Today:TranscriptionRNA ProcessingTranslationREMINDER: If you were in Biology lab last week, you need to check your plates after 2:30 PM in the lab.TRANSCRIPTIONPROKARYOTESHave Sigma factorsEUKARYOTESHave transcription factorsBoth lead to the transcription of specific genesDifferent classes of RNA:• mRNA will dictate the order of amino acids in polypeptide chain (several 1000 mRNAs, ~5% of cell RNA).• rRNA (3) will complex with a number of proteins to formthe ribosome, the site of protein synthesis ~(80%).• tRNA (~40 tRNAs ) will bring amino acids to site ofprotein synthesis (~15%).• snRNA these small nuclear RNA’s are involved inremoving non-coding sequences, introns, in an mRNAmolecule, and splicing together the coding sequences, exons, to generate a functional mRNA molecule.Eukaryotic RNAs must be ProcessedTranscriptionPROKARYOTES VS EUKARYOTESRNA polymerase:Promoter Structure:Transcription Initiation:RNA Processing:Translation• How do various RNAs made during transcriptionparticipate in protein synthesis?• How is information encoded in an mRNAdeciphered?• what factors are required for protein synthesis• mechanism of proteins synthesis• post-translation modification of proteins16S rRNA,~21 differentproteinsStructure 70S Prokaryotic Ribosome: Site of Protein SynthesisThe EndosymbioticTheory70S ribosomesin mitochondria andchloroplasts23S and 5S rRNA~35 different proteins• Gene can be transcribed by more than one RNA pol• Can begin translation before mRNA has beencompletely synthesized.• More than one ribosome can bind to a single mRNA.Role of mRNA in Translation• Sequence of mRNA will dictate the sequence of amino acids in a protein• Many different species of mRNA that will differ in their sequence; multiple copies of a single species of mRNA can be present• In eukaryotes, mRNAs are monocistronic; prokaryotes mRNAs can be monocistronic or Polycistronic: more than one translation start site per mRNA, so more than one messageRole of tRNA• Each tRNA will covalently bind to a specific amino acid. Process called charging• charged tRNA’s will bring amino acids to site of protein synthesis, ribosome.Charging of a tRNA(1) Activation of amino acid:ATP + amino acid ---> aminoacyl(aa) -AMP + P-P ---> 2Pitwo high-energy phosphate bonds are required for activation.(2) Linking activated amino acid to a tRNA:aa-AMP + tRNA -------> aa-tRNA + AMPAminoacyl synthetasesWhere does translation start?How Determine Correct Reading Frame?Eukarya/Archaea- MethionineBacteria- N-formylmethionineWhich AUG?Peptidyl siteAcyl siteExit siteINITIATIONELONGATIONELONGATIONTERMINATIONTermination of Translation• Translation terminates when ribosome reaches astop codon.• Release factor reads stop codon (UAA, UAG, UGA),enters empty A-site, and cleaves polypeptide fromterminal tRNA.Post Translation Modification• Role of Chaperones• Covalently modify protein• Protein localization and secretion• Can cleave off methionine at N-terminal end or a sequence of amino acids at N-terminal end.TranslationSketch out:InitiationElongationTerminationList factors
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