Question

Describe the process of eukaryotic RNA splicing. How are splice sites recognized?

Answer 1

RNA splicing: For most eukaryotic genes and some (prokaryotic ones), the initial RNA that is transcripted from a genes DNA template must be proceed before it becomes a mature mRNA that can direct the synthesis of protein.One of the step in this processing called RNA splicing, involves the removal or splicing "out " of certain sequence reffered to as intervening sequence or introns.RNA splicing was initially discovered in the 1970, over turning years of thought in the field of genes expression.

PROCESS OF EUKARYOTIC RNA SPLICING

The biochemical mechanism by which splicing occurs have been studied in a number of systems and is now fairly well characterised. Introns are removed from primary transcript by cleavage at conserved sequense called splice sites. These sites are found at the 5' and 3' end of introns. most commonly, the RNA sequence that is removed begains with the dinucleotide GU at its 5' end , and ends with AG at its 3' end. These consensus sequence are known to be critical ,because changing one of the conserve nucleotides results in inhibitation of splicing. another important sequence occurs at what is called branch point, located anywhere from 18 to 40 nucleotides upstream from the 3' ends of an intron. THe branch point always contains an adenine, but it is other wise loosely conserve . A typical is YNYYRAY , where Y indicates a pyrimidine , N denotes any neucletide, R denotes any purine, and A denotes any adenine. Rarely, alternate splice site sequence are found that begain with the dinucleotide AU and end with AC; these are spliced through a similar mechanism . Splicing occurs in several steps and is catalysed by small nuclear ribonucleoprotein. FRist, the pre-mRNA is cleaved at the 5; end of the intron following the attachement of a snRNPs called U1 to its complimentary sequence within the intron. The cut end then attaches to the conserved branch point region downstream through pairing of guanine and adenibne nucleotide from the 5'end and the branch ponit , respectively , to form a looped structure known as a lariat . The bounding of guanine and adenine bases takes place via a chemical reaction known as transesterification , in which a hyydroxyl group on a carbon atom of the adenine "attacks " the bond of the guanine nucletide at the splice site . The guanine residue is thus cleaved from the RNA strand and forms a new bond with the adenine.

next, the snRNPs U2 and U4\U6 appeared to contribute to positioning of the end of the 5' end and the branch pont end proximity. With the participation of U5 ,the 3' end of the intron is brought into proximity , cut and joint to the 5, end the step occurs by transterification; in this case, an OH group added 3' end of the exon attacks the phosphodisters bond at 3' splice site.The adjoining exons are covalentely bound , and the resulting lariates is released with U2, U5, and U6 bound to it .

In addition to consensus sequence at the splice sites, eukaryotic gene with long intron also contain exonic splicing enhancers . These sequence , which help position the splicing apparatus, are found in the exons of genes and bind proteins that help recruit splicing mechinery to the correct site. MOstly splicing occurs between exons on a single RNA transcripts , but occasionally trans-splicing occus, in which exons on diffrent , pre-mRNAs ligated together.

The splicing process occurs in cellular mechaines called splisosomes, in which the snRNPs are found alongwith the additional protein. The primary variety of splisosome is one of the most pleantyfull strucyure in the cell , and recently, a secondly type of splisosome has been identifies that processes a minor category of introns . These introns are reffered to as U12-types introns because they depend upon the action of a snRNP called U12 ( the common intron described above afre called U2' type introns) . The role of U12' type introns is not yet defined , but there persistance through out evolution and conservation between homologus genes of widely divergent species.suggests an important functional basis .