Bài giảng Biology - Chapter 16: The Molecular Basis of Inheritance

Chapter 16The Molecular Basis of InheritanceOverview: Life’s Operating InstructionsIn 1953, James Watson and Francis Crick shook the worldWith an elegant double-helical model for the structure of deoxyribonucleic acid, or DNAFigure 16.1DNA, the substance of inheritanceIs the most celebrated molecule of our timeHereditary informationIs encoded in the chemical language of DNA and reproduced in all the cells of your bodyIt is the DNA programThat directs the development of many different types of traitsConcept 16.1: DNA is the genetic materialEarly in the 20th centuryThe identification of the molecules of inheritance loomed as a major challenge to biologistsThe Search for the Genetic Material: Scientific InquiryThe role of DNA in heredityWas first worked out by studying bacteria and the viruses that infect themEvidence That DNA Can Transform BacteriaFrederick Griffith was studying Streptococcus pneumoniaeA bacterium that causes pneumonia in mammalsHe worked with two strains of the bacteriumA pathogenic strain and a nonpathogenic strainGriffith found that when he mixed heat-killed remains of the pathogenic strainWith living cells of the nonpathogenic strain, some of these living cells became pathogenic Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below: Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by anunknown, heritable substance from the dead S cells.EXPERIMENTRESULTSCONCLUSIONLiving S(control) cellsLiving R(control) cellsHeat-killed(control) S cellsMixture of heat-killed S cellsand living R cellsMouse diesMouse healthyMouse healthyMouse diesLiving S cellsare found inblood sample.Figure 16.2Griffith called the phenomenon transformationNow defined as a change in genotype and phenotype due to the assimilation of external DNA by a cellEvidence That Viral DNA Can Program CellsAdditional evidence for DNA as the genetic materialCame from studies of a virus that infects bacteriaViruses that infect bacteria, bacteriophagesAre widely used as tools by researchers in molecular geneticsFigure 16.3PhageheadTailTail fiberDNABacterialcell100 nmAlfred Hershey and Martha ChasePerformed experiments showing that DNA is the genetic material of a phage known as T2The Hershey and Chase experiment In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells.Radioactivity(phage protein)in liquidPhageBacterial cellRadioactiveproteinEmptyprotein shellPhageDNADNACentrifugePellet (bacterialcells and contents)RadioactiveDNACentrifugePelletBatch 1: Phages weregrown with radioactivesulfur (35S), which wasincorporated into phageprotein (pink).Batch 2: Phages weregrown with radioactivephosphorus (32P), which was incorporated into phage DNA (blue).1234Agitated in a blender toseparate phages outsidethe bacteria from thebacterial cells.Mixed radioactivelylabeled phages withbacteria. The phagesinfected the bacterial cells.Centrifuged the mixtureso that bacteria formeda pellet at the bottom ofthe test tube.Measured theradioactivity inthe pellet and the liquid Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material.RESULTSCONCLUSIONEXPERIMENTRadioactivity(phage DNA)in pelletFigure 16.4Additional Evidence That DNA Is the Genetic MateriaPrior to the 1950s, it was already known that DNAIs a polymer of nucleotides, each consisting of three components: a nitrogenous base, a sugar, and a phosphate groupSugar-phosphatebackboneNitrogenousbases5 endO–OPOCH254O–HHOHHH31HOCH3NONHThymine (T)OOPOO–CH2HHOHHHHNNNHNHHAdenine (A)OOPOO–CH2HHOHHHHHHHNNNOCytosine (C)OOPOCH254O–HOHH31OH2HNNNHONNHHHHSugar (deoxyribose)3 endPhosphateGuanine (G)DNA nucleotide2NFigure 16.5Erwin Chargaff analyzed the base composition of DNAFrom a number of different organismsIn 1947, Chargaff reportedThat DNA composition varies from one species to the nextThis evidence of molecular diversity among speciesMade DNA a more credible candidate for the genetic materialBuilding a Structural Model of DNA: Scientific InquiryOnce most biologists were convinced that DNA was the genetic materialThe challenge was to determine how the structure of DNA could account for its role in inheritanceMaurice Wilkins and Rosalind FranklinWere using a technique called X-ray crystallography to study molecular structureRosalind FranklinProduced a picture of the DNA molecule using this technique(a) Rosalind FranklinFranklin’s X-ray diffractionPhotograph of DNA(b)Figure 16.6 a, bFigure 16.7a, cCTAATCGGCACGATATATTACTA0.34 nm3.4 nm(a) Key features of DNA structureG1 nmG(c) Space-filling modelTWatson and Crick deduced that DNA was a double helix Through observations of the X-ray crystallographic images of DNAFranklin had concluded that DNAWas composed of two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interiorThe nitrogenous basesAre paired in specific combinations: