In eucaryotes, most genes are interrupted: the coding

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In eucaryotes, most genes are interrupted the
coding sequences (exons) are split by
intervening sequences (introns). Introns are
removed post transcriptionally.
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1993 Nobel prize for Medicine Discovery of
split genes
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Intron / Exon junctions are defined by a specific
sequence at the 5 terminus (beginning) and 3
terminus (end) of the intron.
Introns ALWAYS begin with GU and end with AG
the GU / AG rule
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The intron exon junctions are recognised by a
collection Of 5 small nuclear RNAs (snRNA) and 50
proteins which form the spliceosome (snRNPs)
SNURPS
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1. Spliceosomes recognise junctions and cleave
   5intron end
2. Intron loops to itself to form a lariat
3. 3 intron end is cleaved and intron is degraded.

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Thalassemia, a form of anaemia common in
Mediterranian countries, is caused by errors in
the splicing process.
Normal red blood cells contain correctly
spliced beta-globin, an important component in
hemoglobin that takes up oxygen in the lungs.
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The red blood cells in thalassemia patients are
distorted and immature, containing a nucleus.
This is due to a point mutation in the
beta-globin gene, which causes an error in splice
site selection. A faulty beta-globin protein is
made, leading to severe anemia.
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A percursor-RNA may often be matured to mRNAs
with alternative structures.An example where
alternative splicing has a dramatic consequence
is somatic sex determination in the fruit fly...
It controls a cascade of alternative RNA
splicing decisions that finally result in female
flies.
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WHY is splicing a feature of eucaryotes and not
procaryotes?
Two theories
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Intron early The earliest
progenote had a spliced genome
Archaebacteria still have spliced genes.
Eubacteria do not and have streamlined their
genome.
Intron
late Progenote was unspliced, and introns were
inserted later
Intron insertion would have destroyed coding
genes
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Is exon shuffling of any benefit to the
eucaryotic organism?
It allows the generation of new proteins