Upload
alanna
View
48
Download
0
Embed Size (px)
DESCRIPTION
Evidence that DNA can transform bacteria. Frederick Griffith (1928) – Streptococcus pneumoniae bacteria – transformation Mouse Experiment Experiment proved that transformation can happen Avery and colleagues (1944) – announced transformation agent was DNA. - PowerPoint PPT Presentation
Citation preview
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evidence that DNA can transform bacteria
• Frederick Griffith (1928) – Streptococcus pneumoniae bacteria – transformation
• Mouse Experiment
• Experiment proved that transformation can happen
• Avery and colleagues (1944) – announced transformation agent was DNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 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.
EXPERIMENT
RESULTS
CONCLUSION
Living S(control) cells
Living R(control) cells
Heat-killed(control) S cells
Mixture of heat-killed S cellsand living R cells
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cellsare found inblood sample.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evidence that viral DNA can program cells
• Alfred Hershey and Martha Chase (1952) – bacteriophages or phages (viruses that infect bacteria) – discovered DNA is the genetic material NOT protein
• Blender experiment
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.3 Viruses infecting a bacterial cell
Phagehead
Tail
Tail fiber
DNA
Bacterialcell
100
nm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Additional evidence that DNA is the genetic material
• Erwin Chargaff (1947) – Chargaff’s rules – The equivalences for any given species between the number of A and T and G and C bases are equal.
• Analyzed DNA from different organisms
– Humans 30.3% of bases were A’s
– E. Coli 26% of bases were A’s
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Rosalind Franklin – (1950’s) – X-ray diffraction photo of DNA – helped Watson and Crick develop their model of DNA structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA
(a) Rosalind Franklin Franklin’s X-ray diffractionPhotograph of DNA
(b)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of DNA
• Watson & Crick – (1953) – 1 page paper in the British journal Nature “Molecular Structure of Nucleic Acids: A Structure for Deoxynucleic acids”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.1 Watson and Crick with their DNA model
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.5 The structure of a DNA strandSugar-phosphate
backboneNitrogenous
bases
5 endO–
O P O CH2
5
4O–
HH
OH
H
H3
1H O
CH3
N
O
NH
Thymine (T)
O
O P O
O–
CH2
HH
OH
HH
HN
N
N
H
NH
H
Adenine (A)
O
O P O
O–
CH2
HH
OH
HH
H
H H
HN
NN
OCytosine (C)
O
O P O CH2
5
4O–
H
O
H
H3
1
OH2
H
N
NN H
ON
N HH
H H
Sugar (deoxyribose)3 end
Phosphate
Guanine (G)
DNA nucleotide
2
N
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.7 The double helix
O
–O O
OHP
H2C
O
–O O
OP
H2C
O
–O O
OP
H2C
O
–OO
OP
O
OH
O
O
OT A
C
GC
A T
O
O
O
O
OH
CH2
O O–
OOP
CH2
OO–
OOP
CH2
OO–
OO
P
CH2
OO–
OOP
5 end
Hydrogen bond3 end
5 end
3 end
C
T
A
A
T
CG
GC
A T
C G
AT
AT
A T
TA
C
TA0.34 nm
3.4 nm
(a) Key features of DNA structure (b) Partial chemical structure (c) Space-filling model
G
1 nm
G
H2C
G
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.8 Base pairing in DNAH
N H O CH3
N
N
O
N
N
N
N H
Sugar
Sugar
Adenine (A) Thymine (T)
N
N
N
N
Sugar
O H N
H
NH
N OH
H
N
Sugar
Guanine (G) Cytosine (C)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Unnumbered Figure p. 298
Purine + Purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: widthConsistent with X-ray data
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA Replication Section 16.2
• Semi-conservative model – each of the two daughter molecules will have one old strand, derived from the parent molecules, and one newly made strand
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
A
C
T
A
G
T
G
A
T
C
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
A
C
T
A
G
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
A
C
T
A
G
A
C
T
A
G
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
A
C
T
A
G
A
C
T
A
G
T
G
A
T
C
T
G
A
T
C
T
G
A
T
C
T
G
A
T
C
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.10 Three alternative models of DNA replication
Conservativemodel. The twoparental strandsreassociate after acting astemplates fornew strands,thus restoringthe parentaldouble helix.
