I. DNA Structure and Genetic Information6.1Macromolecules and Genes6.2The Double Helix6.3Supercoiling6.4Chromosomes and Other Genetic Elements

6.1 Macromolecules and GenesFunctional unit of genetic information is the geneGenes are in cells, and are composed of DNA

6.1 Macromolecules and Genes Three informational macromolecules in cellDNARNAProtein

6.1 Macromolecules and Genes Genetic information flow can be divided into three stagesReplication: DNA is duplicated (Figure 6.3)Transcription: information from DNA is transferred to RNAmRNA (messenger RNA): encodes polypeptidestRNA (transfer RNA): plays role in protein synthesisrRNA (ribosomal RNA): plays role in protein synthesisTranslation: information in RNA is used to build polypeptides

Figure 6.3DNARNAProteinREPLICATIONTRANSCRIPTION OF BOTTOM STRANDTRANSLATION 2012 Pearson Education, Inc.

6.1 Macromolecules and GenesCentral dogma of molecular biologyDNA to RNA to proteinEukaryotes: each gene is transcribed individuallyProkaryotes: multiple genes may be transcribed together

6.2 The Double HelixFour nucelotides found in DNA (Figure 6.1):Adenine (A) Guanine (G) Cytosine (C) Thymine (T)Backbone of DNA chain is alternating phosphates and the pentose sugar deoxyribosePhosphates connect 3-carbon of one sugar to 5-carbon of the adjacent sugar

Figure 6.1Purine basesPyrimidine basesCytosine (C)Thymine (T)Uracil (U)Adenine (A)Guanine (G)PhosphateRiboseH only in DNA5 position3 positionPhosphodiester bondNitrogen base attached to 1 positionDeoxyribose 2012 Pearson Education, Inc.

6.2 The Double HelixAll cells and some viruses have DNA in double- stranded molecule (Figure 6.4)Two strands are antiparallelTwo strands have complementary base sequencesAdenine always pairs with ThymineGuanine always pairs with CytosineTwo strands form a double helix (Figure 6.5)

Figure 6.45-Phosphate3-HydroxylHydrogen bondsPhosphodiester bond5-Phosphate3-Hydroxyl 2012 Pearson Education, Inc.

Figure 6.5Sugar phosphate backboneOne helical turn (10 base pairs)Minor grooveMajor groove3.4 nm 2012 Pearson Education, Inc.

6.2 The Double HelixSize of DNA molecule is expressed in base pairs1,000 base pairs = 1 kilobase pairs = 1 kbp1,000,000 base pairs = 1 megabase pairs = 1MbpE. coli genome = 4.64 MbpEach base pair takes up 0.34 nm of length along the helix10 base pairs make up 1 turn of the helix

6.2 The Double HelixInverted Repeats (Figure 6.6)Repeated sequence that is arranged in an inverse orientationStem Loops (Figure 6.6)Short double-helical regions caused by nearby inverted repeatsCommon in RNA, but not DNA

Figure 6.6Inverted repeatsLoopStemStemloop structure 2012 Pearson Education, Inc.

6.2 The Double HelixHydrogen bonds between DNA strands hold two strands together AdenineThymine pair has two hydrogen bonds and GuanineCytosine pair has three hydrogen bondsGC pairs are stronger than AT pairsHigh heat breaks hydrogen bonds, causing denaturation (melting; Figure 6.7)GC-rich DNA melts at higher temperatures than AT-rich DNA

Figure 6.7Single strandsMeltingTm 85.0Double strand1.21.00.872768084889296CA260 2012 Pearson Education, Inc.

6.3 SupercoilingSupercoiled DNA: DNA is further twisted to save space (Figure 6.8)Negative supercoiling: double helix is underwoundPositive supercoiling: double helix is overwoundRelaxed DNA: DNA has number of turns predicted by number of base pairsNegative supercoiling is predominantly found in natureDNA Gyrase: introduces supercoils into DNA (Figure 6.9)

Figure 6.8Relaxed, covalently closed circular DNASupercoiled circular DNARelaxed, nicked circular DNAChromosomal DNA with supercoiled domainsProteinsSupercoiled domainBreak one strandSealBreak one strandRotate one end of broken strand around helix and sealNick 2012 Pearson Education, Inc.

