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Department
Biology
Course
BIOL 205
Professor
Prof.
Semester
Winter

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Group I: e.g. Tetrahymena rRNA (Tom Cech) • no sequence conservation at splice junction • carry conserved sequences internally • introns self-splice (RNA catalysis, Ribozymes) • Guanosine as cofactor • transesterifications, no external energy input Guanosine attack at HO-G 5’end of intron Exon A Exon B 5’ P 3’ P Exon A 3’ G- P Exon B 3’ 5’ P OH 3’-OH nucleophilic attack Exon A Exon B 5’ P 3’ G-P OH Linear intron released…can circularize Self-splicing reaction Figure 8-18 secondary structure Internal guide sequence (IGS) Group I Introns can be converted to ribozymes Splicesome-mediated Splicing! •GU found at 5’intron-exon junction •AG marks 3’intron-exon junction •A is part of branch site (consensus longer than this) •Intron ends up as lariat •No particular order in which introns are removed Conserved sequences related to intron splicing Complex patterns of eukaryotic mRNA splicing – tropomyosin gene Reactions in exon splicing Figure 8-17 2’ OH nucleophilic attack Creates a unique 5’-2’ linkage Spliceosome assembly and function Figure 8-16 part 1 Contain U1 or U2 small nuclear RNA (snRNA) bind with protein to form small nuclear ribonucleoprotein particles (SNPs) RNA component of U4 and U6 base pair causes U6 to replace U1 at 5’ exon-intron boundary Spliceosome assembly and function F Factors released –allows U5 to bind with U2 Forms active site -first transesterificaton takes place U5 aids in bringing 2 exons together Second transesterification th ARTICLE doi:10.1038/nature12734 November 14 2013 RNA catalyses nuclear pre-mRNA splicing Sebastian M. Fica, Nicole Tuttle, Thaddeus Novak, Nan-Sheng Li, Jun Lu, Prakash Koodathingal, Qing Dai, Jonathan P. Staley & Joseph A. Piccirilli In nuclear pre-messenger RNA splicing, introns are excised by the spliceosome, a dynamic machine composed of both proteins and small nuclear RNAs (snRNAs). Over thirty years ago, after the discovery of self-splicing group II intron RNAs, the snRNAs were proposed to catalyse splicing. However, no definitive evidence for a role of either rescue strategies in spliceosomes from budding yeast, here we show that the U6 snRNA catalyses both of the two splicing reactions by positioning divalent metals that stabilize the leaving groups during each reaction. Notably, all of the U6 catalytic metal ligands we identified correspond to the ligands observed to position catalytic, divalent metals in crystal structures of a group II intron RNA. These findings indicate that group II introns and the spliceosome share common catalytic mechanisms and probably common evolutionary origins. Our results demonstrate that RNA mediates catalysis within the spliceosome. A cascade of alternative RNA splicing regulates sex determination in Drosophila mRNA-binding proteins repress mRNA translation to determine cell lineages Figure 12-24a GLP-1 protein expressed in two anterior cells (green) but not in posterior cells where glp-1 mRNA is regulated by GLD-1 protein C. elegans 4 cell stage embryo mRNA-binding proteins repress mRNA translation to determine cell lineages Figure 12-24b Translational Control Spatial Control region Translation of glp-1 blocked in posterior cells by binding of GLD-1 protein to SCR GLD-1 is enriched in posterior blastomeres YOU ARE HERE ! RNAPOLYMERASE II TRANSCRIBED GENES most cellular mRNAs 5’Flanking region DNA cis-acting control elements Insulators Enhancer Activators TATA Element Repressors BOX -19 to -27 Control elements : Different in almost every gene Each recognized by protein factors known as Ts factors Hot tubs treatment during early pregnancy -1 four weeks Embryo can’t launch a Heat shock response -may lead in encephaly Neural tube defects Steroid hormones, thyroid hormone, vitamin D3 and Retinoic acid (Vitamin A) work through similar receptors These compounds have dissimilar structures! Steroid hormone receptors have a conserved structure! Steroid receptors Variable region •Transactivation of gene expression •Nuclear localization signal •Regions for protein interaction DNA binding region •Rich in basic amino acids •Cysteine rich, zinc fingers •Confers target specificity Steroid receptor activation and action HSP90 Hormone binding domain is sufficient to carry reporter into nucleus A typical signal-transduction pathway Figure 12-17 Signal transduction can be complicated. Development : Chapter 12 • What are the number, identity and function of genes in development? 1. What genes are important? 2. Where in the developing animal and at what times are they active? 3. How is the expression of developmental genes regulated? 4. Through what molecular mechanisms do gene products affect development? Pattern formation a key to developmental positioning and identity You distinguish an arm from a leg by shape:
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