Exons are termed as 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.
Some of the important difference between introns and exons are the following:
Difference Between Introns and Exons
What are Introns?
Introns play a 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.
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.
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 an equivalent exons, since different introns within the pre-mRNA are often removed by the method of other splicing. Exonization is the creation of a replacement exon, as a result of mutations in introns.