Introns and exons

INTRONS AND EXONS

In this picture we can see a segment of DNA, where introns and exons ar reprensented:

 

The exon is the region of a gene that is not separated during the splicing process, remaining in the messenger RNA.

 

 

An intron is a part of the gene that does not code for any amino acid. In plant and animal cells, most of the sequences that code for genes are split by one or more introns. 

The areas of the gene sequence that are expressed in proteins are called exons because they are expressed, while those that are not expressed are called introns because they are found between exons.

 

	DNA initially transcribed into immature messenger RNA (mRNA) consists of coding (exons) and non-coding (introns) sequences.
	. The introns have been excised from the immature mRNA to form mature mRNA, leaving only the exons that will eventually code for the amino acid product

Introns are very large pieces of RNA within a messenger RNA molecule that interfere with the coding of exons. These introns are removed from the RNA molecule to leave a series of exons linked together so that the correct amino acids can be encoded.

What are introns and exons?

Introns and exons are a series of nucleotides within a gene.

Introns are removed by splicing RNA as RNA matures, meaning that they are not expressed in the final messenger RNA (mRNA) product, while exons continue to be covalently glued to one another to create mature mRNA.

Introns can be considered as intervention strings, and exons as expressed strings.

 

There are an average of 8.8 exons and 7.8 introns per human gene.

 

What are exons?

Exons are the series of nucleotides in DNA and RNA that are conserved in the creation of mature RNA. The process by which DNA is used as a pattern to create mRNA is called transcription.

the mRNA then works in conjunction with ribosomes and transfer RNA (tRNA), both present in the cytoplasm, to create proteins in a process known as translation.

Exons generally include the 5 '- and 3' - untranslated regions of mRNA, which contain start and stop codons, in addition to any protein coding sequences.

 

 

What are introns?

Introns are nucleotide strings in DNA and RNA that do not directly code for proteins, and are removed during the precursor messenger RNA (pre-mRNA) stage of mRNA maturation by splicing of RNA.

Introns can range in size from 10's of low pairs to 1000's of low pairs, and can be found in a wide variety of genes that generate RNA in most living organisms, including viruses.

Four different types of introns have been determined:

• Introns in protein coding genes, removed by spliceosomes

• Introns in tRNA genes, which are removed by proteins

• Self-splicing introns, which catalyze their own removal of mRNA, tRNA, and rRNA precursors using guanosine-5'-triphosphate (GTP), or another nucleotide cofactor (group 1)

• Self-splicing introns, which do not require GTP to be removed (group 2)

 

It is vital that the introns are precisely removed, as any excess nucleotides from the intron, or the deletion of nucleotides from the exon, can result in a defective protein being produced. This is because the amino acids that make up proteins are assembled together based on codons, which consist of three nucleotides. Inaccurate removal of the intron can thus result in a frameshift mutagen, which means that the genetic code would be read incorrectly.

This can be explained by using the following phrase as a metaphor for an exon: "THE BIG CAT OF THE TAN IS WICKED." If the intron before this exon was imprecisely removed, so that the "B" was no longer present, then the series became unreadable: "ANC FROM OBT HEB IGT AT ..."

How RNA splices

RNA splicing is the method by which pre-mRNA is made into mature mRNA, the removal of introns and the joining together of exons. Various methods of splicing exist, depending on the organism, the type of RNA or intron structure, and the presence of catalysts.

Introns possess a highly conserved series of GU at their 5 'end, known as the donor site, and a highly conserved series of AG at the 3' end, called the acceptor site. A large RNA-protein complex, the spliceosome, made up of five small nuclear ribonucleoproteins (snRNPs) recognizes the start and end points of the intron thanks to these sites, and catalyzes the removal of the intron accordingly. It connects the spliceosome forms the intron into a loop that can be easily cleaved, and the RNA remaining on each side of the intron. Other types of spliceosomes that recognize unusual or transformed intron series also exist, known as minor spliceosomes.

tRNA splicing is far rarer, although it does occur in all three major domains of life, bacteria, archaea and eukarya. Multiple enzymes fill the role of snRNPs in a gradual process, which can vary wildly between organisms.

Self-splicing introns are generally found in RNA molecules that are thought to catalyze biochemical reactions, ribozymes. Group 1 introns are attacked at the 5 'site by a nucleotide cofactor, which from the splice may be free in the biological environment or a part of the intron itself, leading to the 3' OH of the adjacent exon to become nucleophilic and thus from the linkage to the 5 'end of another exon, following the formation of the intron in a loop. Group 2 introns are spliced in a similar way, albeit with the use of a specific adenosine that targets the 5 'splice site.

 

Alternative splicing

Option splicing refers to the way that different combinations of exons can be joined together, resulting in a single gene coding for multiple proteins. Walter Gilbert first put this idea forward, and he proposed that different permutations of exons could produce different protein isoforms. These in turn would have different chemical substance and biological activities.

It is now thought that between 30 and 60% of human genes undergo alternative splicing. Moreover, over 60% of disease-causing mutations in humans are related to splice deviations, rather than errors in coding sequences.

An example of a human gene that undergoes splicing of choice is fibronectin, a glycoprotein that extends from the cell into the extracellular matrix. Over 20 different isoforms of fibronectin have been discovered. These have all been produced from different combinations of the exons of the fibronectin gene.

The way to remove the intron have been established IV distinct groups.

The maturation processes that are now known to lead primary transcripts to become mature transcripts, shown on an mRNA, are: