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Lecture 10

BIO206 Lecture 10.pdf

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George S Espie

Lecture 10 October-15-13 2:12 PM Enhancers & Repressors Upstream transcription factors – activating or repressor - Associated with 4 areas of chromosomes (they need to be all in place before polymerase can begin transcription) RNA splicing: Intron Removal • introns removed & exons spliced together • requires small nuclear ribonucleoprotein particles (snRNPs - “snurps”) = proteins + snRNAs • Collectively: complex of snurps = spliceosome • occurs in nucleus after capping and before tailing • incompletely processed mRNAs are not exported from nucleus Cotranscriptional processing Introns need to be physically removed, exons need to be spliced together (done by spliceosome) Exon/Intron junction Conserved splice sites RNA Splicing - Reaction Mechanism Spliceosome Identifies & Excises Introns- Splices Exons Forms during transcription Macromolecular Protein & RNA super-complex An assembly of 5 snRNPs complexes smallnuclearR ibN ucleaP roteins (snurp) snRNP U1 = proteins + snRNA U1 smallnuclearRNA (snRNA) - 5 snRNA + more than 60 polypeptides RNA splicing: The players Previously reported consensus branch point sequences Splicing process: Note how U2 binds to the 2 bases on either side of the branch point adenosine & leaves that one sticking out. U1 has complimentary sequence to 5’ splice site SnRNP bind complimentary to the RNA sequence SR proteins are Serine arginine rich proteins that bind to RNA After Pol2 has finished and dissociates from the DNA, the phosphates that were added by TF2H are stripped off by phosphatases. Co transcriptional processing overview: - Done primarily to increase stability - Aid in the exportation of the nucleus - Identification CTD is loading dock for post transcriptional processing. Alternative splicing: - Exons can be joined in different patterns - Increases the # of different proteins that can be encoded by a gene - Alternative splicing in different cell types leads to different proteins from the same pre- mRNA transcript - Increased the speed at which the emergence of new useful proteins. (inclusion of makes genetic recombination more likely) - Ex. alpha tropomyosin: o Coiled protein that regulates muscle cell contractions. o Tropomyosin part of thin filament of muscle. Sits in myosin binding grove of Actin filament o o Changing way you splice, one gene can code for many different types of proteins o Similar proteins that perform similar functions in different areas of the body. o More bang for the buck for a single gene sequence in DNA Exportation to cytoplasm: Only properly processed mRNA strands can pass through the nuclear pore complex SR = serine arginine rich EJC = exon junction complex hnRNP = heteronuclear ribo nuclear particle Nuclear export receptor Helps mRNA cross the nuclear pore and goes back after its done. mRNA eventually degraded by RNases. Varies on how long it takes for the to degrade. 3 (bacteria) – 30 (eukaryotic) CBC (Cap binding complex) eventually removed by another protein Waste products of transcription (introns, etc) are degraded inside the nucleus and reused. Evolutionary theory: early prokaryotic cells used to have introns, and over time shed those introns in favour of adopting a smaller genome to allow for more rapid and efficient reproduction. (Eukaryotes that reproduce rapidly also have very few introns) Another theory: eukaryotic cells were invaded by parasitic genetic elements that became introns later on and never were removed completely from the genome. Central dogma of Molecular Biology: Protein synthesis: - Protein are made of a limited # of subunits (20 amino acids) - Sub units are added one at a time o N-C terminus o mRNA translated in 5’ to 3’ direction - Specific starting point, growth proceeds in one direction - Primary polypeptide product is usually modified Sequence of events: Translation (converting to another language, 4 base pair code into 20 amino acid code) - Process by which the base sequence of mRNA is used to order and join the amino acids in a protein in a protein - Involves over 100 macromolecules - 3 types or RNA o mRNA (only intermediate step, all other RNA types are final products) o rRNA (ribosomal) : forms part of the structures of ribosomes and participates in protein synthesis o tRNA (transfer): used in proteins synthesis as an adapter between mRNA and amino acids - Small RNAs: used in pre mRNA splicing, transport of proteins to ER and other cellular processes Genetic Code: RULE
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