Defect in Snurps causes ?

m-RNA processingProkaryotic mRNA is functional immediately upon synthesis, i.e. prokaryotic primary transcript of mRNA is functional. Thus it does not require post-transcriptional modification. In Eukaryotes the primary transcript of mRNA is the hn RNA (hetrogeneous nuclear RNA). After transcription hnRNA is extensively modified to form functional mRNA. These modifications are as follows.1) The 5'-capping :- This is the first processing reaction. 51-end of mRNA is capped with 7-methylguansosine. This cap helps in initiation of translation (protein synthesis) and stabilizes the structure of mRNA by protecting from 5'-exonuclease.2) Addition of poly 'A' tail :- As the name suggests, multiple 'A' (adenylate) residues are added at 3'end.This poly-A tail is not transcribed from DNA, but rather added after transcription. These tails helps to stabilize the mRNA (by protecting from 3'-exonuclease), facilitate exit from the nucleus, and aid in translation. After mRNA enters the cytosol, the poly-A tail is gradually shoened. Some mRNAs do not have poly-A tail, e.g. mRNAs of histones and some interferons.3) Removal of introns (splicing) :- Eukaryotic genes contain some coding sequences which code for protein and some intervening non-coding sequences which do not code for protein. The coding sequences are called `exons' and intervening non-coding sequences are called `introns'. The process by which introns are excised and exons are linked to form functional mRNA is called splicing. Thus mature mRNA does not contain introns.Splicesome Splicesome is an assembly made up of small nuclear RNA (snRNA), some proteins and hnRNA. snRNA combines with proteins to form small nuclear ribnonucleoprotein paicles (snRNPs or snurps) that mediate splicing. It is snRNA component of snurps that catalyzes splicing. Snurps are U4, U5 and U6.Only about 1-5% of human DNA has coding sequence (exons). Remaining is non-coding (introns).4) Alternate splicing :- The hn-RNA molecules from some genes can be spliced in alternative way in different tissues. Thus two or more different mRNA (and therefore 2 or more proteins) can be synthesized from same hnRNA. For example, difference isoforms of tropomyosin in different tissues in due to alternate splicing.