(a)
Semiconserva-tive model.The two strandsof the parentalmolecule separate, 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 cellFirstreplication
Secondreplication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.13 Incorporation of a nucleotide into a DNA strand
New strand Template strand
5’ end 3’ end
Sugar A TBase
C
G
G
C
A
C
T
PP
P
OH
P P
5’ end 3’ end
5’ end 5’ end
A T
C
G
G
C
A
C
T
3’ end
Nucleosidetriphosphate
Pyrophosphate
2 P
OH
Phosphate
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
1
2
3
Origin of replication
Bubble
Parental (template) strand
Daughter (new) strand
Replication fork
Two daughter DNA molecules
In 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 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Parental DNA
DNA pol Ill elongatesDNA strands only in the5 3 direction. 3
5
5
3
35
21
Okazakifragments
DNA pol III
Templatestrand
Leading strandLagging strand
32 1
Templatestrand DNA ligase
Overall direction of replication
One new strand, the leading strand,can elongate continuously 5 3as the replication fork progresses.
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).
DNA ligase joins Okazakifragments by forming a bond betweentheir free ends. This results in a continuous strand.
2
3
1
4
Figure 16.14 Synthesis of leading and lagging strands during DNA replication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.15 Synthesis of the lagging strand
Overall direction of replication
3
3
3
35
35
35
35
35
35
35
3 5
5
1
1
21
12
5
5
12
35
DNA ligase forms a bond between the newest DNAand the adjacent DNA of fragment 1.
6 The lagging strand in this region is nowcomplete.
7
DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2.
5
After the second fragment is primed. DNA pol III adds DNAnucleotides until it reaches the first primer and falls off.
4
After reaching the next RNA primer (not shown), DNA pol III falls off.
3
DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.
2
Primase joins RNA nucleotides into a primer.
1
Templatestrand
RNA primer
Okazakifragment
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Table 16.1 Bacterial DNA replication proteins and their functions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overall direction of replication
Helicase 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.
Leadingstrand Origin of replication
Laggingstrand
Laggingstrand
LeadingstrandOVERVIEW
Leadingstrand
Replication fork
DNA pol III
Primase
PrimerDNA pol III Lagging
strand
DNA pol I DNA ligase
1
2 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.
7
Parental DNA
53
43
21
5
3
Figure 16.16 A summary of bacterial DNA replication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Proofreading and repairing DNA
• Errors do occur
– 1 in 10 billion nucleotides on entire DNA
– 1 in 100,000 for incoming nucleotides
• Proofreading is done by DNA pol III as it attaches new nucleotides
• Mismatch repair cells use special enzymes to fix mismatched nucleotides
– A-C for example
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.17 Nucleotide excision repair of DNA damage
Nuclease
DNApolymerase
DNAligase
A thymine dimerdistorts the DNA molecule.1
Repair synthesis bya DNA polymerasefills in the missingnucleotides.
3
DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.
4
A nuclease enzyme cutsthe damaged DNA strandat two points and thedamaged section isremoved.
2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Telomeres
• Repeated units of bases
– TTAGGG in humans
• Do NOT contain genes
• They protect the genes from being eroded (getting shorter and shorter) through DNA replication rounds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.18 Shortening of the ends of linear DNA molecules
End of parentalDNA strands
Leading strandLagging strand
Last fragment Previous fragment
RNA primer
Lagging strand
Removal of primers andreplacement with DNAwhere a 3 end is available
Primer removed butcannot be replacedwith DNA becauseno 3 end available
for DNA polymerase
Second roundof replication
New leading strand
New lagging strand 5
Further roundsof replication
Shorter and shorterdaughter molecules
5
3
5
3
5
3
5
3
3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 16.19 Telomeres
1 µm