Intron & Exon : Definition, Characteristics, Distribution, Classification Function and significance

  A gene is a section of DNA with information to construct a protein. Most of the potion of a gene in eukaryotes consist of non-coding DNA. That interrupts the relatively short segment of coding DNA. A gene mainly contains two parts one is Introns and another one is Exons.

Exons are the coding sequence
  of the gene.
Introns are the non-coding
  sequence of the gene.

Introns & Exons

The mRNA molecule synthesized from such a gene by transcription is called a primary RNA or pre mRNA transcript.

* Introns :

An intron is any non-coding nucleotide sequence within a gene, which is represented in the primary transcript of the gene but is removed by RNA Splicing in final mRNA product.

Introns are first discovered in 1977 by Phillip Sharb and Richard Roberts independently.

Characteristics :

- The term intron refers to both the DNA sequence within gene and RNA corresponding sequence in primary transcripts.
- Introns are not universal, because they are not present in Prokaryotes but present in most genes of higher eukaryotes.
- Introns range in size from about 50 nucleotides to >100,000 nucleotides.
- Large genes consist of a long string of alternating exons and introns with most of genes consisting of introns.

Distribution :

Introns are rare in Prokaryotic genome and not found in most genes of lower simple eukaryotes such as yeast.
Many genes like interferons, histone genes, ribo nuclease genes, heat shock protein genes, many of G-Protein couppled receptor lack introns. Which indicates the intros are not essential for gene function in eukaryotes.
  Introns are also found in nuclear, mitochondrial & chloroplast genome.

Frequency :
The frequency of intron within different genomes is observed to very widely across the spectrum of biological organisms.
a). Extremely common in nuclear genome of vertebrates specially human and mice. In many vertebrates protein coding genes contain multiple introns.
b). Introns are rare in nuclear genes of bakers yeast.
c). Mitochondrial genome of eukaryotic microorganisms contain many introns, while mitochondrial genome of vertebrates does not contain introns.

• Longest eukaryotic intron found in Drosophila, located on dhc-7 gene, the length of intron is >3.6Mb.
• The shortest intron found in human, located on MST1L gene which is 30 bp long.

Classification of introns :
  Introns are classified into four types depending upon their splicing machanism and location.

Type 1 ( AU-AG or AU-AC Introns)
  This type of introns are found in nuclear eukaryotic pre mRNA.
  This type of intron is non self splicing. It requires spliceosome for splicing.

Type 2 (group-I introns)
  This type of introns are found in nuclear, eukaryotic pre rRNA genes and organelles RNAs.
  This types of introns are self splicing.

Type 3 ( group-II introns)
  This type of introns are found in chloroplast, mitochondrial genes and some Prokaryotic RNAs.
  This type of introns are self splicing.

Type 4 ( tRNA introns)
  This type of introns are found in eukaryotic nuclear pre rRNA.
  These introns are non self splicing and they requires enzymes for splicing.

Introns usually do not code for proteins. However certain introns of group I and II class contain ORF (open reding fragment) whose expression allows the protein to be mobile.

Significance of introns :

1) Alternate splicing

  Alternate splicing isThe processing of an RNA transcript into different mRNA molecules, and a singal gene might encode many proteins. Thus aquisition of introns have been positively selected as a source of functional diversity.

2) Regulating gene expression
- Introns contain functional elements like regulatory elements and alternative promoters.
- First introns provides binding site for transcription factor or may act as transcriptional enhancers/repressors.
- Introns act as internal pramoter to produce alternate RNA.
- Introns are required for RNA editing (5' cap & 3' tail).
- sequence introns serves as guide for the chemical alteration of exonic nucleotides by RNA editing.

3) gene splicing by mi RNA &
si RNA
- Introns releases trans acting factors such as micro RNA (mi RNA) and small nucleolar RNA (sno RNA).
- mi RNA targets include transcription factors and genes involved in stress response, hormone signalling and cell metabolism.

4) Exon shuffling
  Introns play important role in evolution by facilitating recombination exons of different genes.

* Exons

Exons is any part of a gene that
encode a part of the final mature RNA. Produced by that genes (RNA) after introns have been removed by RNA splicing.

Introduction :

- The term exon is derived from expressed region and coined by Walter Gilbert in 1978.
- The term exon refers to both the DNA sequence with in a gene and corresponding sequence in RNA transcript.
- In RNA splicing introns are removed and exons are covalently joined to generate mature mRNA.
- The entire set of exons is called exome, just like gene and genome.
- In human genome only 1.1% genome is exon, 24% introns and 75% of genome is intragenic DNA.

Structure and Size of Exons :
- In protein coding genes, the exons include both protein coding sequence and 5' and 3' untranslated regions (UTR).
- Mostly first exon includes both the 5' UTR and dirt part coding sequence. rarely UTRs may contain introns.
- Some non-coding RNA transcript also have exons and introns.

Functions of Exons :
- Exons are piece of coding DNA that encode proteins.
- Different exons encode different domains of a protein. These domain may be encode by a single exon or multiple exons spliced together.
- Presence of exons and introns allows greater molecular evolution by exon shuffling.
• Exon shuffling - exons on sister chromatids are exchanged during recombination.

Alternative splicing :
  This process allows the exons to be arranged in different combinations, when introns are removed.
  Exons also allows for multiple proteins to be translated from same gene.
  Alternate splicing and deftect in splicing can result in disease,   example- Alcoholism and cancer,
Human slo gene - The gene consist of 35 exons, which combine to form over 500 mRNA. The different mRNAs control which sound frequencies can be heard.