and introns.

Mature mRNAs originating from an equivalent gene needn't include equivalent exons, since different introns within the pre-mRNA are often removed by the method of another splicing. Exonization is the creation of a replacement exon, as a result of mutations in introns.

Some of the important differences between introns and exons are the following:

Difference between Introns and Exons

Conclusion

This is all about the meaning, explanation, and differences between introns and exons. Concentrate on their structural features and functions to grab hold of this biology topic and establish your conceptual foundation.