adenine with thymine, and cytosine with guanineO–OOOHO–OOOH2CO–OOOH2CO–OOOOHOOOTACGCATOOOCH2OO–OOCH2CH2CH25 endHydrogen bond3 end3 endGPPPPOOHO–OOOPPO–OOOPO–OOOP(b) Partial chemical structureH2C5 endFigure 16.7bOWatson and Crick reasoned that there must be additional specificity of pairingDictated by the structure of the basesEach base pair forms a different number of hydrogen bondsAdenine and thymine form two bonds, cytosine and guanine form three bondsNHOCH3NNONNNNHSugarSugarAdenine (A)Thymine (T)NNNNSugarOHNHNHNOHHNSugarGuanine (G)Cytosine (C)Figure 16.8HConcept 16.2: Many proteins work together in DNA replication and repairThe relationship between structure and functionIs manifest in the double helixThe Basic Principle: Base Pairing to a Template StrandSince the two strands of DNA are complementaryEach strand acts as a template for building a new strand in replicationIn DNA replicationThe parent molecule unwinds, and two new daughter strands are built based on base-pairing rules(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.(b) The first step in replication is  separation of the two DNA  strands.(c) Each parental strand now  serves as a template that  determines the order of  nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands.  Each “daughter” DNA  molecule consists of one parental strand and one new strand.ACTAGACTAGACTAGACTAGTGATCTGATCACTAGACTAGTGATCTGATCTGATCTGATCFigure 16.9 a–dFigure 16.10 a–cConservativemodel. The twoparental strandsreassociate after acting astemplates fornew strands,thus restoringthe parentaldouble helix.Semiconservativemodel. The two strands of the parental moleculeseparate, and each functionsas a templatefor synthesis ofa new, comple-mentary strand.Dispersivemodel. Eachstrand of bothdaughter mol-ecules containsa mixture ofold and newlysynthesizedDNA.Parent cellFirstreplicationSecondreplicationDNA replication is semiconservativeEach of the two new daughter molecules will have one old strand, derived from the parent molecule, and one newly made strand(a)(b)(c)Experiments performed by Meselson and StahlSupported the semiconservative model of DNA replicationFigure 16.11 Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15N. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15N medium. Meselson and Stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria.EXPERIMENTThe bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask in step 2, one sample taken after 20 minutes and one after 40 minutes.RESULTSBacteriacultured inmediumcontaining15NBacteriatransferred tomediumcontaining14N21DNA samplecentrifugedafter 20 min(after firstreplication)3DNA samplecentrifugedafter 40 min(after secondreplication)4LessdenseMoredenseCONCLUSION Meselson and Stahl concluded that DNA replication follows the semiconservative model by comparing their result to the results predicted by each of the three models in Figure 16.10. The first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminated the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated the dispersive model and supported the semiconservative model.First replicationSecond replicationConservativemodelSemiconservativemodelDispersivemodelDNA Replication: A Closer LookThe copying of DNAIs remarkable in its speed and accuracyMore than a dozen enzymes and other proteinsParticipate in DNA replicationGetting Started: Origins of ReplicationThe replication of a DNA moleculeBegins at special sites called origins of replication, where the two strands are separatedA eukaryotic chromosomeMay have hundreds or even thousands of replication originsReplication begins at specific siteswhere the two parental strandsseparate and form replicationbubbles.The bubbles expand laterally, asDNA replication proceeds in bothdirections.Eventually, the replicationbubbles fuse, and synthesis ofthe daughter strands iscomplete.123Origin of replicationBubbleParental (template) strandDaughter (new) strandReplication forkTwo daughter DNA moleculesIn eukaryotes, DNA replication begins at many sites along the giantDNA molecule of each chromosome.In this micrograph, three replicationbubbles are visible along the DNA ofa cultured Chinese hamster cell (TEM).