Figure 6.9Relaxed circleSupercoiled DNADNA gyrase makes double-strand breakDouble-strand break resealedOne part of circle is laid over the otherHelix makes contact in two placesUnbroken helix is passed through the breakFollowing DNA gyrase activity, two negative supercoils result 2012 Pearson Education, Inc.

6.4 Chromosomes and Other Genetic ElementsGenome: entire complement of genes in cell or virusChromosome: main genetic element in prokaryotesOther genetic elements include virus genomes, plasmids, organellar genomes, and transposable elements

6.4 Chromosomes and Other Genetic Elements Viruses contain either RNA or DNA genomesCan be linear or circularCan be single or double strandedPlasmids: replicate separately from chromosomeGreat majority are double strandedMost are circularGenerally beneficial for the cell (e.g., antibiotic resistance)NOT extracellular, unlike viruses

6.4 Chromosomes and Other Genetic Elements Chromosome is a genetic element with housekeeping genesPresence of essential genes is necessary for a genetic element to be called a chromosomePlasmid is a genetic element that is expendable and rarely contains genes for growth under all conditions

6.4 Chromosomes and Other Genetic Elements Transposable ElementsSegment of DNA that can move from one site to another site on the same or a different DNA moleculeInserted into other DNA moleculesThree main types:Insertion sequencesTransposonsSpecial viruses

II. Chromosomes and Plasmids6.5 The Escherichia coli Chromosome6.6 Plasmids: General Principles6.7 The Biology of Plasmids

6.5 The Escherichia coli ChromosomeEscherichia coli is a useful model organism for the study of biochemistry, genetics, and bacterial physiology The E. coli chromosome from strain MG1655 has been mapped using conjugation, transduction, molecular cloning, and sequencing (Figure 6.10)

Figure 6.10Origin of replicationlac operon (lactose degradation)trp operon (tryptophan biosynthesishis operon (histidine biosynthesis)Not1 restriction sites, in kbpEscherichia coli K-12100/0908070605040302010HfrHP804HfrCHfr44KL14 2012 Pearson Education, Inc.

6.5 The Escherichia coli ChromosomeSome features of the E. coli chromosomeMany genes encoding enzymes of a single biochemical pathway are clustered into operonsOperons equally distributed on both strands~5 Mbp in size~40% of predicted proteins are of unknown functionAverage protein contains ~300 amino acidsInsertion sequences (IS elements)

6.6 Plasmids: General PrinciplesPlasmids: genetic elements that replicate independently of the host chromosome (Figure 6.11)Small circular or linear DNA molecules Range in size from 1 kbp to >1 Mbp; typically less than 5% of the size of the chromosomeCarry a variety of nonessential, but often very helpful, genesAbundance (copy number) is variable

Figure 6.11 2012 Pearson Education, Inc.

6.6 Plasmids: General PrinciplesA cell can contain more than one plasmid, but it cannot be closely related genetically due to plasmid incompatibilityMany incompatibility (Inc) groups recognizedPlasmids belonging to same Inc group exclude each other from replicating in the same cell but can coexist with plasmids from other groups

6.6 Plasmids: General PrinciplesSome plasmids (episomes) can integrate into the cell chromosome; similar to situation seen with prophagesRemoval (curing) plasmids from host cells can result from various treatmentsConjugative plasmids can be transferred between suitable organisms via cell-to-cell contactConjugal transfer controlled by tra genes on plasmid

6.7 The Biology of PlasmidsGenetic information encoded on plasmids is not essential for cell function under all conditions but may confer a selective growth advantage under certain conditions

6.7 The Biology of PlasmidsR plasmidsResistance plasmids; confer resistance to antibiotics and other growth inhibitors (Figure 6.12)Widespread and well-studied group of plasmidsMany are conjugative

Figure 6.12Replication functionsmersulstrcattetoriTtraIS1IS1IS10IS10Tn1094.3/0 kbp75 kbp25 kbp50 kbp 2012 Pearson Education, Inc.

