| What is Alternative splicing? |
ALTERNATIVE Splicing : Generally after transcription of eukaryotic genes , pre mRNAs undergo processing . So that the non coding introns are excised & exons are ligated together & thus, without any variation of the nature RNA , the cell produced a specific of protein such splicing is called constitutive splicing . But from virus to mammals, production of two or more mRNAs from a single pre-RNA by joining together varieties of mRNA are produced from a single pre- mRNA transcription. Such a phenomenon is called Alternative splicing . By this method different types of proteins are synthesised from a single pre-mRNA in a cell. This primary transcripts produced from complex transcription units can be processed is alternative ways to yield different mRNA in a cell. This primary transcripts produced from complex transcription units can be processed is alternative ways to yield different mRNA . The expression of both simple & complex transcription units could be controlled by on-off regulation of cleavage & polyadenylation of their pre- mRNAs . OFF-ON Mechanism of post transcription regulation:- The off-an regulation of cleavage & polydenylation step in RNA processing are coordinated by U1A protein with U1 snRNA assembly & they play a crucial role in RNA splicing. The pre-mRNA encoding U1A itself contains two copies of U1A- binding site (7 base sequence) just upstream of its polyadenylation signal. When a U1AA protein binds to each copy of this 7 base sequence in a U1A pre mRNA , containing poly A polymerase & other factors. Only cleavage occurs but polyadenylation doesn’t both occur. Thus a free 3’ end is generated & in absence of polyadenylation, both cleavage products are degraded rapidly by exonucleus & no U1A- mRNA is produced. As a result U1A protein synthesis is decreased until all the excess is used is formation of new U1 SnRNPs . Once this occurs , no U1A protein is available to bind to newly mode U1A pre mRNA which them can be processed normally . Therefore U1A pre mRNA splicing is autoregulated by its own protein. U1A which inhibits polyaderylation to its cleavage sie.
Eg; 1- Tissue – specific RNA splicing is filoronectin . 2- Multiple protein isoforms is vertebrate nervous system & Ig diversification. 3- Splicing control – Drosophila sex differentiation . |