Intron

Intron . It was discovered by Phillip Allen Sharp and Richard J. Roberts , winning with them the Nobel Prize in Physiology or Medicine in 1993 .

Summary

[ hide ]

• 1 Origin
• 2 Physiology
• 3 Classification
  • 1 Group I introns
  • 2 Introns of group II and III
  • 3 Nuclear, spliceosomal introns or group IV introns
• 4 Where are they located?
• 5 Gallery
• 6 References
• 7 Source

Origin

The term intron was introduced by the American biochemist Walter Gilbert in 1978 . Introns can represent an alternative splicing site , being able to give different types of proteins . Splicing control is regulated by a wide variety of molecular signals . Introns can also contain “ancient information”, that is, gene fragmentsthat were probably expressed but not currently expressed. It has been speculated as to the origin of the introns and that they may have come from transposons, due to their ability to insert themselves within coding sequences. This relationship is based on the fact that the system for removing introns from the RNA sequence and some transposons is similar, the introns have repeated sequences on the sides that allow them to be circularized and eliminated by endonucleases related to those of transposons.

Physiology

Introns are the sequences of DNA or RNA sequences that separate exons. Both are transcribed from DNA to RNA , and from that primary transcript (or pre-mRNA) the introns will be removed during the maturation of the RNA . These sequences do not contain information on the amino acids that will give rise to the protein, but they are functionally important, since they are partly responsible, for example, for exon shuffling. Introns are characteristic of eukaryotes , although some have been found in prokaryotes, do not represent such an important part of their genome. The frequency of introns in the genome is highly variable depending on the organism that we study. For example, higher vertebrates such as humans or cows have a very high percentage of introns and sequences that do not encode proteins. In bacteria and archeas they are rarer but there are also. There appears to be some kind of relationship between the number of introns and overall non-coding sequences and the evolutionary complexity of a species. In contrast, the mitochondrial genomes have no introns. In fact, the mitochondria have some genes with the overlapping base sequence, something unusual in eukaryotes . Introns are removed from the RNA sequence messenger by the process called alternative splicing, which encompasses both the elimination of the intron by specialized proteins and the subsequent union of the exons that are loose.

Classification

Four classes of introns are currently recognized:

Intron group I

1. They self-process in vitro although some need the intervention of the protein that is encoded by the intron ORF: the madurases. Some of them also have retrotranscriptase or endonuclease activity that serve for transposition and homing of the intron.
2. They require an external cofactor that is a guanine nucleoside or nucleotide (guanosine, GMP, GDP, or GTP) in which 3’OH acts as a nucleophile.
3. No energy source is needed because energy from a particular type of transesterification is used: Transphosphorylation.
4. The union of the external cofactor triggers the process, without the formation of ties (lariat).
5. There are no clear consensus sequences to produce the splice or to recognize the edges of the intron.
6. Small sequences have been identified that are often important, directly or indirectly, to processing. Its conservation suggests that a common ancestor migrated to the mitochondria from the nucleus, or vice versa. These sequences form two typical structures:

• one is formed by the mating of 6- 7 nt between the P and Q sequences of 10 nt in length (P 4 in figure). In the case of mitochondrial introns they are called « 9 L» sequences.
• The other structure is formed by the 5nt pairing between the 12 nt R and S elements (P 7 in the figure). In the case of mitochondrial introns, they are called ” 2 “.

Group II and III introns

1. They self-process in vivo although some also need ripening, and using the energy of the transfosphorylation
2. They do not need an external cofactor since they use the 2’OH of a residue of A of the intron itself
3. By using a nucleotide of the intron as a cofactor, they give rise to a loop-shaped intermediate (lariat).
4. Although the nucleotide sequence is not conserved, they have a secondary structure made up of 6 hairpins named d1 to d6. On d6 is the A that will start the nucleophilic attack. The catalytic center of the intron is defined by d1 and d5.
5. They often behave like moving elements, which is why they are considered a particular type of retrotransposon. To jump to a new place (homing) they use both endonucleases and retrotranscriptases encoded by the intron itself or other introns from the same group.

Their structure and splice mechanism closely resemble nuclear introns, and are therefore thought to be evolutionarily related. There is a subset of these introns that lack the d2 to d4 domains and are often called group III introns, not to be confused with nuclear introns (see below).

Nuclear, spliceosomal or group IV introns

It is the most numerous, with introns present in most eukaryotic nuclear mRNAs. The splicing is similar to that of group II, but the intervention requires a very large set of proteins and RNA to form the ayustosoma —with ATP expenditure. None of this type has yet been found in prokaryotes. It groups the introns present in the tRNAs. Unlike the previous three, endonucleases are required to remove the intron and ligases to link the exons. They have been found in eukaryotes and archaea.

Where are they located?

They are found common in multicellular eukaryotes, such as humans. They are less common in single-celled eukaryotes, such as yeast, and even rarer in bacteria. It has been suggested that the number of introns that an organism’s genes contain is positively related to its complexity.