“The coding portion of a gene is known as exons while the non-coding DNA sequences are known as introns.”
Genes are known to us so far, after all this blog is about DNA, genes, chromosomes, and all those things. If you don’t know! Go and search in the search box for what you want to read.
Technically speaking, a gene is a polynucleotide chain of DNA that makes a protein via mRNA. Or
It’s a functional part of the genome.
The gene is made up of three different types of sequences: promoter region, introns and exons. Each one has a different function to perform.
The promoters are leading nucleotides that help a gene to form a protein like a signal entry gate that allows various enzymes to take part in the protein formation. Exons encode amino acids and introns regulated gene expression.
Each DNA sequencing has a specific triplet code known as genetic code that makes an amino acid. To know more about exons and introns we should first learn how the entire mechanism works briefly.
The DNA is copied in replication and forms mRNA during transcription by taking the help of polymerase.
Then it leaves the nucleus and forms protein at the ribosome using the help of tRNA and rRNA. In the final step of translation, the chain of amino acids is formed. Now let us understand what the exons and introns are!
Similarities between exon and intron:
Exon and introns are the nucleic acid sequence made up of the polynucleotide chain. However, the structure of introns is more complex with high GC-rich regions and repetitive sequences.
Both the exons and introns take part in the protein formation.
Both follow the DNA packaging mechanism to fit inside a cell. As we said, structurally, there is no difference between them. Both interact with histone proteins, form the nucleosome assembly, chromatins and finally a chromosome.
Both are located on chromosomes. Note that the introns are situated on the heterochromatin region and henceforth wrapped tightly. While the exon parts are located in the euchromatin region.
Related article: Genotype vs Phenotype.
Differences: Exon vs Intron
The differences between the exons and introns are a lot more than similarities.
First of all, The exons are belong to the coding portion of a genome which is up to 3% of the total genome.
While the introns are included in the non-coding junk portion of a genome which can’t form protein but do something else.
The exon makes proteins while the intron regulates gene expression.
Note: the exons and introns are located one after another in a gene. The number of exons and introns also differs in different genes.
The final product of the exon is protein hence it is present in DNA as well as mRNA which means it is present during all DNA metabolic actions like replication, transcription and translation.
On the other side, The introns are only present in DNA and in the primary mRNA transcript but not in the final mRNA.
The exons take part in the transition of genetic code but the introns can’t! What does it mean?
After replication, the entire DNA is copied and makes mRNA via transcription. The mRNA is a type of ribonucleic acid that has uracil instead of thymine in it and contains the genetic information to make protein.
Once the introns are removed, the exons get involved in the transition of DNA to mRNA means the entire coding DNA sequencing got converted into the mRNA transcript.
As we said above the introns and exons are located one after another in a gene to make a transcript, introns must be removed first.
The process known as splicing removes introns and joins exons. Henceforth, exons remain at their place but the introns are removed from the transcript.
Do you know there is an entire genetic field that studies only the mRNA or transcript known as transcriptomics? Read related article: What Is Transcriptomics?
So before leaving the nucleus, the gene sheds off introns from the mRNA and moves to the ribosome so the introns remain in the nucleus only and don’t translate while the exons move out of the nucleus, to the cytoplasm and do the translation.
The function of exons is to translate protein and make various enzymes, chaperons, surface peptides, receptors and other amino acids. While the introns that are often known as intervening sequences regulate gene expression.
Though introns aren’t involved in protein formation, their function is as important as the exons. It regulates the expression of genes means it controls which gene makes how much protein, is it less important than making a protein?
If some genes are over or under-expressed it imbalances cell division, cell cycle and causes cancer.
One of the key differences between both is the conserve sequences in it. The sequences in the exons are more conserved since evolution than the introns. The reason is that after every generation, the same protein must be manufactured.
But to adjust to the environment, a cell may change its expression in order to survive. So the introns are less conserved.
The exons are present in both prokaryotes and eukaryotes while the introns are only present in eukaryotes. The reason for that is also clear!
The eukaryotes are multicellular organisms and follow a higher level of complex genomic arrangement while the prokaryotes are unicellular and follow less complex genomic arrangements.
The genome of prokaryotes is of a few hundred to a few thousand nucleotides to make all the functional proteins and gene regulation. So the non-functional (intervening sequences) are less in it. Their genes only contain exons, unlike eukaryotes.
If we analyze the sequences of exons and introns, those are also different.
The exonic sequences are less complex, less repetitive and less GC rich while the intronic sequences are highly complex with high repetitive DNA and GC content.
What is exon?
The coding portion of a gene involved in making a protein is known as exon or exonic sequences. It’s a chain of nucleotides that are conserved and arranged loosely to allow transcription.
Different genes have a different number of exons. Some exons also share sequence similarities with other exons. Also, the length of exons is different.
As we said it performs transcription in a nucleus and makes the primary transcript which converts into the mature mRNA and leaves the nucleus.
After the transition from DNA to mRNA, the exons reaches the ribosome and decode the information to make a chain of amino acid.
What is intron?
The introns are also DNA sequences known as intervening sequences present between the exons. Much like the exons their length and number also vary from gene to gene.
It’s less conserved and believed to be non-function, although it regulates gene expression. The introns are methylated sequences that are tightly wrapped and don’t allow to work.
Introns are more important as like the exons, new gene or genotype combinations are constructed by introducing exons in new between or between unknown introns.
It was believed that in ancient times the introns are also present in prokaryotes but removed entirely to make the cell simpler.