Presentation on theme: "The Molecular Basis of Inheritance"— Presentation transcript:
1 The Molecular Basis of Inheritance Chapter 16The Molecular Basis of Inheritance
2 Figure 16.1 Watson and Crick with their DNA model
3 Figure 16.2 Can the genetic trait of pathogenicity be transferred between bacteria? Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because theyhave a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsuleand 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 RHeat-killed(control) S cellsMixture of heat-killed S cellsand living R cellsMouse diesMouse healthyLiving S cellsare found inblood sample.1922 GriffithTransformation, a change in genotype and phenotype due to the assimilation of foreign DNA by a cell.In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transformingsubstance was DNA.
5 Figure 16.4 Is DNA or protein the genetic material of phage T2? In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfurand phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells.EXPERIMENTRadioactivity(phage protein)in liquidPhageBacterial cellRadioactiveproteinEmptyprotein shellDNACentrifugePellet (bacterialcells and contents)Pellet(phage DNA)in pelletBatch 1: Phages weregrown with radioactivesulfur (35S), which wasincorporated into phageprotein (pink).Batch 2: Phages werephosphorus (32P), whichwas incorporated intophage 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 andthe liquid
6 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.RESULTSCONCLUSION
7 Figure 16.5 The structure of a DNA strand Sugar-phosphatebackboneNitrogenousbases5 endO–OPCH254H31CH3NThymine (T)Adenine (A)Cytosine (C)OH2Sugar (deoxyribose)3 endPhosphateGuanine (G)DNA nucleotide
8 Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA (a) Rosalind FranklinFranklin’s X-ray diffractionPhotograph of DNA(b)
9 Figure 16.7 The double helix OHPH2CTACGCH2O–5 endHydrogen bond3 end0.34 nm3.4 nm(a) Key features of DNA structure(b) Partial chemical structure(c) Space-filling model1 nm
10 Unnumbered Figure p. 298 Purine + Purine: too wide Pyrimidine + pyrimidine: too narrowPurine + pyrimidine: widthConsistent with X-ray dataChargaff’s rules (1947)In all organisms, the number of adenines was approximately equal to the number of thymines (%T = %A).Also (%G = %C).Erwin Chargaff
11 Figure 16.8 Base pairing in DNA HNHOCH3SugarAdenine (A)Thymine (T)Guanine (G)Cytosine (C)
12 Figure 16.9 A model for DNA replication: the basic concept (layer 1) (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.ACTG
13 Figure 16.9 A model for DNA replication: the basic concept (layer 2) (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.ACTG
14 Figure 16.9 A model for DNA replication: the basic concept (layer 3) (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.ACTG
15 Figure 16.9 A model for DNA replication: the basic concept (layer 4) (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.ACTGThe second paper of Watson and Crick
16 Figure 16.10 Three alternative models of DNA replication Conservativemodel. The twoparental strandsreassociateafter acting astemplates fornew strands,thus restoringthe parentaldouble helix.(a)Semiconserva-tive model.The two strandsof the parentalmoleculeseparate,and each functionsas a templatefor synthesis ofa new, comple-mentary strand.(b)Dispersivemodel. Eachstrand of bothdaughter mol-ecules containsa mixture ofold and newlysynthesizedDNA.(c)Parent cellFirstreplicationSecond
17 Figure 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model? Late 50sMatthew Meselson and Franklin Stahl cultured E. coli bacteria for several generationson a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15N. The bacteriaincorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium withonly 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would belighter than the parental DNA made in the 15N medium. Meselson and Stahl could distinguish DNA of differentdensities by centrifuging DNA extracted from the bacteria.EXPERIMENTThe bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flaskin step 2, one sample taken after 20 minutes and one after 40 minutes.RESULTSBacteriacultured inmediumcontaining15Ntransferred to14N21DNA samplecentrifugedafter 20 min(after firstreplication)3after 40 min(after second4LessdenseMore
18 CONCLUSIONMeselson and Stahl concluded that DNA replication follows the semiconservativemodel by comparing their result to the results predicted by each of the three models in FigureThe first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminatedthe conservative model. A second replication produced both light and hybrid DNA, a result that eliminatedthe dispersive model and supported the semiconservative model.First replicationSecond replicationConservativemodelSemiconservativeDispersive
19 Figure 16.12 Origins of replication in eukaryotes Replication 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 µmE. coli: 5 million/25 minHuman cell: 6 billion/few hoursThe rate of elongation is about 500 nucleotides per second in bacteriaand 50 per second in human cells
20 Figure 16.13 Incorporation of a nucleotide into a DNA strand Why antiparallel?New strandTemplate strand5’ end3’ endSugarATBaseCGPOHNucleosidetriphosphatePyrophosphate2 PPhosphateThe exergonic hydrolysis of pyrophosphate to two inorganic phosphate molecules drives the polymerization of the nucleotide to the new strand.