6.7 The Biology of PlasmidsIn several pathogenic bacteria, virulence characteristics are encoded by plasmid genesVirulence factorsEnables pathogen to colonizeEnables pathogen to cause host damageHemolysinEnterotoxin

6.7 The Biology of PlasmidsBacteriocinsProteins produced by bacteria that inhibit or kill closely related species or even different strains of the same speciesColicin, nisinGenes encoding bacteriocins are often carried on plasmids

III. DNA Replication6.8 Templates and Enzymes6.9 The Replication Fork6.10 Bidirectional Replication and the Replisome6.11 The Polymerase Chain Reaction (PCR)

6.8 Templates and Enzymes DNA replication is semiconservative (Figure 6.13)Each of the two progeny double helices have one parental and one new strand Precursor of each nucleotide is a deoxynucleoside 5-triphosphate (dNTP; Figure 6.14)Replication ALWAYS proceeds from the 5 end to the 3 endAnimation: DNA Replication: Synthesis

Figure 6.13Parental strandNew strandSemiconservative replication 2012 Pearson Education, Inc.

Figure 6.14Growing pointDeoxyribonucleoside triphosphateDNA polymerase activityBaseBaseBase 2012 Pearson Education, Inc.

6.8 Templates and Enzymes DNA polymerases catalyze the addition of dNTPsFive different DNA polymerases in E. coliDNA polymerase III is primary enzyme replicating chromosomal DNADNA polymerases require a primerPrimer made from RNA by primase

6.9 The Replication ForkDNA synthesis begins at the origin of replication in prokaryotesReplication fork: zone of unwound DNA where replication occurs (Figure 6.16)DNA helicase unwinds the DNAExtension of DNAOccurs continuously on the leading strand Discontinuously on the lagging strandOkazaki fragments are on lagging strand

Figure 6.16RNA primerPrimaseSingle-strand binding proteinDNA polymerase IIIRNA primerHelicaseFree 3-OHLagging strandLeading strand 2012 Pearson Education, Inc.

6.10 Bidirectional Replication and the ReplisomeDNA synthesis is bidirectional in prokaryotesTwo replication forks moving in opposite directions (Figure 6.20)DNA Pol III adds 1,000 nucleotides per secondReplisome: complex of multiple proteins involved in replication (Figure 6.22)DNA pulled through the replisome

Figure 6.20Replication forkMovement of forkReplication forkOriginOrigin (DnaA binding site)LaggingLaggingLeadingLeading 2012 Pearson Education, Inc.

Figure 6.22DNA polymerase IIINewly synthesized strandDNA helicaseDNA gyraseParental DNARNA primerDNA primaseSingle-strand DNA-binding proteinsRNA primerDNA polymerase IIINewly synthesized strandTauLeading strand templateLagging strand templateDirection of new synthesisDirection of new synthesis 2012 Pearson Education, Inc.

6.10 Bidirectional Replication and the ReplisomeDNA replication is extremely accurateProofreading helps to ensure high fidelityMutation rates in cells are 1081011 errors per base insertedPolymerase can detect mismatch through incorrect hydrogen bondingProofreading occurs in prokaryotes, eukaryotes, and viral DNA replication systems

6.11 The Polymerase Chain ReactionThe polymerase chain reaction (PCR) is basically DNA replication in a test tubeConceived by Kary MullisAlso called DNA amplification

6.11 The Polymerase Chain ReactionSteps in PCR (Figure 6.24)Add DNAAdd primerAdd DNA polymeraseTaq polymerase or Pfu polymeraseHeat and coolHeat and cool

Figure 6.24Target sequenceDNA polymeraseHeatPrimersPrimer extensionRepeat cycleRepeat cyclePCR cycleCopies of target sequence0112238Number of PCR cyclesCopies of target sequence108107106105104103102102 4 6 8 10 12 14 16 18 204 2012 Pearson Education, Inc.

6.11 The Polymerase Chain ReactionApplications of PCRPhylogenetic studiesSurveying different groups of environmental organismsAmplifying small amounts of DNAIdentifying a specific bacteriaLooking for a specific gene

IV. RNA Synthesis: Transcription6.12 Overview of Transcription6.13 Sigma Factors and Consensus Sequences6.14 Termination of Transcription6.15 The Unit of Transcription

6.12 Overview of TranscriptionTranscription (DNA to RNA) is carried out by RNA polymeraseRNA polymerase uses DNA as templateRNA precursors are ATP, GTP, CTP, and UTPChain growth is 5 to 3 just like DNA replication

6.12 Overview of TranscriptionOnly one of the two strands of DNA are transcribed by RNA polymerase for any geneGenes are present on both strands of DNA, but at different locationsRNA polymerase has five different subunitsRNA polymerase recognizes DNA sites called promoters

