DNA: The Carrier of Genetic Information
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Transcript of DNA: The Carrier of Genetic Information
DNA: DNA: The Carrier of Genetic The Carrier of Genetic
InformationInformation
Chapter 12 Chapter 12
Learning Objective 1Learning Objective 1
• What evidence was accumulated during What evidence was accumulated during the 1940s and early 1950s demonstrating the 1940s and early 1950s demonstrating that DNA is the genetic material?that DNA is the genetic material?
The Mystery of GenesThe Mystery of Genes
• Many early geneticists thought genes were Many early geneticists thought genes were proteinsproteins• Proteins are complex and variableProteins are complex and variable• Nucleic acids are simple moleculesNucleic acids are simple molecules
Evidence for DNAEvidence for DNA
• DNA (deoxyribonucleic acid)DNA (deoxyribonucleic acid)• TransformationTransformation experiments experiments
• DNA of one strain of bacteria can transfer DNA of one strain of bacteria can transfer genetic characteristics to related bacteriagenetic characteristics to related bacteria
Bacteriophage ExperimentsBacteriophage Experiments
• BacteriophageBacteriophage (virus) infects bacterium (virus) infects bacterium• only DNA from virus enters the cellonly DNA from virus enters the cell• virus reproduces and forms new viral particles virus reproduces and forms new viral particles
from DNA alonefrom DNA alone
KEY CONCEPTSKEY CONCEPTS
• Beginning in the 1920s, evidence began to Beginning in the 1920s, evidence began to accumulate that DNA is the hereditary accumulate that DNA is the hereditary materialmaterial
Learning Objective 2Learning Objective 2
• What questions did these classic What questions did these classic experiments address? experiments address? • Griffith’s transformation experimentGriffith’s transformation experiment• Avery’s contribution to Griffith’s workAvery’s contribution to Griffith’s work• Hershey–Chase experimentsHershey–Chase experiments
Griffith’s Transformation ExperimentGriffith’s Transformation Experiment
• Can a genetic trait be transmitted from one Can a genetic trait be transmitted from one bacterial strain to another? bacterial strain to another?
• Answer: YesAnswer: Yes
Griffith’s Transformation ExperimentGriffith’s Transformation Experiment
Fig. 12-1, p. 261
Experiment 1 Experiment 2 Experiment 3 Experiment 4
R cells injected
S cells injected
Heat-killed S cells injected
R cells and heat-killed S cells injected
Mouse lives Mouse dies Mouse lives Mouse dies
Animation: Griffith’s ExperimentAnimation: Griffith’s Experiment
CLICKTO PLAY
Avery’s ExperimentsAvery’s Experiments
• What molecule is responsible for bacterial What molecule is responsible for bacterial transformation? transformation?
• Answer: DNAAnswer: DNA
Hershey–Chase ExperimentsHershey–Chase Experiments
• Is DNA or protein the genetic material in Is DNA or protein the genetic material in bacterial viruses (phages)? bacterial viruses (phages)?
• Answer: DNAAnswer: DNA
Hershey–Chase Hershey–Chase ExperimentsExperiments
Fig. 12-2, p. 262
35S
Bacterial viruses grown in 35S to
label protein coat or 32P to label DNA
32 P
Viruses infect bacteria
1
2
Fig. 12-2, p. 262
Agitate cells in blender
Agitate cells in blender
Separate by centrifugation
Separate by centrifugation
35S32 P
Bacteria in pellet contain 32P-labeled DNA
35S-labeled protein in supernatant
3
4
5
Fig. 12-2, p. 262
Viral reproduction inside bacterial cells
from pellet
7
32PCell lysis
6
5
6
7
Learning Objective 3Learning Objective 3
• How do How do nucleotidenucleotide subunits link to form a subunits link to form a single DNA strand?single DNA strand?
