In eucaryotes, most genes are interrupted: the coding sequences (exons) are split by intervening sequences (introns). Introns are removed post transcriptionally.

1993 Nobel prize for Medicine – Discovery of split genes: Richard Roberts Philip Sharp

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

The intron exon junctions are recognised by a collection Of 5 small nuclear RNAs (snRNA) and 50 proteins which form the spliceosome (snRNPs) “SNURPS”

Spliceosomes recognise junctions and cleave 5’intron end; Intron loops to itself to form a “lariat” 3’ intron end is cleaved and intron is degraded.

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.

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. Aberrant Normal

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.

WHY is splicing a feature of eucaryotes and not procaryotes? Two theories : Intron early The earliest progenote had a spliced genome Intron late Progenote was unspliced, and introns were inserted later

Intron early Intron late 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

Is exon shuffling of any benefit to the eucaryotic organism? It allows the generation of new proteins