Introns or the intervening sequence are considered as the non-coding part of the genes, while the exons or the expressed sequence are known to be as the coding part for proteins of the genes. Introns are the common attribute found in the genes of the multicellular eukaryotes like humans, while exons are found in both prokaryotes and eukaryotes.
The traditional method for the flow of biological information in the living being is that the DNA makes RNA and then RNA makes proteins. These methods are also known by their name as Replication, Transcription, and Translation.
Starting from the replication, which is known as the process of copying of deoxyribose nucleic acid (DNA) to produce the identical copy of the DNA molecules itself. Then comes the transcription which is the synthesis of the ribonucleic acid (RNA) from the DNA. Finally, the stored genetic information is expressed in the form of proteins, this is known as translation.
Targetting the transcription where the entire DNA is copied into pre-mRNA (primary transcripts), and these sequences are made up of introns (the non-coding regions) and exons (the coding region), notably, in the eukaryotic genes.
Further, this pre-mRNA undergoes many alterations like end modifications, splicing, etc., which are collectively called as post-transcriptional modifications. Here the introns are removed, and exons are joined to form a contiguous coding sequence. This process is performed to convert pre-mRNA into its active form called as mature mRNA, which is ready for the translation.
At this moment, we will discuss in differences between the introns and exons followed by a brief explanation.
Content: Introns Vs Exons
|Basis for Comparison||Introns||Exons|
|Meaning||The transcribed part of the nucleotide sequence in mRNA, which is known to carry the non-coding part for the proteins.||The transcribed part of the nucleotide sequence in mRNA, responsible for the protein synthesis.|
|Found in||In eukaryotes only.||In both prokaryotes and in eukaryotes.|
|Part of||Non-coding DNA.||Coding DNA.|
|Other features||1. These bases are situated between two exons. |
2. Introns remain in the nucleus, even after the mRNA splicing.
3. These are the less conserved sequence.
4. They are present in DNA as well as in mRNA primary transcript.
|1. These are the bases which are mainly known for coding the amino acid sequence for the protein.
2. Exons move to the cytoplasm from the nucleus, when mature mRNA is produced.
3. These are the highly conserved sequence.
4. They mark their presence in DNA as well as in mature mRNA.
Definition of Introns
An intron is a nucleotide sequence present in DNA and RNA; these are the intervening or interrupting sequence found between the two exons. They range from the 10’s to 1000’s of base pairs. These are found in the eukaryotes like humans.
Introns do not code for protein directly, but they are the part of transcribed pre-mRNA (primary transcripts). Introns are needed to be removed before the mRNA converts into the proteins. So for this, the pre-mRNA undergoes the process called splicing.
Splicing or RNA splicing is one of the post-transcriptional modifications steps for the removal of introns; it is the important process which is done very precisely. This modification is supported by the small nuclear ribonucleoprotein particles (snRNPs) or snurps. These snRNPs are formed with the association of the small nuclear RNA (snRNA) with proteins. Together they are called as the spliceosome.
Splicing occurs at particular splicing sites, and they begin with the nucleotides present as GU at 5′ ends and AG at the 3′ end. The snurps binds at both the ends of the intron and forms the loop, and then the intron is removed from the sequence, and the exons are joined together. Post-transcriptional modifications occur in the nucleus, after which the mature RNA (mRNA) moves to the cytosol to perform the function of translation.
Why is introns removal essential?
As we discussed before, that introns are non-coding part of the nucleotide sequence as well as not highly conserved. So it is necessary to splice off or remove the introns to avoid the production of the wrong or incorrect protein. As if any introns got left or any exon got deleted, all the faulty proteins will be produced.
This occurs because the amino acids which make the proteins are based on the codons left over after the post-transcriptional modifications. The three nucleotides present in the sequence, make up the amino acid and proceeds with protein production.
Definition of Exons
Exons are the coding part of the nucleotide sequence, which encodes for the amino acid sequence for the protein. These are the only parts, which are transcribed and converted into mature mRNA after post-transcriptional modification. These further moved to the cytoplasm, where they are translated into proteins, this happens with the support of another molecule known as tRNA.
Key Differences Between Introns and Exons
Following points presents the significant differences between the two regions of the nucleotide sequence:
- Introns also are known as the intervening sequence, are known as the non-coding region of the nucleotide sequence and are present between the two exons. On the other hand exons or expressed sequence, are known as the coding region of the nucleotide sequence, and they are only responsible for the synthesis of proteins in the cytosol.
- Introns are found in eukaryotes only, while exons are found in both prokaryotes and in eukaryotes.
- In comparison to introns, exons are the highly conserved sequence and mark their presence in DNA as well as in mature mRNA. Introns are limited to DNA and in the primary transcript or pre mRNA.
- As introns are the non-coding part, so they remain in the nucleus only after the splicing, on the other hand, exons move to the cytosol for protein synthesis after RNA splicing.
- Exons mark their presence in DNA as well as in mature mRNA, but introns are present in DNA and in the primary transcript or pre-mRNA only.
The journey from the genes to the making of protein is complex and is performed with high fidelity to make the right and functional proteins. Though there are many confusing terms like the introns and exons, and their meaning is sometimes get interchanged.
From the above content, we conclude that till now the function of exons is very clear, but still, researches are going on to know much about the introns and their function in the nucleotide sequence.