LE 17-23Wild-type hemoglobin DNAmRNA3′ 5′ 5′3′5′ 3′ 3′5′Mutant hemoglobin DNAmRNANormal hemoglobin Sickle-cell hemoglobinLE 17-24Base-pair substitutionNo effect on amino acid sequenceU instead of CMissenseA instead of GNonsenseU instead of AStopAmino endProtein5′ 3′Carboxyl endStopStopStopmRNAWild typeLE 17-25Base-pair insertion or deletionFrameshift causing immediate nonsenseExtra UMissingFrameshift causingextensive missenseInsertion or deletion of 3 nucleotides:no frameshift but extra or missing amino acidMissingStopStopAmino endCarboxyl endStopWild typemRNAProtein5′ 3′Mutagens• Spontaneous mutations can occurduring DNA replication, recombination,or repair• Mutagens are physical or chemicalagents that can cause mutationsWhat is a gene? revisiting thequestion• A gene is a region of DNA whose finalproduct is either a polypeptide or an RNAmoleculeLE 17-26TRANSCRIPTIONRNA PROCESSINGRNAtranscript5′ExonNUCLEUSFORMATION OFINITIATION COMPLEXCYTOPLASM3′DNARNApolymeraseRNA transcript(pre-mRNA)IntronAminoacyl-tRNAsynthetaseAminoacidtRNAAMINO ACID ACTIVATION3′mRNAAPERibosomalsubunits5′GrowingpolypeptideEAActivatedamino acidAnticodonTRANSLATIONCodonRibosomeConcept 18.4: Individual bacteriarespond to environmental change byregulating their gene expression• A bacterium can tune its metabolism to thechanging environment and food sources• This metabolic control occurs on two levels:– Adjusting activity of metabolic enzymes– Regulating genes that encode metabolicenzymesLE 18-20Regulation of enzymeactivityRegulation of enzymeproductionEnzyme 1Regulation of gene expressionEnzyme 2Enzyme 3Enzyme 4Enzyme 5Gene 2Gene 1Gene 3Gene 4Gene 5TryptophanPrecursorFeedbackinhibitionOperons: The Basic Concept• In bacteria, genes are often clustered intooperons, composed of– An operator, an “on-off” switch– A promoter– Genes for metabolic enzymes• An operon can be switched off by a proteincalled a repressor• A corepressor is a small molecule thatcooperates with a repressor to switch anoperon offLE 18-21aPromoterPromoterDNAtrpRRegulatorygeneRNApolymerasemRNA3′5′ProteinInactiverepressorTryptophan absent, repressor inactive, operon onmRNA 5′trpEtrpDtrpCtrpB trpAOperatorStart codonStop codontrp operonGenes of operonEPolypeptides that make upenzymes for tryptophan synthesisDCBALE 18-21b_1DNAProteinTryptophan(corepressor)Tryptophan present, repressor active, operon offmRNAActiverepressorLE 18-21b_2DNAProteinTryptophan(corepressor)Tryptophan present, repressor active, operon offmRNAActiverepressorNo RNA madeRepressible and Inducible Operons: TwoTypes of Negative Gene Regulation• A repressible operon is one that is usuallyon; binding of a repressor to the operatorshuts off transcription• The trp operon is a repressible operon• An inducible operon is one that is usuallyoff; a molecule called an inducer inactivatesthe repressor and turns on transcription• The classic example of an inducible operonis the lac operon, which contains genescoding for enzymes in hydrolysis andmetabolism of lactoseLE 18-22aDNAlaclRegulatorygenemRNA5′3′RNApolymeraseProteinActiverepressorNoRNAmadelacZPromoterOperatorLactose absent, repressor active, operon offLE 18-22bDNA laclmRNA5′3′lac operonLactose present, repressor inactive, operon onlacZlacY lacARNApolymerasemRNA 5′ProteinAllolactose(inducer)Inactiverepressorβ-GalactosidasePermeaseTransacetylase• Inducible enzymes usually function in catabolicpathways• Repressible enzymes usually function in anabolicpathways• Regulation of the trp and lac operons involvesnegative control of genes because operons areswitched off by the active form of the repressorOverview: How EukaryoticGenomes Work and Evolve• Two features of eukaryotic genomes are amajor information-processing challenge:– First, the typical eukaryotic genome is muchlarger than that of a prokaryotic cell– Second, cell specialization limits the expressionof many genes to specific cells• The DNA-protein complex, called chromatin,is ordered into higher structural levels thanthe DNA-protein complex in prokaryotesConcept 19.1: Chromatinstructure is based on successivelevels of DNA packing• Eukaryotic DNA is precisely combined witha large amount of protein• Eukaryotic chromosomes contain anenormous amount of DNA relative to theircondensed lengthNucleosomes, or “Beads on aString”• Proteins called histones are responsible forthe first level of DNA packing in chromatin• The association of DNA and histones seemsto remain intact throughout the cell cycle• In electron micrographs, unfolded chromatinhas the appearance of beads on a string• Each “bead” is a nucleosome, the basic unitof DNA packingLE 19-2aDNA double helixHistonetailsHis-tonesLinker DNA(“string”)Nucleosome(“bead”)10 nm2 nmHistone H1Nucleosomes (10-nm fiber)Higher Levels of DNA Packing• The next level of packing forms the 30-nm chromatin fiberLE 19-2b30 nmNucleosome30-nm fiber• In turn, the 30-nm fiber forms loopeddomains, making up a 300-nm fiberLE 19-2c300 nmLoopsScaffoldProtein scaffoldLooped domains (300-nm fiber)• In a mitotic chromosome, the looped domainscoil and fold, forming the metaphasechromosomeLE 19-2dMetaphase chromosome700 nm1,400 nm• Interphase chromatin is usually much lesscondensed than that of mitotic chromosomes• Much of the interphase chromatin is present as a10-nm fiber, and some is 30-nm fiber, which insome regions is folded into looped domains• Interphase chromosomes have highly condensedareas, called heterochromatin, and lesscompacted areas, called euchromatinConcept 19.2: Gene expressioncan be regulated at any stage,but the key step is transcription• All organisms must regulate which genes areexpressed at any given time• A multicellular organism’s cells undergo celldifferentiation, specialization in form and functionDifferential Gene Expression• Differences between cell types result fromdifferential gene expression, the expression ofdifferent genes by cells within the same genome• In each type of differentiated cell, a unique subsetof genes is expressed• Many key stages of gene expression can beregulated in eukaryotic cellsLE 19-3SignalNUCLEUSDNARNAChromatinGene availablefor transcriptionGeneExonIntroTranscriptionPrimary transcriptRNA processingCapTailmRNA in nucleusTransport to cytoplasmCYTOPLASMmRNA in cytoplasmTranslationDegradationof mRNAPolypeptideCleavageChemical modificationTransport to