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Chapter 16: The Molecular Basis of Inheritance (DNA)
Zooming in on DNA
LE 16-2
Living S cells(control)
Living R cells(control)
Heat-killedS cells (control)
Mixture of heat-killedS cells and livingR cells
Mouse dies
Living S cellsare found in blood sample
Mouse healthy Mouse healthy Mouse dies
RESULTS
LE 16-3
Bacterialcell
Phagehead
Tail
Tail fiber
DNA
100
nm
LE 16-4
Bacterial cell
Phage
DNA
Radioactiveprotein
Emptyprotein shell
PhageDNA
Radioactivity(phage protein)in liquid
Batch 1:Sulfur (35S)
RadioactiveDNA
Centrifuge
Pellet (bacterialcells and contents)
PelletRadioactivity(phage DNA)in pellet
Centrifuge
Batch 2:Phosphorus (32P)
Activity: Hersey Chase Experiment
LE 16-5Sugar–phosphate
backbone
5 end
Nitrogenousbases
Thymine (T)
Adenine (A)
Cytosine (C)
DNA nucleotidePhosphate
3 endGuanine (G)
Sugar (deoxyribose)
LE 16-6
Franklin’s X-ray diffractionphotograph of DNA
Rosalind Franklin
DNAi.org
LE 16-UN298
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: widthconsistent with X-ray data
DNAi.org
LE 16-8
Adenine (A) Thymine (T)
Guanine (G) Cytosine (C)
Sugar
Sugar
Sugar
Sugar
LE 16-7
5 end
3 end
5 end
3 end
Space-filling modelPartial chemical structure
Hydrogen bond
Key features of DNA structure
0.34 nm
3.4 nm
1 nm
Activity: DNA & RNA Structure
Activity: DNA Double HelixDNAi.org
LE 16-9_4
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.
The first step in replication is separation of the two DNA strands.
Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.
The nucleotides are connected to form the sugar-phosphate back-bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.
LE 16-10
Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix.
Semiconservative model. The two strands of the parental moleculeseparate, and each functions as a template for synthesis of a new, comple-mentary strand.
Dispersive model. Each strand of both daughter molecules contains a mixture of old and newly synthesized DNA.
Parent cellFirstreplication
Secondreplication
LE 16-11
Bacteriacultured in mediumcontaining15N
DNA samplecentrifugedafter 20 min(after firstreplication)
DNA samplecentrifugedafter 40 min(after secondreplication)
Bacteriatransferred tomediumcontaining14N
Lessdense
Moredense
Conservativemodel
First replication
Semiconservativemodel
Second replication
Dispersivemodel
Meselson & Stahl
LE 16-12
In eukaryotes, DNA replication begins at may sitesalong the giant DNA molecule of each chromosome.
Two daughter DNA molecules
Parental (template) strand
Daughter (new) strand0.25 µm
Replication fork
Origin of replication
Bubble
In this micrograph, three replicationbubbles are visible along the DNAof a cultured Chinese hamster cell(TEM).
Activity: DNA Replication: An Overview
LE 16-13
New strand
5 end
Phosphate Base
Sugar
Template strand
3 end 5 end 3 end
5 end
3 end
5 end
3 end
Nucleosidetriphosphate
DNA polymerase
Pyrophosphate
Activity: DNA Replication: A Closer Look
LE 16-14
Parental DNA
5
3
Leading strand
35
3
5
Okazakifragments
Lagging strand
DNA pol III
Templatestrand
Leading strand
Lagging strand
DNA ligase Templatestrand
Overall direction of replication
LE 16-15_6
53
Primase joins RNAnucleotides into a primer.
Templatestrand
5 3
Overall direction of replication
RNA primer3
5
35
DNA pol III addsDNA nucleotides to the primer, formingan Okazaki fragment.
Okazakifragment
3
5
5
3
After reaching thenext RNA primer (not
shown), DNA pol IIIfalls off.
33
5
5
After the second fragment isprimed, DNA pol III adds DNAnucleotides until it reaches thefirst primer and falls off.
33
5
5
DNA pol I replaces the RNA with DNA,adding to the 3 endof fragment 2.
33
5
5
DNA ligase forms abond between the newestDNA and the adjacent DNAof fragment 1.
The lagging strand in the regionis now complete.
LE 16-16
5
3Parental DNA
3
5
Overall direction of replication
DNA pol III
Replication fork
Leadingstrand
DNA ligase
Primase
OVERVIEW
PrimerDNA pol III
DNA pol I
Laggingstrand
Laggingstrand
Leadingstrand
Leadingstrand
LaggingstrandOrigin of replication
Activity: DNA Replication: A ReviewDNAi.org
LE 16-17
DNA ligase
DNA polymerase
DNA ligase seals thefree end of the new DNAto the old DNA, making thestrand complete.
Repair synthesis bya DNA polymerasefills in the missingnucleotides.
A nuclease enzyme cutsthe damaged DNA strandat two points and the damaged section isremoved.Nuclease
A thymine dimerdistorts the DNA molecule.
Proofreading and Repair
LE 16-18
End of parentalDNA strands
5
3
Lagging strand 5
3
Last fragment
RNA primer
Leading strandLagging strand
Previous fragment
Primer removed butcannot be replacedwith DNA becauseno 3 end available
for DNA polymerase5
3
Removal of primers andreplacement with DNAwhere a 3 end is available
Second roundof replication
5
3
5
3Further roundsof replication
New leading strand
New leading strand
Shorter and shorterdaughter molecules
Telomeres
