2 Word Roots anti- = opposite exo- = out, outside, without anticodon: specialized base triplet on one end of a tRNA molecule that recognizes particular complementary codon on mRNA moleculeexo- = out, outside, withoutexon: coding region of eukaryotic gene that is expressedintro- = withinintron: noncoding, intervening sequence within eukaryotic genemuta- = change; -gen = producingmutagen: physical or chemical agent that causes mutationspoly- = manypoly-A-tail: modified end of 3’ end of mRNA molecule consisting of addition of adenine nucleotidestrans- = across; -script = writetranscription: synthesis of RNA on DNA template
11 Fig. 17-3 http://highered.mcgraw-hill.com/olc/dl/120077/bio25.swf DNATRANSCRIPTIONmRNARibosomeTRANSLATIONPolypeptideBacterial cell (lacks nucleus so mRNA produce by transcriptionimmediately translated without additional processing)NuclearenvelopeDNATRANSCRIPTIONPre-mRNARNA PROCESSINGFigure 17.3 Overview: the roles of transcription and translation in the flow of genetic informationmRNATRANSLATIONRibosomePolypeptide(b) Eukaryotic cell (nucleus provides separate compartment for transcription; original RNAtranscript, called pre-RNA, processed in various ways before leaving nucleus as RNA)
16 First mRNA base (5 end of codon) Third mRNA base (3 end of codon) Fig. 17-5Second mRNA baseFirst mRNA base (5 end of codon)Third mRNA base (3 end of codon)Figure 17.5 The dictionary of the genetic code
20 signals initiation of RNA synthesis Fig The initiation of transcription at a eukaryotic promoter1Eukaryotic promoter includesTATA box (~25 nucleotidesupstream from transcriptionalstart point) which is crucial informing initiation complex(signals initiation of RNAsynthesis)Promotersignals initiation of RNA synthesisTemplate5335TATA boxStart pointTemplateDNA strandSeveral transcription factors (collection of proteins onrecognizing TATA box) must bind to DNA before RNApolymerase II can do so and transcription can begin2TranscriptionfactorsRNA Polymerase Binding and Initiation of Transcription53353Additional transcription factors bind to DNA alongwith RNA polymerase II, forming the transcriptioninitiation complex. DNA helix unwinds, RNAsynthesis begins at start point on template strandFigure 17.8RNA polymerase IITranscription factors53355RNA transcriptTranscription initiation complex
33 3 end is attachment site for amino acid Figure The structure of transfer RNA (tRNA)Form fits function3 end is attachment site for amino acidAmino acidattachment site5HydrogenbondsAnticodon written 3’ 5’ to align properlyw/codons written 5’ 3’ (for base pairing, RNAstrands must be antiparallel, like DNA. ForExample, anticodon 3’-AAG-5” pairs w/mRNAcodon 5’-UUC-3’(a) Two-dimensional structure5Amino acidattachment site3Hydrogenbonds35AnticodonAnticodon(b) Three-dimensional structure(c) Symbol usedin this book
35 Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA driven by hydrolysis of ATPAminoacyl-tRNA synthetase (enzyme)Active site binds amino acid w/ATPATP loses two P groups/joins amino acid as AMPAmino acidPPPAdenosineATPPAdenosinetRNAPPiAminoacyl-tRNAsynthetasePiPitRNAAppropriate tRNA covalently bondsto amino acid, displacing AMPPAdenosineAMPComputer modeltRNA charged w/amino acid is release by enzymeAminoacyl-tRNA(“charged tRNA”)
37 Ribosome has three binding sites for tRNA FigGrowingpolypeptideExit tunneltRNAmoleculesLargesubunitEPASmallsubunit5mRNA3(a) Computer model of functioning ribosomeP site (Peptidyl-tRNA binding site) holds tRNA that carries growing polypeptide chainA site (Aminoacyl-tRNA binding site) holds tRNAthat carries next amino acid to be added to chainE site (Exit site) where dischargedtRNAs leave ribosomeEPALargesubunitmRNAbinding siteSmallsubunitRibosome has three binding sites for tRNA(b) Schematic model showing binding sitesFigure The anatomy of a functioning ribosomeAmino endGrowing polypeptideNext amino acidto be added topolypeptide chainEtRNAmRNA35Codons(c) Schematic model with mRNA and tRNA
39 Ribosome Association and Initiation of Translation Initiation stage of translation brings together mRNA/tRNA w/1st amino acid/two ribosomal subunits (rRNA)First, small ribosomal subunit binds w/mRNA and special initiator tRNA carrying methionine (N-terminus)Then small subunit moves along mRNA until it reaches start codon (AUG)/binds to it using hydrogen bondProteins (initiation factors) bring in large subunit that completes translation initiation complexGTP provides energy for assemblyInitiator tRNA is in P site; A site available to tRNA bearing next amino acidContinues in one direction to final amino acid at carboxyl end (C-terminus)
41 2. Peptide bond formation: rRNA molecule of large subunit FigAmino end Codon recognition: anticodonof polypeptide of incoming aminoacyl tRNAbase-pairs w/complementarymRNA codon in A site.Hydrolysis of GTP increasesaccuracy/efficiency of step.E3mRNARibosome ready fornext aminoacyl tRNA3. Translocation: Ribosometranslocates tRNA in A site toP site. At same time, empty tRNAin P site moved to E site, where itis released. mRNA moves alongw/its bound tRNAs, bringing nextcodon to be translated into A site.PsiteAsite5GTPGDP2. Peptide bond formation:rRNA molecule of large subunitcatalyzes formation of peptidebond between new amino acid inA site and carboxyl end of growingpolypeptide in P site. Thisremoves polypeptide from tRNAin P site and attaches it to aminoacid on tRNA in A site.EEPAPAFigure The elongation cycle of translationGDPGTPEPA