6.12 Overview of TranscriptionPromoters: site of initiation of transcriptionRecognized by sigma factor of RNA polymeraseTranscription stops at specific sites called transcription terminatorsUnlike DNA replication, transcription involves smaller units of DNAOften as small as a single geneAllows cell to transcribe different genes at different rates

Figure 6.25Sigma factorRNA polymerase (core enzyme)Promoter regionGene(s) to be transcribed light green strand)SigmaRNADNAShort transcriptsLonger transcriptsSigma recognizes promoter and initiation siteTranscription begins; sigma released. RNA chain growsTermination site reached; chain growth stopsPolymerase and RNA released 2012 Pearson Education, Inc.

Transcription: The ProcessAnimation: The Process of Transcription

6.13 Sigma Factors and Consensus SequencesSigma factors recognize two highly conserved regions of promoter (Figure 6.26)Two regions within promoters that are highly conserved:Pribnow box: located 10 bases before the start of transcription (10 region)35 region: located ~35 bases upstream of transcription

Figure 6.26RNA polymerase (core enzyme)TranscriptionSigmamRNA start35 sequencePribnow boxConsensusPromoter sequence1.2.3.4.5.6. 2012 Pearson Education, Inc.

6.14 Termination of TranscriptionTermination of RNA synthesis is governed by a specific DNA sequenceIntrinsic terminators: transcription is terminated without any additional factorsRho-dependent termination: Rho protein recognizes specific DNA sequences and causes a pause in the RNA polymerase

6.15 The Unit of TranscriptionUnit of transcription: unit of chromosome bounded by sites where transcription of DNA to RNA is initiated and terminatedMost genes encode proteins, but some RNAs are not translated (i.e., rRNA, tRNA)Three types of rRNA: 16S, 23S, and 5SrRNA and tRNA are very stabletRNA cotranscribed with rRNA or other tRNAmRNAs have short half-lives (a few minutes)

6.15 The Unit of TranscriptionProkaryotes often have genes clustered togetherThese genes are transcribed all at once as a single mRNAAn mRNA encoding a group of cotranscribed genes is called a polycistronic mRNAOperon: a group of related genes cotranscribed on a polycistronic mRNA Allows for expression of multiple genes to be coordinated

V. Protein Structure and Synthesis6.16 Polypeptides, Amino Acids, and the Peptide Bond6.17 Translation and the Genetic Code6.18 Transfer RNA6.19 Steps in Protein Synthesis6.20 The Incorporation of Selenocysteine and Pyrrolysine6.21 Folding and Secreting Proteins

6.16 Polypeptides, Amino Acids, and the Peptide BondProteins play a major role in cell functionCatalytic proteins (enzymes)Structural proteinsProteins are polymers of amino acidsAmino acids are linked by peptide bonds to form a polypeptide (Figure 6.30)

Figure 6.30N-terminusC-terminusPeptide bond 2012 Pearson Education, Inc.

6.16 Polypeptides, Amino Acids, and the Peptide BondThe linear array of amino acids in a polypeptide is called its primary structure The chemical properties of the amino acid are related to their side chain (Figure 6.29)The diversity of amino acids make possible an enormous number of unique proteins with different biochemical properties

Figure 6.29Amino group-CarbonCarboxylic acid groupGeneral structure of an amino acidStructure of the amino acid R groups(Note: Because proline lacks a free amino group, the entire structure of this amino acid is shown, not just the R group).Ionizable: acidicIonizable: basicNonionizable polarNonpolar (hydrophobic)Ser Serine (S)Thr Threonine (T)Asn Asparagine (N)Gln Glutamine (Q)Cys Cysteine (C)Sec Selenocysteine (U)Tyr Tyrosine (Y)Asp Aspartate (D)Glu Glutamate (E)Lys Lysine (K)Pyl Pyrrolysine (O)Arg Arginine (R)His Histidine (H)Gly Glycine (G)Ala Alanine (A)Val Valine (V)Leu Leucine (L)Ile Isoleucine (I)Met Methionine (M)Phe Phenylalanine(F)Trp Tryptophan(W)Pro Proline (P) 2012 Pearson Education, Inc.

