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17 Cards in this Set
- Front
- Back
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splicing- Transesterification |
two transesterification reactions in pre-mRNA splicing -2'OH group branch-site adenosine attacks the phosphate group at the 5'splice site intron -transestification occurs 3'OH group at the 5' spice site attacks phosphate at 3' spice site- transesterification intron removed known as branched lariat, converted to linear RNA by a debranching enzyme and is rapidly degraded by nuclear exosome |
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Consensus sequence |
defines the splice sites in eukaryotic pre-mRNAs |
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The spliceosome -what is it -what does it consist of |
A large ribonucleoprotein comples that mediates pre-mRNA slicing -contains 5 small nuclear RNS (snRNAs) U1, U2, U4, U5, U6 between 107-210 nts long and contains ~170 proteins snRNAs bind to consensus sequence on pre-mRNA to define spice-site selection |
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Spliceosome Mechanism |
-U1 snRNP assembles at 5' splice site -U2 snRNP assembles at 3' splice site -Splicing factor 1 (SF1) assembles at branch point A -A trimeric snRNP complex (U4, U5 and U6) joins to form the splicesome -rearrangement of base-pairing interactions to form catalytically active spliceosome -U1 andU4 snRNPs released -catalytic core catalyses the first transesterification reaction- forms intron lariat -Further rearrangement joins the two exons in a second transesterification reaction releasing the lariat intron as well as U2, U5 and U6 -The excised lariat intron is converted to linear RNA by a debranching enzyme and degraded by the exosome |
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Exons Vs introns size structure |
exons- 10 times shorter and more uniformed than introns 10 exons per gene introns- 1.5KB long largest 1.1 Mb long |
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Exons definition hypothesis |
introns very long so additional factors (SR proteins) help to guide snRNPs to splice-sites -serine-arginine-rich (SR) proteins preferentially bind to exon sequences in the pre-mRNA -hnRNPs (heterogenous ribonucleoprotein particles may preferentially bind to intron sequences |
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ß-thalassemia -what is it? -cause |
-inherited blood disorder -severe anemia due to aberrant (not normal) haemoglobin synthesis -common in eastern mediteranean due to heterozygotes thought to be protected from malaria -caused by mutations in the beta-globin gene -causes portions of exons to be deleted and not transcribed |
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Alternative splicing |
-forms different mRNA variants from the same gene -may code for a different protein or have different mRNA properties eg stability -therefore more mRNA variations than gene |
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Most extreme example of alternative RNA splicing |
The drosophila DSCAM gene can generate 38,016 possible isoforms from one gene!!! |
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Patterns of alternative splicing |
-exon skipping -intron retention -alternative 5' splice site -alternative 3' splice site -mutually exclusive exons |
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α-tropomyosin- alternative splicing |
α-tropomyosin coiled coil protein regulates muscle contraction alternative splicing cases variants of α-tropomyosin in different tissue |
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Control of alternative slicing by proteins |
Activator proteins- bind to transcript and activate splicing that wouldn't happen otherwise repressor proteins- binds to transcript inhibiting splicing by hindering access of the splicesome |
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RNA editing |
changes in the nucleotide sequence of the mRNA -insertion/deletion- of uridine (uracil nucleotide) in mitochondria of pathogenic trypanosomas (lower eukaryotes) -substitution editing- by base deanimation- c to U A to inosine (i) |
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C to U editing example |
-C to U editing of APOB pre-mRNA -Tissue-specific isoforms of apolipoprotein B, a protein required for uptake and transport of cholesterol eg c to u editing occurs in intestine but not liver to give different forms •Site-specific deamination in the intestine changes the CAA codon (coding for Gln) to a UAA stop codon •Reaction is catalysed by a cytidine deaminase that selectively recognise the RNA sequence around deamination site. |
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A to I editing example |
A to I editing in pre-mRNAs of GluR-B -GluR-B- a glutamate receptor subunit -A to I editing can occur here in inotrophic glutamate recptor AMPA -I is read as G during translation -effects Ca2+ permeability of the receptor -A to I deanimation done by 'adenosine deaminase acting on RNA' (ADARs) |
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RNA export |
-Nuclear export factors (NXF1/T1) assemble with mRNA in the nucleus -NXF/T are released in the cytoplasm and reimported to the nucleus -Nuclear cap-binding protein (CBC) and PABPN1 are exchanged for cytoplasmic cap-binding protein (eIF4e) and PABPC1 in the cytoplasm |
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What drives the direction of mRNP export |
-reversible phosphorylation of SR proteins |