42 Termination of Translation FigTermination of TranslationReleasefactorFreepolypeptide5333255GTP2 GDPRelease factor causes addition of water molecule instead of amino acid/ promotes hydrolysis of bond between tRNA in P site and last amino acid of polypeptide, freeing polypeptide from ribosomeStop codon (UAG, UAA,or UGA) reached A siteaccepts “release factor,”protein shaped like tRNA,instead ofaminoacyl tRNAFigure The termination of translationTwo ribosomal subunits/other components of translation assembly dissociate (requires 2 GTPs)
47 Ribosome mRNA Signal peptide ER membrane Signal peptide removed Fig2. SRP (signal-recognition particle) binds to signal peptide, halting synthesis momentarily, cues ribosome to attach to ER.Synthesis finishes in cytosol unless polypeptide signals ribosome to attach to ER/polypeptides destined for ER or for secretion marked by signal peptide (~20 amino acids at/near leading end).4. SRP leaves, polypeptide synthesis resumes w/simultaneous translocation across membrane (signal peptide stays attached to translocation complex)1. Polypeptide synthesis always begins on free ribosome in cytosolRibosomemRNASignalpeptideERmembraneSignalpeptideremovedSignal-recognitionparticle (SRP)ProteinCYTOSOLTranslocationcomplexFigure The signal mechanism for targeting proteins to the ERER LUMENSRPreceptorprotein5. Signal-cleaving enzyme cuts off signal peptide6. Rest of completed polypeptide leaves ribosome and folds into final conformation. It may become part of ER membrane or be exported from cell.3. SRP binds to receptor protein in ER membrane. Receptor part of protein translocation complex that has membrane pore and signal-cleaving enzyme.
48 A Science Odyssey: You Try It: DNA Workshop Justify the role of DNA replication being the starting point toward the goal of protein synthesis. Manipulate online models to create representations of DNA replication, transcription, and translation.Use construction paper, markers, and scissors to construct a model of DNA using at least 24 nucleotides. Use the model to distinguish between DNA and RNA; to model and explain the processes of replication, transcription, and translation; and to predict (with justification) the effects of change (mutation) on the original nucleotide sequence.
50 Mutations: any changes in genetic material of cell or virus not due to segregation or to normal recombination of genetic materialPoint mutations: chemical changes in just one base pair of geneChange of single nucleotide, if present in protein-coding region in DNA template strand, can change amino acid sequence of polypeptide and can lead to production of abnormal or nonfunctioning proteinMutations can alter levels of gene expression/be silentIf in gamete or cell giving rise to gamete, can be transmitted to offspring (genetic disorder or heredity disorder if mutation has adverse effect on phenotype)
51 Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5 Figure The molecular basis of sickle-cell disease: a point mutationWild-type hemoglobin DNAMutant hemoglobin DNA3CTT53CAT55GAA35GTA3mRNAmRNA5GAA35GUA3Normal hemoglobinSickle-cell hemoglobinGluVal
53 Silent (no effect on amino acid sequence owing to redundancy) Fig a Base-pair substitutionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endA instead of G3553Figure Types of point mutationsU instead of C53StopSilent (no effect on amino acid sequence owing to redundancy)
54 Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Fig b Base-pair substitutionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endT instead of C3553Figure Types of point mutationsA instead of G53StopMissense (Substitutions that change one amino acid to another. May have little effect onprotein if new amino acid has properties similar to those of amino acid it replaces, or it maybe in region of protein where exact sequence of amino acids not essential to protein’s function.)
55 Nonsense (Change codon for amino acid into stop codon. Fig c Base-pair substitutionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endA instead of T3553Figure Types of point mutationsU instead of A53StopNonsense (Change codon for amino acid into stop codon.Translation terminated prematurely resulting in polypeptidethat will be shorter. Nearly all lead to nonfunctioning proteins.)
56 Frameshift causing immediate nonsense (1 base-pair insertion) Fig d Base-pair insertion or deletionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endExtra A3553Figure Types of point mutationsExtra U53StopFrameshift causing immediate nonsense (1 base-pair insertion)
57 Frameshift causing extensive missense (1 base-pair deletion) Fig e Base-pair insertion or deletionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endmissing3553Figure Types of point mutationsmissing53Frameshift causing extensive missense (1 base-pair deletion)
58 Wild type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Fig f Base-pair insertion or deletionWild typeDNA templatestrand3553mRNA53ProteinStopAmino endCarboxyl endmissing3553Figure Types of point mutationsmissing53StopNo frameshift, but one amino acid missing (3 base-pair deletion)
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