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Biological Sciences
Aarti Ashok

Splicing of mRNAs *How does intron splicing occur?  pre-mRNAs become associated with ribonucleoproteins (protein-RNA complexes) that aid in the removal of introns o mature RNA is then transported out of the nucleus to the cytosol Self-Splicing mRNAs  Self-Splicing Introns o fungal mitochondria, plant chloroplasts and several bacteria/archaea o known as Group II Introns, they fold into a complex structure *STEPS 1. 2’ OH of adenosine in intron attacks ste 5’ splice site of exon 1 2. A 2’-5’ phosphodiester linkage links the 1 nucleotide of the intron to the A=lariat 3. The 3’ OH of the exon 1 attacks the 3’ splice site and exons 1+2 are joined together Splicing of mRNAs  Eukaryotic mRNAs are similar in structure to Type II introns but are not self-splicing o require several small nuclear RNAs (snRNAs) and their associated proteins o snRNAs + associated proteins = snRNPs (“snurps”)  when hnRNAs are synthesized in the nucleus, they become associated w/ a large macromolecular complex known as the spliceosome  made up of several snRNPs that assemble onto the pre-mRNA  once assembled, components of the splicesome then carry out the cutting and pasting reactions required to remove the intron and paste the exons together – spliced introns are degraded within the nucleus  the intron removal process requires the function of U1, U2, U5 and U4/6 snRNPs  these snRNPs contain RNA helicases (use ATP) and mediate various dynamic interactions and rearrangements b/w RNA molecules Step 1  the U1 snRNP binds to the 5’ splice site of the pre-mRNA – protein components help select the splice site  specific base pairing b/w U1 and the 5’ splice site Step 2  the U2 snRNP binds to the pre-mRNA next  base pairing occurs in such a way as to allow a particular A residue to bulge out o this will be branch point of lariat Step 3  the U4/U6 pair and U5 snRNPs are the next to enter into the splicing reaction  note that at first, U4 and U6 are extensively base-paired together o U6 is thought to be a ribozyme and U4 is thought to inhibit its catalytic activity o Later, they are stripped apart and U6 becomes paired with U2  initially, the entry of U4/6 + U5 releases U1 from the assembled complexes  once U4 has been released, U6 is now catalytically active Step 4  in the first reaction at the 5’ splice site, results in: o a free 5’ exon o a lariat intron-3’ exon intermediate  U2 and U6 base pair (U6 is catalytically active)  U5 snRNA keeps the “free” 5’ exon in place and also interacts with the 3’ exon Step 5  in the second step of the U6 catalyzed reaction, the lariat is released and the 2 exons are joined together Splicing of mRNAs  The spliceosome is thought to be ribozyme (RNA acting as an enzyme) o that is, the RNAs (U6) are catalytically active o proteins play supplementary roles:  splice site recognition  maintaining the right 3-D structure  transport of spliced mRNAs to the nuclear envelope  What’s the evidence for this? 1. The chemical reactions that occur in the self-splicing of type II introns and the ones that occur during spliceosome mediated removal of introns from pre-mRNAs are identical. 2. The snRNAs in the spliceosome closely resemble the parts of the type II introns imp for splicing to occur. A giant “mRNA factory”  capping enzymes, splicesome components and polyadenylation factors are all recruited by RNA polymerase II o travel with it as the transcript is being synthesized o RNA processing and transcription are closely linked   ALL these processes occur within the NUCLEUS The RNA world  typically, nucleic acids are thought to store information and proteins are thought to catalyze reactions o perhaps early during evolution, RNA performed both these roles due to the absence of DNA and protein at that point = RNA world hypothesis o splicing may be a legacy of RNA from those ancient times The value of Introns 1. Alternative Splicing: The process of splicing can be regulated such that different versions of a mature mRNA can be generated. In one pathway, a sequence may be an intron, but in a different pathway, it may be an exon. – sam
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