Watson and CrickWatson and Crick
• DNA ModelDNA Model • DemonstratedDemonstrated
• how information is stored in molecule’s how information is stored in molecule’s structurestructure
• how DNA molecules are how DNA molecules are templatestemplates for their for their own replication own replication
NucleotidesNucleotides
• DNA is a polymer of DNA is a polymer of nucleotides nucleotides • Each nucleotide subunit containsEach nucleotide subunit contains
• a nitrogenous basea nitrogenous base• purinespurines ( (adenineadenine or or guanineguanine))• pyrimidinespyrimidines ( (thyminethymine or or cytosinecytosine) )
• a pentose sugar (a pentose sugar (deoxyribosedeoxyribose))• a phosphate groupa phosphate group
Forming DNA ChainsForming DNA Chains
• BackboneBackbone• alternating sugar and phosphate groupsalternating sugar and phosphate groups• joined by covalent joined by covalent phosphodiester linkagesphosphodiester linkages
• Phosphate group attaches toPhosphate group attaches to• 55′′ carbon of one deoxyribose carbon of one deoxyribose• 33′′ carbon of the next deoxyribose carbon of the next deoxyribose
DNA DNA NucleotidesNucleotides
Fig. 12-3, p. 264
Thymine
Adenine Nucleotide
CytosinePhosphate group
Phosphodiester linkage
Guanine
Deoxyribose (sugar)
Animation: Subunits of DNAAnimation: Subunits of DNA
CLICKTO PLAY
KEY CONCEPTSKEY CONCEPTS
• The DNA building blocks consist of four The DNA building blocks consist of four nucleotide subunits: T, C, A, and Gnucleotide subunits: T, C, A, and G
Learning Objective 4Learning Objective 4
• How are the two strands of DNA oriented How are the two strands of DNA oriented with respect to each other?with respect to each other?
DNA MoleculeDNA Molecule
• 2 polynucleotide chains2 polynucleotide chains• associated as associated as double helixdouble helix
DNA MoleculeDNA Molecule
Fig. 12-5, p. 266
Sugar–phosphate backbone
Minor groove
3.4 nm Major groove
0.34 nm
2.0 nm
= hydrogen= oxygen
= carbon
= atoms in base pairs = phosphorus
Double HelixDouble Helix
• AntiparallelAntiparallel • chains run in opposite directionschains run in opposite directions
• 55′′ end end • phosphate attached to 5phosphate attached to 5′′ deoxyribose carbon deoxyribose carbon
• 33′′ end end • hydroxyl attached to 3hydroxyl attached to 3′′ deoxyribose carbon deoxyribose carbon
KEY CONCEPTSKEY CONCEPTS
• The DNA molecule consists of two strands The DNA molecule consists of two strands that wrap around each other to form a that wrap around each other to form a double helix double helix
• The order of its building blocks stores The order of its building blocks stores genetic informationgenetic information
Animation: DNA Close UpAnimation: DNA Close Up
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Learning Objective 5Learning Objective 5
• What are the base-pairing rules for DNA?What are the base-pairing rules for DNA?• How do complementary bases bind to How do complementary bases bind to
each other?each other?
Base PairsBase Pairs
• Hydrogen bondingHydrogen bonding• between specific base pairsbetween specific base pairs• binds two chains of helix binds two chains of helix
• Adenine (Adenine (AA) with thymine () with thymine (TT))• forms two hydrogen bondsforms two hydrogen bonds
• Guanine (Guanine (GG) with cytosine () with cytosine (CC))• forms three hydrogen bondsforms three hydrogen bonds
Base Pairs and Hydrogen BondsBase Pairs and Hydrogen Bonds
Fig. 12-6a, p. 267
Fig. 12-6b, p. 267
Adenine Thymine
Deoxyribose Deoxyribose
Guanine Cytosine
Deoxyribose Deoxyribose
Chargaff’s RulesChargaff’s Rules
• Complementary base pairingComplementary base pairing • between A and T; G and Cbetween A and T; G and C• therefore A = T; G = Ctherefore A = T; G = C
• If base sequence of 1 strand is knownIf base sequence of 1 strand is known• base sequence of other strand can be base sequence of other strand can be
predictedpredicted
KEY CONCEPTSKEY CONCEPTS
• Nucleotide subunits pair, based on precise Nucleotide subunits pair, based on precise pairing rules: T pairs with A, and C pairs pairing rules: T pairs with A, and C pairs with G with G
• Hydrogen bonding between base pairs Hydrogen bonding between base pairs holds two strands of DNA togetherholds two strands of DNA together
Learning Objective 6Learning Objective 6
• What evidence from Meselson and Stahl’s What evidence from Meselson and Stahl’s experiment enabled scientists to experiment enabled scientists to differentiate between differentiate between semiconservative semiconservative replicationreplication of DNA and alternative models? of DNA and alternative models?
