Splicing of pre-mRNA
Removing non-coding introns sequences: Splicing of pre-mRNA
In the late 1970s electron-microscopic studies of DNA-RNA hybrids showed that mRNA molecules lack stretches of sequences which are present on the chromosomal DNA. When the DNA is transcribed, the primary RNA product shows a sequence that is an exact copy of the DNA template. However, genes and thus also pre-mRNAs are interrupted by stretches of sequences (introns) that are not translated into the amino-acid chain of a protein. In order to produce a functional protein these introns are removed from the primary RNA by splicing: the intervening sequences are cut out and the coding sequences (exons, carrying the genetic information) are fused together.
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Splicing of introns in pre-mRNA
The introns are cut out and the exons are fused together. The mature mRNA works as a template for the production of proteins in the next step of gene expression (translation).
Splicing is carried out by huge cellular machine – the spliceosome. This is a complex of RNA molecules and around 150 different proteins. The spliceosome is assembled on each intron and is guided to the correct position by subcomplexes which recognise specific sequences and signals on the pre-mRNA defining the start and the end of the intron. The spliceosome catalyses the excision of intronic sequences followed by connection of the exons. After the reaction the intron is released in form of a lariat which is subsequently degraded.
Sequence elements defining the start and the end of an intron.
The 5’ splice site indicates the beginning of the introns and the 3’ splice site marks the end of the intron. The branch point nucleotide is necessary for the splicing reaction (see biochemistry of the splicing reaction below)
Besides spliceosomal splicing there are two additional, highly specialized ways that splicing of particular introns types can occur: autocatalytic splicing and enzymatic splicing. The three possible mechanisms for splicing only differ in the executing unit the biochemical reaction is the same.
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