BIOL 302 week 2.23-2.25
BIOL 302 week 2.23-2.25 BIOL 302
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This 7 page Class Notes was uploaded by Michaela Sanner on Friday February 26, 2016. The Class Notes belongs to BIOL 302 at University of South Carolina taught by Erin Connolly in Spring 2016. Since its upload, it has received 34 views. For similar materials see Cell and Molecular Biology in Biology at University of South Carolina.
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Date Created: 02/26/16
2/23/16 5) TF II H phosphorylates RNA Polymerase II TF II H has protein kinase phosphorylation of RNA Polymerase II causes a change in shape (30) > RNA Polymerase II is released from complex and transcription begins phosphorylation occurs at CTD ( Cterminal domain) CTD long tail/extension that extends from the body of RNA polymerase II 6) General transcription factors are released from the DNA and can be recycled mRNA Processing in Eukaryotes* mRNA processing mRNA> modified chemically>then ready for translation Eukaryotes: DNA in nucleus transcription occurs in nucleus translation in cytosol on ribosomes transcription & translation are separated in space and time mRNA processing helps to link transcription and translation Prokaryotes: no nucleus; single compartment transcription & translation occur in same compartment translation can begin before transcription of a RNA is complete **2 processes are linked Eukaryotic cells: Transcription (nucleus) > primary transcript(nucleus)> mRNA processing(nucleus)> mature transcript(nucleus)> export of mature transcript from nucleus to cytosol (occurs via nuclear pores) > translation (cytosol on ribosomes) mRNA processing 3 steps occur to primary transcript (only in eukaryotes occur in nucleus; only mRNAs so RNA Polymerase II transcripts) 1) mRNA capping 2) polyadenylation 3) splicing result in mature transcript RNA Capping: modification of the 5' end of transcript; 5' end is "capped" by the addition of a special nucleotide (7methyl guanosine) cell adds this cap before transcription is complete **Transcription and RNA Processing are coupled processes phosphorylation of CTD of RNA polymerase II enables RNA Processing RNA processing enzymes bind to phosphorylated CTD of RNA Polymerase II Polyadenylation: modification to the 3' end of the transcript; 3' is trimmed (at particular sequence) add series of adenine ribonucleotides at 3' end of transcript series of adenines at 3' end is "poly A tail" only found on eukaryotic mRNAs generally between ~150250 nucleotides long Functions of mRNA modifications** stabilize the mRNAs aid in export from nucleus used by translation machinery to indicate that a transcript is complete and intact Splicing: eukaryotic genes are often interrupted by noncoding sequences Interona region of a eukaryotic gene that does not** code for protein, but that is part of the primary transcript **interons are excised from RNA by splicing to generate mature transcript Exons segments of eukaryotic genes that are part of both the primary and mature transcript and do* code for proteins How does the cell know which parts to splice out/remove? **special sequences in mRNA recognized by enzymes that perform splicing** Interon coding: R=A or G Y=C or U N=A, G, C, or U (nonspecific position) Splicing Requirements** 3 special sequences in mRNA enzymes that recognize the special sequences (enzyme complexed called snRNPs "snurps") snRNPs small nuclear ribonucleoprotein particles are splicing enzymes form the core of spliceosome spliceosome large assembly of RNA and protein that performs RNA splicing Splicing mechanismSimplified 1) branch point A in interon attacks the 5' splicing site/junction and cuts the sugar phosphate backbone at the junction 2) 5' end of interon is covalently linked to the 2'OH of the ribose of the branch point A (formation of a lariat) 3) the free 3'OH of the 1st exon then reacts with the beginning of 2nd exon, joining the 2 Exons together, and releasing the interon Benefits of Splicing* alternative splicing ~60% human genes are spliced production of different proteins from the same RNA transcript by splicing it in different ways **allows eukaryotic organisms to increase the coding potential of their genomes** 2/25/16 mRNA Export from nucleus to cytosol: mRNAs leave nucleus via nuclear pore complex requires proteins: poly A binding protein cap binding protein } recognize chemical modifications (Poly A, cap, exon) exon junction complex nuclear transport receptor mRNA turnover / RNA degradation > RNA stability mRNAs are degraded at different rates ~10min>~12 hours stability is controlled by signals (sequences within RNA) within the RNA itself often UTRs are important for stability of transcript UTRUntranslated region Translation: decoding the information in mRNA to produce polymers constructed of amino acids mRNA> protein 4 nucleotide (RNA 4 letter alphabet) 20 amino acids (proteins 20 letter alphabet) an mRNA is decoded in sets of 3 nucleotides (codon) governed by genetic code : sets of rules that speciify correspondence between nucleotide triplets (codon) and amino acids in proteins 64 combinations of triplets not equal to 20 amino acids **genetic code is redundant** meaning can be more than one codon per amino acid codonsequence of nucleotides in a DNA or RNA that represents the instructions for incorporation of a particular amino acid into a polypeptide chain amino acids are coded by between 1 and 6 different codons there are 3 STOP codons (causes gene termination) (code for no amino acid) > 61 codons that code for amino acids one start codonAUG>methionine triplets that code for the same amino acid tend to have the same nucleotides @ position 1 and 2 and tend to differ at position 3 Ex: Glycine 1 2 3 G G A G G C G G G G G U G G N"wobble" @ 3rd position Reading Frame: one of 3 possible ways/methods to "read" a nucleotide sequence in RNA as sets of nonoverlapping triplets RNA 5' AUG. CCG. UUU. GCU. AUA. AAA. CUG. 3' Met Pro Phe Ala Ile Lys Leu first reading frame Cys Arg Leu Glu Stop second reading frame (decode starting at 2nd nt.) Ala Val Cys Tur Lys Thr third reading frame(start @ 3rd nt.) What is required for translation?** mRNA 2 types of adaptor molecules 1) tRNAs 2) amino acyl tRNA synthetases tRNAs adaptors that recognize both the RNA triplet in the mRNA and the correct corresponding amino acid ~80 nucleotides long fold up into 3D structures called "clover leaf" anticodon loop basepairs with a complementary codon in mRNA 3' end of tRNA covalent attachment of amino acid 61 codons not equal to 20 amino acids some tRNAs can basepair with more than 1 codon in RNA some tRNAs can tolerate a mismatch at the 3rd position # of tRNAs dependent on species Amino acyl tRNA synthetases enzymes that recognize and attach the correct amino acid to a given tRNA tRNAs minus amino acid "uncharged" tRNAs tRNAs plus amino acid "charged" tRNAs Translation mRNA tRNA + amino acyl tRNA synthetases ribosomes large protein + RNA complexes decodes mRNA and synthesizes protein Ribosomes have 2 subunits ( higher order) large subunit 49 proteins and 3 RNAs (rRNAs) small subunit 33 proteins and 1 RNA (rRNA) Total: 82 protein and 4 RNA (rRNAs) Ribosomes function: small subunit matches tRNAs to codons in a mRNA large subunit catalyze formation of peptide bonds Ribosome Binding Site 1) mRNA binding sitesmall subunit 2) A site amino acyl tRNA "charged" tRNAs enter ribosome 3) P Site peptidge tRNA peptide bonds made 4) E site exit "uncharged" tRNAs exit complex here 24 are tRNA binding sites: only 2 tRNA binding sites may be occupied at one time Translation 3 Stages 1) Initiation not covered ** 2) Elongation 3) Termination not covered Steps of translation elongation start with a chain of 3 amino acids >addition of amino acid #4 to pre existing chain 1) amino acyl tRNA binds in A site (carrying amino acid #4) 2) 3) 4)
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