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BIOCHEMISTRYCELL BIOLOGY

What is Difference Between Exons and Introns?

Difference Between Exons and Introns is that Introns and exons are considered as two characteristics of a gene that contains coding regions known as exons, which are interrupted by non-coding regions known as introns. The exons encode proteins and the regions of DNA between the exons are introns. Difference Between Exons and Introns

For some time it was thought that the genes present along the DNA of higher organisms (eukaryotes) were arranged continuously, as is the case in bacteria where polypeptide chains are encrypted by an array of codons or triplets in the molecule of DNA

However, it was in 1977 when several researchers – including JW Roberts, PA Sharp, Nobel Prizes in 1993, and P. Chambon – in different laboratories discovered that some genes are discontinuous, that is, they are interrupted within the sequence; for example, the gene for the β chain of hemoglobin is interrupted by interposed sequences that do not encode and are formed by a region of 550 base pairs and a shorter one of 120 base pairs.

This means that the gene for β-globin is fragmented into three coding sequences.

Only eukaryotes contain introns in the coding region. In eukaryotes, both introns and exons are transcribed to form the primary mRNA transcript. During mRNA processing, introns are removed from the primary mRNA transcript, producing a mature mRNA, which leaves the nucleus in the cytoplasm to be translated into an amino acid sequence.

the main difference between introns and exons is that introns remain within the nucleus, keeping the DNA safe in genes while exons leave the nucleus to be translated into a protein.

What are introns?

An intron is a nucleotide sequence that is found in both DNA and RNA, which interrupts the gene sequence. Introns are found both in the intergenic regions of the gene and in the primary mRNA transcript.

The word intron means “in the nucleus.” Therefore, RNA splicing within the nucleus is a universal feature in introns. Therefore, mature RNA lacks introns. On the other hand, prokaryotes lack RNA splicing mechanisms.

Therefore, specific regions such as exons and introns cannot be identified in prokaryotes. The structure of the primary mRNA transcript is also called pre-mRNA

Introns can be classified into four main classes: splenic intronic introns, tRNA introns, group I introns and group II introns. Spliceosomal introns are found in the protein-coding genes, eliminated by the spliceosomes.

tRNA introns are the tRNA segments removed from the anticodon loop of tRNA precursors. The introns of group I and group II are automatically spliced ​​from a wide variety of protein-coding and other types of mRNA, forming a 3D architecture.

The biological function of introns is not clearly known. The introns in the genome serve as a substantial fraction of the DNA, keeping the DNA in the genome safe. Alternative intron splicing promotes the production of a wide variety of proteins from a single primary mRNA transcript

What are exons?

An exon is the coding region of the gene, which encodes an amino acid sequence of a functional protein. Exons are interrupted by introns in eukaryotic genes. But after undergoing processing, mature mRNA is only composed of exons.

The intron removal process is known as splicing. The alternative splice promotes the production of different combinations of amino acid sequences by combining different combinations of exons together. Therefore, exons are responsible for the amino acid sequence of the polypeptide. The whole set of exons in the genome is known as the exome.

In the human genome, the exome consists of only 1.1% of the complete genome, while introns consist of 24% of the genome and 75% of the genome consists of intergenic regions. Both protein-coding regions and the 5 ‘and 3′ untranslated regions (UTR) are contained by exons. The 5’-UTR is contained in the first exon.

Difference between introns and exons

Definition

Introns: Introns are segments of DNA that do not encode any amino acid sequence in the coding region.

Exons: Exons are the segments of DNA that encode a part of an amino acid sequence of a complete protein.

Encode the DNA

Introns: Introns belong to non-coding DNA.

Exons: Exons belong to the DNA encoder.

Transcription

Introns: Introns are considered as the bases located between two exons.

Exons: Exons are the bases that encode an amino acid sequence of a protein.

Presence

Introns: Introns are only found in eukaryotes.

Exons: Exons are found in both prokaryotes and eukaryotes.

Movement in Nucleus

Introns: Introns remain in the nucleus by splicing the primary mRNA transcript during mRNA processing within the nucleus.

Exons: Exons leave the nucleus towards the cytoplasm after the production of mature mRNA.

Sequence conservation

Introns: Sequences in introns are less conserved compared to exons.

Exons: The sequences in the exons are highly conserved.

Presence in the genome

Introns: Introns are found in the primary transcription of DNA and mRNA.

Exons: Exons are found in both DNA and mRNA.

Function

Introns: The function of introns is not clearly known, but it is considered to be a substantial fraction of the DNA.

Exons: The function of exons is to translate into a protein.

conclusion

A gene is a segment of DNA that produces a functional product, either a polypeptide or an RNA. The intergenic regions of a gene are composed of introns. This means that a gene in eukaryotes consists of a coding region structure, which is divided into segments called exons; Introns can be found between two exons. Introns belong to non-coding DNA.

All exons together with the intergenic regions are transcribed by RNA polymerase in the primary mRNA transcript. Introns are removed from the primary transcript during mRNA processing. Therefore, a mature mRNA consists only of exons.

Exon splicing can occur in an alternative way in polycistronic mRNAs in prokaryotes, producing more than one type of mature mRNA from a single primary mRNA transcript. Introns in the genome are considered a substantial fraction of the DNA, while exons encode proteins. Therefore, the main difference between introns and exons is their function in the genome.

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