6.17 Translation and the Genetic CodeTranslation: the synthesis of proteins from RNAGenetic code: a triplet of nucleic acid bases (codon) encodes a single amino acidSpecific codons for starting and stopping translationDegenerate code: multiple codons encode a single amino acidAnticodon on tRNA recognizes codonWobble: irregular base pairing allowed at third position of tRNA (Figure 6.31)

Figure 6.31Alanine tRNAKey bases in codon: anticodon pairingWobble position; base pairing more flexible heremRNACodonAnticodon 2012 Pearson Education, Inc.

6.17 Translation and the Genetic CodeStop codons: terminate translation (UAA, UAG, and UGA)Start codon: translation begins with AUGReading frame: triplet code requires translation to begin at the correct nucleotide (Figure 6.32)ShineDalgarno sequence: ensures proper reading frameOpen reading frame (ORF): AUG followed by a number of codons and a stop codon in the same reading frame

Figure 6.32mRNACorrectIncorrectIncorrect011 2012 Pearson Education, Inc.

6.17 Translation and the Genetic CodeCodon bias: multiple codons for the same amino acid are not used equallyVaries with organismCorrelated with tRNA availabilityCloned genes from one organism may not be translated by recipient organism because of codon biasSome organelles and a few cells have slight variations of the genetic code (e.g., mitochondria of animals, Mycoplasma, and Paramecium)

6.18 Transfer RNATransfer RNA: at least one tRNA per amino acidBacterial cells have 60 different tRNAsMammalian cells have 100110 different tRNAsSpecific for both a codon and its cognate amino acidtRNA and amino acid brought together by aminoacyl-tRNA synthetasesATP is required to attach amino acid to tRNAtRNA is cloverleaf in shape (Figure 6.33)

Figure 6.33Acceptor endAcceptor stemD loopTC loopAnticodon stemAnticodonCodonmRNAD loopTC loopAcceptor stemAcceptor endAnticodon stemAnticodonAnticodon loopAA mG 2012 Pearson Education, Inc.

6.18 Transfer RNAAnticodon: three bases of tRNA that recognize three complementary bases on mRNAFidelity of recognition process between tRNA and aminoacyl-tRNA synthetase is critical (Figure 6.34)Incorrect amino acid could result in a faulty or nonfunctioning protein

Figure 6.34Uncharged tRNA-specific for valine (tRNAVal)Anticodon regionAmino acid (valine)AMPAMPValineAminoacyl-tRNA synthetase for valineAnticodon looptRNA acceptor stemCharged valyl tRNA, ready for protein synthesisC A CLinkage of valine to tRNAValC A C 2012 Pearson Education, Inc.

6.19 Steps in Protein SynthesisRibosomes: sites of protein synthesisThousands of ribosomes per cellComposed of two subunits (30S and 50S in prokaryotes)S = Svedberg unitsCombination of rRNA and proteinE. coli has 52 distinct ribosomal proteins

6.19 Steps in Protein SynthesisTranslation (Figure 6.35) is broken down into three main steps: 1. Initiation: two ribosomal subunits assemble with mRNABegins at an AUG start codon 2. Elongation: amino acids are brought to the ribosome and are added to the growing polypeptideOccurs in the A and P sites of ribosomeTranslocation: movement of the tRNA holding the polypeptide from the A to the P site

6.19 Steps in Protein SynthesisSteps of Translation (contd)3. Termination: occurs when ribosome reaches a stop codonRelease factors (RF): recognize stop codon and cleave polypeptide from tRNARibosome subunits then dissociateSubunits free to form new initiation complex and repeat processPolysomes: a complex formed by ribosomes simultaneously translating mRNA (Figure 6.36)

Figure 6.35TRANSLATION: InitiationTRANSLATION: ElongationInitiator tRNAmRNARibosome binding site (RBS)Initiation complexGTPGTPGTPSmall 30S subunitP siteE siteA siteLarge 50S subunitSmall 30S subunitAdd large subunitE sitemRNAIncoming tRNAA siteCodon recognitionPeptide bond formationTranslocationCycle continues three timesP siteA siteGrowing polypeptideP siteP siteP siteP siteA siteA siteA site 2012 Pearson Education, Inc.

Figure 6.36mRNAGrowing polypeptideNearly finished polypeptide 2012 Pearson Education, Inc.

