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Biochemistry 2280A
Eric Ball

Lecture 3 - Brandl Formation of mRNAs – RNA processing (eukaryotic cells):  Primary RNA transcript  processed to become a translatable mRNA  exported out of the nucleus  translation  5’ capping  3’ polyadenylation  Splicing concerted processes (they happen together)  Requires specific enzymes Capping of the 5’ end:  5’end of the message is capped with 7 methyl guanosine  Capping is required for RNA export from the nucleus (marks the end of the mRNA as being intact), aids in stabilizing the mRNA from being degraded and acts as a translational signal 3’ Polyadenylation:  The addition of a long A-tract to the 3’ end of the RNA by enzymes downstream  The RNA is first cleaved ~30 bases following an AAUAAA sequence which is 3’ to the coding region – a string of A residues is then added  Enzymes recognize AAUAAA sequence  clips  then polyA tail is added  The polyA tail helps protect the 3’end of the mRNA from degradation o Provides additional stability to the mRNA by reducing effects of 3’ endonucleases  Indicates that the 3’ end of the mRNA is intact and therefore is important for export out of the nucleus and for translation o If you have the cap to the polyA tail, the cell wants to export that RNA into the cytoplasm and ultimately translate it RNA splicing:  In eukaryotic cells, protein coding sequences are interrupted by one or more non- coding sequences called introns o Genes are not continuous, they are interrupted o Introns = interruptions  The coding sequences are called exons o Exons = expressed  Introns are spliced out of the primary transcript to form the mature mRNA  It is unclear if prokaryotes lost introns or if eucaryotes gained introns Why is splicing important?  Introns provide the opportunity for differential splicing  Differential splicing: a signal RNA can be spliced in different ways to create functionally related, but distinct proteins  Splicing enhances the coding capacity of the genome  Tissue specific forms of a protein can be created, but splicing requires specific sequences in the RNA: o 5’ splice junction o 3’ splice junction o Branch point  Note: adenine is essential for splicing How are introns removed from the primary RNA?  Specific sequences within and surrounding the intron target the intron for removal  These are the 5’ junction, branch point (or acceptor site) and the 3’ junction o Loss of these sites results in defects in splicing o In fact, some of the thalasemias (genetic disorders resulting in anemia) are the result of the loss of splice junctions in the globin genes Spliceosome:  Introns are removed as the result of the catalytic activity found in small nuclear ribonucleoprotein particles (snRNPs = snurps) o The snRNPs as the name s
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