(b)(a)0.25 µmFigure 16.12 a, bFigure 16.13New strandTemplate strand5 end3 endSugarATBaseCGGCACTPPPOHPP5 end3 end5 end5 endATCGGCACT3 endPyrophosphate2 POHPhosphateElongating a New DNA StrandElongation of new DNA at a replication forkIs catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3 end of a growing strandNucleosidetriphosphateAntiparallel ElongationHow does the antiparallel structure of the double helix affect replication?DNA polymerases add nucleotidesOnly to the free 3end of a growing strandAlong one template strand of DNA, the leading strandDNA polymerase III can synthesize a complementary strand continuously, moving toward the replication forkTo elongate the other new strand of DNA, the lagging strandDNA polymerase III must work in the direction away from the replication forkThe lagging strandIs synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligaseParental DNA DNA pol Ill elongatesDNA strands only in the5 3 direction.1OkazakifragmentsDNA pol IIITemplatestrandLagging strand32TemplatestrandDNA ligaseOverall direction of replication One new strand, the leading strand,can elongate continuously 5 3 as the replication fork progresses.2 The other new strand, thelagging strand must grow in an overall3 5 direction by addition of shortsegments, Okazaki fragments, that grow5 3 (numbered here in the orderthey were made).3 DNA ligase joins Okazakifragments by forming a bond betweentheir free ends. This results in a continuous strand.4Figure 16.1435533521Leading strand1Synthesis of leading and lagging strands during DNA replicationPriming DNA SynthesisDNA polymerases cannot initiate the synthesis of a polynucleotideThey can only add nucleotides to the 3 endThe initial nucleotide strandIs an RNA or DNA primerOnly one primer is needed for synthesis of the leading strandBut for synthesis of the lagging strand, each Okazaki fragment must be primed separately Overall direction of replication33335353535353535355112112551235TemplatestrandRNA primerOkazakifragmentFigure 16.15Primase joins RNA nucleotides into a primer.1DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.2After reaching the next RNA primer (not shown), DNA pol III falls off.3After the second fragment is primed. DNA pol III adds DNAnucleotides until it reaches the first primer and falls off.4DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2.5DNA ligase forms a bond between the newest DNAand the adjacent DNA of fragment 1.6The lagging strand in this region is nowcomplete.7Other Proteins That Assist DNA ReplicationHelicase, topoisomerase, single-strand binding proteinAre all proteins that assist DNA replicationTable 16.1Figure 16.16Overall direction of replicationLeadingstrandLaggingstrandLaggingstrandLeadingstrandOVERVIEWLeadingstrandReplication forkDNA pol IIIPrimasePrimerDNA pol IIILaggingstrandDNA pol IParental DNA53432Origin of replicationDNA ligase153 Helicase unwinds theparental double helix.1 Molecules of single-strand binding proteinstabilize the unwoundtemplate strands.2 The leading strand issynthesized continuously in the5 3 direction by DNA pol III.3 Primase begins synthesisof RNA primer for fifthOkazaki fragment.4 DNA pol III is completing synthesis ofthe fourth fragment, when it reaches theRNA primer on the third fragment, it willdissociate, move to the replication fork,and add DNA nucleotides to the 3 endof the fifth fragment primer.5 DNA pol I removes the primer from the 5 endof the second fragment, replacing it with DNAnucleotides that it adds one by one to the 3 endof the third fragment. The replacement of thelast RNA nucleotide with DNA leaves the sugar-phosphate backbone with a free 3 end.6 DNA ligase bondsthe 3 end of thesecond fragment tothe 5 end of the firstfragment.7A summary of DNA replicationThe DNA Replication Machine as a Stationary ComplexThe various proteins that participate in DNA replicationForm a single large complex, a DNA replication “machine”The DNA replication machineIs probably stationary during the replication processProofreading and Repairing DNADNA polymerases proofread newly made DNAReplacing any incorrect nucleotidesIn mismatch repair of DNARepair enzymes correct errors in base pairingFigure 16.17NucleaseDNApolymeraseDNAligase A thymine dimerdistorts the DNA molecule.1 A nuclease enzyme cutsthe damaged DNA strandat two points and thedamaged section isremoved.2 Repair synthesis bya DNA polymerasefills in the missingnucleotides.3 DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.4In nucleotide excision repairEnzymes cut out and replace damaged stretches of DNAReplicating the Ends of DNA MoleculesThe ends of eukaryotic chromosomal DNAGet shorter with each round of replicationFigure 16.18End of parentalDNA strandsLeading strandLagging strandLast fragmentPrevious fragmentRNA primerLagging strandRemoval of primers andreplacement with DNAwhere a 3 end is availablePrimer removed butcannot be replacedwith DNA becauseno 3 end availablefor DNA polymeraseSecond roundof replicationNew leading strandNew lagging strand 5Further roundsof replicationShorter and shorterdaughter molecules535353533Eukaryotic chromosomal DNA moleculesHave at their ends nucleotide sequences, called telomeres, that postpone the erosion of genes near the ends of DNA moleculesFigure 16.191 µmIf the chromosomes of germ cells became shorter in every cell cycleEssential genes would eventually be missing from the gametes they produceAn enzyme called telomeraseCatalyzes the lengthening of telomeres in germ cells