RNA Splicing

This is true for most eukaryotic genes (as well as some prokaryotic ones). A technique known as RNA splicing is used to remove or “splice out” certain segments known as intervening sequences, or introns, from the RNA molecule during one of the steps in this processing. As a result, the final mRNA is composed of the leftover sequences, which are referred to as exons, which are linked together by the splicing procedure. RNA splicing was first identified in the 1970s, causing a reversal of decades of conventional wisdom in the field of gene expression.

Bacterial Research from the Beginning

It was in very simple bacterial systems that gene regulation was initially investigated in great depth. The majority of bacterial RNA transcripts do not undergo splicing; these transcripts are referred to as collinear, meaning that their DNA directly encodes their RNA sequences. For the most part (with the exception of the 5′ and 3′ noncoding regions), a one-to-one connection of bases exists between the gene and the mRNA that is produced from the gene. But in 1977, multiple groups of researchers who were studying with adenoviruses, which are viruses that infect and reproduce in mammalian cells, came up with some unexpected findings. Using this method, the researchers discovered a series of viral RNA molecules that they dubbed “mosaics,” each of which comprised sequences from noncontiguous places in the viral genome. These mosaics were discovered late in the course of a viral infection. Researchers discovered long primary RNA transcripts that contained all of the sequences from the late RNAs as well as what were dubbed the intermediate sequences during their investigations of early infection (introns).

Following the discovery of the adenoviral intron, introns were discovered in a large number of additional viral and eukaryotic genes, including those encoding haemoglobin and immunoglobulin. RNA splicing was then detected in a number of in vitro systems derived from eukaryotic cells, including the removal of introns from transfer RNA in yeast cell-free extracts, and in a number of other systems. These findings bolstered the concept that splicing of large initial transcripts did, in fact, result in the production of mature messenger RNA. Another hypothesis proposed that the DNA template looped in some fashion or developed a secondary structure that allowed transcription to occur from noncontiguous sections of the DNA template.

The Process of Splicing

1. A pre-mRNA molecule goes through numerous processes that are catalysed by small nuclear ribonucleoproteins (SNRPs) before it becomes an mRNA molecule (snRNPs). Following the binding of the U1 snRNP to the 5′ splice site, the 5′ end of the intron base couples with the downstream branch sequence, resulting in the formation of a lariat structure. When the exon is cut at the 3′ end, it is joined to the branch site by a hydroxyl (OH) group at the 3′ end of the exon, which attacks the phosphodiester bond at the 3′ splice site, the exon is considered complete. This results in the covalent bonding of the exons (L1 and L2) and the release of the lariat containing the intron from the exons (L1).

2. It takes multiple steps for splicing to take place, and each step is catalysed by tiny nuclear ribonucleoproteins (snRNPs, commonly pronounced “snurps”). For starters, the pre-mRNA is cleaved at the 5′ end of the intron after it has been attached to its corresponding sequence within the intron by a snRNP known as U1 (small nuclear RNA polymerase I). This looped structure known as a lariat is formed when the cut end joins the conserved branch point area downstream through pairing of guanine and adenine nucleotides from the 5′ end and the branch point, respectively. The guanine and adenine bases are joined together through a chemical event known as transesterification, in which a hydroxyl (OH) group attached to one of the adenine nucleotides’ carbon atoms attacks the bond formed between the guanine and adenine bases at the splice site. This results in the guanine residue being cleaved away from the RNA strand and forming an additional binding with the adenine.

Conclusion 

Therefore it can be concluded,  Following that, the snRNPs U2 and U4/U6 appear to be involved in the positioning of the 5′ end and the branch point in close proximity to one another. With the assistance of U5, the 3′ end of the intron is brought into close proximity to the 5′ end of the intron, which is then cut and linked to the 5′ end. In this situation, an OH group at the 3′ end of the exon attacks the phosphodiester bond located at the 3′ site of splicing, resulting in the formation of a phosphodiester bond. In this case, the surrounding exons are covalently linked, and the resulting lariat is released, with the exons U2, U5, and U6 still attached to it.