Models of DNA Models of DNA ReplicationReplication
Fig. 12-7a, p. 268
(a) Hypothesis 1: Semiconservative replication
Parental DNA First generation Second generation
Fig. 12-7b, p. 268
(b) Hypothesis 2: Conservative replication
Parental DNA First generation Second generation
Fig. 12-7c, p. 268
(c) Hypothesis 3: Dispersive replication
Parental DNA First generation Second generation
Meselson-StahlMeselson-Stahl ExperimentExperiment
• E. coli E. coli • grown in medium containing heavy nitrogen grown in medium containing heavy nitrogen
((1515N)N)• incorporated incorporated 1515N into DNAN into DNA
• Transferred from Transferred from 1515N to N to 1414N mediumN medium• after one or two generations, DNA density after one or two generations, DNA density
supported supported semiconservative replication semiconservative replication
Meselson-StahlMeselson-Stahl ExperimentExperiment
Fig. 12-8a, p. 269
Bacteria are grown in 15N (heavy) medium. All
DNA is heavy.
Some cells are transferred to
14N (light) medium.
Some cells continue to grow in 14N medium.
First generation Second generation
Cesium chloride (CsCl)
High density
Low density
DNA
DNA is mixed with CsCl solution, placed in an ultracentrifuge, and centrifuged at very high speed for about 48 hours. 14N (light)
DNA
14N – 15N hybrid DNA
15N (heavy) DNA
DNA molecules move to positions where their density equals that of the CsCl solution.
The greater concentration of CsCl at the bottom of the tube is due to sedimentation under centrifigal force.
Fig. 12-8b, p. 269
14N (light) DNA
14N – 15N hybrid DNA
14N – 15N hybrid DNA
15N (heavy) DNA
Before transfer to 14N
One cell generation after transfer to 14N
Two cell generations after transfer to 14N
The location of DNA molecules within the centrifuge tube can be determined by UV optics. DNA solutions absorb strongly at 260 nm.
Semiconservative ReplicationSemiconservative Replication
• Each daughter double helix consists ofEach daughter double helix consists of• 1 original strand from parent molecule1 original strand from parent molecule• 1 new complementary strand1 new complementary strand
Learning Objective 7Learning Objective 7
• How does DNA replicate?How does DNA replicate?• What are some unique features of the What are some unique features of the
process?process?
DNA ReplicationDNA Replication
• 2 strands of double helix unwind2 strands of double helix unwind• each is template for complementary strandeach is template for complementary strand
• Replication is initiatedReplication is initiated• DNA primaseDNA primase synthesizes synthesizes RNA primerRNA primer
• DNA strandDNA strand growsgrows• DNA polymeraseDNA polymerase adds nucleotide subunits adds nucleotide subunits
DNA ReplicationDNA Replication
Fig. 12-10, p. 271
Nucleotide joined to growing chain by DNA polymerase
Phosphates released
Base
Other EnzymesOther Enzymes
• DNA helicasesDNA helicases• open the double helixopen the double helix
• TopoisomerasesTopoisomerases • prevent tangling and knottingprevent tangling and knotting
KEY CONCEPTSKEY CONCEPTS
• DNA replication results in two identical DNA replication results in two identical double-stranded DNA moleculesdouble-stranded DNA molecules• molecular mechanism passes genetic molecular mechanism passes genetic
information from one generation to the nextinformation from one generation to the next
Learning Objective 8Learning Objective 8
• What makes What makes DNA replicationDNA replication (a) (a) bidirectional and (b) continuous in one bidirectional and (b) continuous in one strand and discontinuous in the other?strand and discontinuous in the other?