6.19 Steps in Protein SynthesisMany antibiotics inhibit translation by interacting with ribosomesStreptomycin, chloramphenicol, tetracycline, etc.Many antibiotics are specific for organisms from one or two domains (e.g., chloramphenicol is specific for Bacteria)Animation: The Process of Translation

6.20 The Incorporation of Selenocysteine and PyrrolysineUniversal genetic code encodes 20 amino acidsMore than 100 different amino acids have been found in proteinsMost are made through posttranslational modificationOthers are inserted during protein synthesis

6.21 Folding and Secreting ProteinsOnce formed, a polypeptide folds to form a more stable structure. Secondary structure Interactions of the R groups force the molecule to twist and fold in a certain way (Figure 6.38) Tertiary structure3-dimensional shape of polypeptide (Figure 6.39)Quaternary structureNumber and types of polypeptides that make a protein

Figure 6.38Amino terminusHydrogen bonds between distant amino acidsAmino acids in a polypeptideHydrogen bonds between nearby amino acids-Helix-Sheet 2012 Pearson Education, Inc.

Figure 6.39A chainB chain-Sheet-HelixInsulinRibonuclease 2012 Pearson Education, Inc.

6.21 Folding and Secreting ProteinsDenaturation When proteins are exposed to extremes of heat, pH, or certain chemicals Causes the polypeptide chain to unfold Destroys the secondary, tertiary, and/or quaternary structure of the proteinThe biological properties of a protein are usually lost when it is denatured

6.21 Folding and Secreting ProteinsMost polypeptides fold spontaneously into their active formSome require assistance from molecular chaperones or chaperonins for folding to occur (Figure 6.40)They only assist in the folding, are not incorporated into proteinCan also aid in refolding partially denatured proteins

Figure 6.40Improperly folded proteinProperly folded (active) proteinProperly folded (active) proteinADPADPATPATPDnaK DnaJGroELGroES 2012 Pearson Education, Inc.

6.21 Folding and Secreting ProteinsSignal sequences: found on proteins requiring transport from cell (Figure 6.41)1520 residues longFound at the beginning of the protein moleculeSignal the cells secretory system (Sec system)Prevent protein from completely folding

Figure 6.41Translational apparatusRibosomemRNAProteinSecAMembranePeriplasmSignal recognition particleMembrane secretion systemProtein contains signal sequenceProtein does not contain signal sequenceProtein secreted into periplasmProtein inserted into membrane 2012 Pearson Education, Inc.

6.21 Folding and Secreting ProteinsSecretion of Folded Proteins: The Tat SystemProteins that fold in the cytoplasm are exported by a transport system distinct from Sec, called the Tat protein export systemIronsulfur proteinsRedox proteins

*Figure 6.3 Synthesis of the three types of informational macromolecules.Figure 6.1 Components of the nucleic acids.Figure 6.4 DNA structure.Figure 6.5 A computer model of a short segment of DNA showing the overall arrangement of the double helix.Figure 6.6 Inverted repeats and the formation of a stemloop.Figure 6.7 Thermal denaturation of DNA.Figure 6.8 Supercoiled DNA.Figure 6.9 DNA gyrase.Figure 6.10 The chromosome of Escherichia coli strain K-12.Figure 6.11 The bacterial chromosome and bacterial plasmids, as seen in the electron microscope.Figure 6.12 Genetic map of the resistance plasmid R100.Figure 6.13 Overview of DNA replication.Figure 6.14 Extension of a DNA chain by adding a deoxyribonucleoside triphosphate at the 3 end.Figure 6.16 Events at the DNA replication fork.Figure 6.20 Dual replication forks in the circular chromosome.Figure 6.22 The replisome.Figure 6.24 The polymerase chain reaction (PCR).Figure 6.25 Transcription.Figure 6.26 The interaction of RNA polymerase with the promoter.Figure 6.30 Peptide bond formation.Figure 6.29 Structure of the 22 genetically encoded amino acids.Figure 6.31 The wobble concept.Figure 6.32 Possible reading frames in an mRNA.Figure 6.33 Structure of a transfer RNA.Figure 6.34 Aminoacyl-tRNA synthetase.Figure 6.35 The ribosome and protein synthesis.Figure 6.36 Polysomes.Figure 6.38 Secondary structure of polypeptides.Figure 6.39 Tertiary structure of polypeptides.Figure 6.40 The activity of molecular chaperones.Figure 6.41 Export of proteins via the major secretory system.