21 Overall direction of replication Figure 16.14 Synthesis of leading and lagging strands during DNA replicationParental DNADNA pol Ill elongatesDNA strands only in the5 direction.3521OkazakifragmentsDNA pol IIITemplatestrandLeading strandLagging strand3DNA ligaseOverall direction of replicationOne new strand, the leading strand,can elongate continuously 5 as the replication fork progresses.The other new strand, thelagging strand must grow in an overall3 direction by addition of shortsegments, Okazaki fragments, that grow5 (numbered here in the orderthey were made).DNA ligase joins Okazakifragments by forming a bond betweentheir free ends. This results in acontinuous strand.4
22 Figure 16.15 Synthesis of the lagging strand Overall direction of replication3512DNA ligase forms a bond between the newest DNA and the adjacent DNA of fragment 1.6The lagging strand in this region is now complete.7DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2.5After the second fragment is primed. DNA pol III adds DNA nucleotides until it reaches the first primer and falls off.4After reaching the next RNA primer (not shown), DNA pol III falls off.3DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.Primase joins RNA nucleotides into a primer.Template strandRNA primerOkazaki fragment
23 Table 16.1 Bacterial DNA replication proteins and their functions
24 Figure 16.16 A summary of bacterial DNA replication Overall direction of replicationHelicase unwinds theparental double helix.Molecules of single-strand binding proteinstabilize the unwoundtemplate strands.The leading strand issynthesized continuously in the5 3 direction by DNA pol III.LeadingstrandOrigin of replicationLaggingOVERVIEWReplication forkDNA pol IIIPrimasePrimerDNA pol IDNA ligase123Primase begins synthesisof RNA primer for fifthOkazaki fragment.4DNA 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 end of the fifth fragment primer.5DNA 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.6DNA ligase bondsthe 3 end of thesecond fragment tothe 5 end of the firstfragment.7Parental DNA53
25 Arthur Kornberg (born March 3, 1918) is an American biochemist who won the Nobel Prize in Physiology or Medicine 1959 for his discovery of "the mechanisms in the biological synthesis of deoxyribonucleic acid (DNA)" together with Dr. Severo Ochoa of New York University.Identified “DNA polymerase”
26 Reiji Okazaki (岡崎令治 Okazaki Reiji, 1930 – 1975) was a Japanese molecular biologist known for his research in the DNA replication and the discovery of Okazaki fragments.Reiji Okazaki was born in Hiroshima, Japan. He graduated in 1953 from Nagoya University, and worked as a professor there after 1963.Okazaki died of leukemia only a few years after his discovery. He was exposed to heavy radiation when he was in Hiroshima when the first atomic bomb was dropped.Tsuneko Okazaki (wife and collaborator)
27 Figure 16.17 Nucleotide excision repair of DNA damage NucleaseDNApolymeraseligaseA thymine dimerdistorts the DNA molecule.1Repair synthesis bya DNA polymerasefills in the missingnucleotides.3DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.4A nuclease enzyme cutsthe damaged DNA strandat two points and thedamaged section isremoved.2Mutations are caused byEnzymatic mistakesHigh energy radiation (X-ray, UV)Spontaneous changesDefects in repair cause cancers (i.e. skin cancer)
28 Figure 16.18 Shortening of the ends of linear DNA molecules End of parentalDNA strandsLeading strandLagging strandLast fragmentPrevious fragmentRNA primerRemoval 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 5Further roundsShorter and shorterdaughter molecules53
30 4. The ends of DNA molecules are replicated by a special mechanism. The ends of eukaryotic chromosomal DNA molecules, the telomeres, have special nucleotide sequences.Multiple repetitions of one short nucleotide sequence. TTAGGG in humanTelomerase catalyzes the lengthening of telomeres in eukaryotic germ cells, restoring their original length.Telomerase is not present in most cells of multicellular organisms.Normal shortening of telomeres may protect organisms from cancer by limiting the number of divisions that somatic cells can undergo.Short telomeres in Werner syndromeActive telomerase has been foundin some cancerous somatic cells.Telomerase may provide a usefultarget for cancer diagnosis and chemotherapy.
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