Bidirectional ReplicationBidirectional Replication
• Starting at Starting at origin of replicationorigin of replication• proceeding in both directionsproceeding in both directions
• Eukaryotic chromosomeEukaryotic chromosome• may have multiple origins of replicationmay have multiple origins of replication• may replicate at many points at same timemay replicate at many points at same time
Bidirectional ReplicationBidirectional Replication
Fig. 12-11a, p. 272
DNA polymerase
Origin of replication on DNA molecule
3’
5’3’
5’
Fig. 12-11b, p. 272
Twist introduced into the helix by unwinding
Single-strand binding proteinsRNA primer
DNA polymerase
DNA helicase
RNA primer
Direction of replication
3’
5’
3’
5’3’3’
Fig. 12-11c, p. 272
3’
5’
3’
5’
3’
5’
3’
5’
DNA SynthesisDNA Synthesis
• Always proceeds in 5Always proceeds in 5′′ →→ 3 3′′ direction direction• Leading strandLeading strand
• synthesized continuouslysynthesized continuously
• Lagging strandLagging strand• synthesized discontinuouslysynthesized discontinuously• forms short forms short Okazaki fragmentsOkazaki fragments• DNA primaseDNA primase synthesizes RNA primers synthesizes RNA primers• DNA ligaseDNA ligase links Okazaki fragments links Okazaki fragments
DNA SynthesisDNA Synthesis
Fig. 12-12a, p. 273
DNA helixRNA primer
Leading strand
DNA polymerase
Lagging strand (first Okazaki fragment)
Direction of replication
Replication fork
3’
5’
3’
5’
3’5’
3’5’
Fig. 12-12b, p. 273
Leading strand
RNA primers
Two Okazaki fragments
3’5’
3’5’
5’3’
3’
5’3’
5’
Fig. 12-12c, p. 273
Leading strand
DNA ligase Third Okazaki fragment
Lagging strand
3’
5’
3’5’
3’
5’3’5’
3’5’
Replication in Bacteria and Replication in Bacteria and EukaryotesEukaryotes
Fig. 12-13a, p. 274
Template DNA (light blue)
New DNA (dark blue)
3’5’
3’
5’
Fig. 12-13b, p. 274
340 nm
Fig. 12-13c, p. 274
Replication “bubbles”
Single replication bubble formed from two merged bubbles
Replication fork
3’
5’
3’5’
Animation: Overview of DNA Animation: Overview of DNA replication and base pairingreplication and base pairing
CLICKTO PLAY
Learning Objective 9Learning Objective 9
• How do How do enzymesenzymes proofread and repair proofread and repair errors in DNA?errors in DNA?
DNA PolymerasesDNA Polymerases
• Proofread each new nucleotideProofread each new nucleotide• against template nucleotide against template nucleotide
• Find errors in base pairing Find errors in base pairing • remove incorrect nucleotideremove incorrect nucleotide• insert correct oneinsert correct one
DNA MutationDNA Mutation
Fig. 12-9, p. 270
Mutation
Stepped Art
Fig. 12-9, p. 270
Mutation
Mismatch RepairMismatch Repair
• Enzymes recognize incorrectly paired Enzymes recognize incorrectly paired nucleotides and remove themnucleotides and remove them
• DNA polymerases fill in missing DNA polymerases fill in missing nucleotidesnucleotides
Nucleotide Excision RepairNucleotide Excision Repair
• Repairs DNA lesionsRepairs DNA lesions• caused by sun or harmful chemicalscaused by sun or harmful chemicals
• 3 enzymes3 enzymes• nucleasenuclease cuts out damaged DNA cuts out damaged DNA• DNA polymeraseDNA polymerase adds correct nucleotides adds correct nucleotides• DNA ligaseDNA ligase closes breaks in sugar–phosphate closes breaks in sugar–phosphate
backbonebackbone
Nucleotide Excision RepairNucleotide Excision Repair
Fig. 12-14, p. 275
Nuclease enzyme bound to DNA DNA lesion
DNA polymeraseDNA ligase
New DNA
3’ 5’
3’ 5’
3’ 5’
3’ 5’
3’ 5’
3’ 5’
Learning Objective 10Learning Objective 10
• What is a What is a telomeretelomere?? • What are the possible connections What are the possible connections
between between telomerasetelomerase and cell aging, and and cell aging, and between telomerase and cancer? between telomerase and cancer?
TelomeresTelomeres
• Eukaryotic chromosome endsEukaryotic chromosome ends• noncoding, repetitive DNA sequences noncoding, repetitive DNA sequences
• Shorten slightly with each cell cycleShorten slightly with each cell cycle• Can be extended by Can be extended by telomerasetelomerase
Replication at TelomeresReplication at Telomeres
Fig. 12-15a, p. 276
DNA replication
RNA primerRNA primer
Removal of primer
3’
3’
3’
3’
3’
3’
3’
3’
3’
3’
5’
5’
5’
5’
5’
5’
5’
5’
5’
5’
+
+
Fig. 12-15b, p. 276
3’
5’
Cell AgingCell Aging
• May be caused by absence of telomerase May be caused by absence of telomerase activity activity
• Cells lose ability to divideCells lose ability to divide• after a limited number of cell divisionsafter a limited number of cell divisions
Cancer CellsCancer Cells
• Have Have telomerasetelomerase• to maintain telomere length and possibly to maintain telomere length and possibly
resist apoptosisresist apoptosis
• Including human cancersIncluding human cancers• breast, lung, colon, prostate gland, pancreasbreast, lung, colon, prostate gland, pancreas