Chapter 16Additional Evidence That DNA Is the Genetic MaterialSlide 3Slide 4Slide 5Slide 6DNA replicationSemi-conservative replicationSlide 9Elongating a New DNA StrandSlide 11Slide 12Other Proteins That Assist DNA ReplicationSlide 14Proofreading and Repairing DNAReplicating the Ends of DNA MoleculesTelomeresYou should now be able to:PowerPoint Lectures for Biology, Eighth EditionNeil Campbell and Jane ReeceChapter 16Chapter 16The Molecular Basis of InheritanceAdditional Evidence That DNA Is the Genetic Material•Prior to the 1950s, it was already known that DNA–Is a polymer of nucleotides, each consisting of three components: a nitrogenous base, a sugar, and a phosphate groupSugar-phosphatebackboneNitrogenousbases5 endO–OPOCH254O–HHOHHH31HOCH3NONHThymine (T)OO POO–CH2HHOHHHHNNNHNHHAdenine (A)OOPOO–CH2HHOHHHHHHHNNNOCytosine (C)OOPOCH254O–HOHH31OH2HNNNHONNHHHHSugar (deoxyribose)3 endPhosphateGuanine (G)DNA nucleotide2NFigure 16.5•Maurice Wilkins and Rosalind Franklin–Were using a technique called X-ray crystallography to study molecular structure•Rosalind Franklin–Produced a picture of the DNA molecule using this technique(a) Rosalind FranklinFranklin’s X-ray diffractionPhotograph of DNA(b)Figure 16.6 a, b•In 1953, James Watson and Francis Crick shook the world–With an elegant double-helical model for the structure of deoxyribonucleic acid, or DNAFigure 16.1Figure 16.7a, cCTAATCGGCACGATATA TTACTA0.34 nm3.4 nm(a) Key features of DNA structureG1 nmG(c) Space-filling modelT•Watson and Crick deduced that DNA was a double helix –Through observations of the X-ray crystallographic images of DNAO–OOOHO–OOOH2CO–OOOH2CO–OOOOHOOOTACGCATOOOCH2OO–OOCH2CH2CH25 endHydrogen bond3 end3 endGPPPPOOHO–OOOPPO–OOOPO–OOOP(b) Partial chemical structureH2C5 endFigure 16.7bO•Each base pair forms a different number of hydrogen bonds–Adenine and thymine form two bonds, cytosine and guanine form three bondsNHOCH3NNONNNN HSugarSugarAdenine (A)Thymine (T)NNNNSugarOHNHNHNOHHNSugarGuanine (G)Cytosine (C)Figure 16.8HDNA replication–The 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–dSemi-conservative replicationConservativemodel. 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 cellFirstreplicationSecondreplication(a)(b)(c)•A eukaryotic chromosome–May 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 strand Template strand5 end3 endSugarATBaseCGGCACTPPPOHPP5 end3 end5 end 5 endATCGGCACT3 endPyrophosphate2 POHPhosphateElongating a New DNA Strand•Elongation of new DNA at a replication fork–Is catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3 end of a growing strandNucleosidetriphosphateParental 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 16.1435533521Leading strand1•Synthesis of leading and lagging strands during DNA replicationOverall direction of replication33335353535353535355112112551235TemplatestrandRNA 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 Replication•Helicase, topoisomerase, single-strand binding protein–Are all proteins that assist DNA replicationTable 16.1Figure 16.16Overall direction of replicationLeadingstrandLaggingstrandLaggingstrandLeadingstrandOVERVIEWLeadingstrandReplication forkDNA pol IIIPrimasePrimerDNA pol IIILaggingstrandDNA pol IParental DNA53432Origin of replicationDNA ligase153 Helicase unwinds theparental double helix.1 Molecules of single-strand binding proteinstabilize the unwoundtemplate strands.2 The leading strand issynthesized
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