Embed Size (px)
Citation preview
DNA
The Molecule of Life
Overview: Life’s Operating Instructions• 1953
– James Watson and Francis Crick
– Structure of the molecule of inheritance
– Deoxyribonucleic acid
• Rosalind Franklin– Used X-ray
crystalography to take the first picture of the molecule
© 2011 Pearson Education, Inc.
Figure 16.4-1
Bacterial cell
Phage
Batch 1:Radioactivesulfur(35S)
DNA
Batch 2:Radioactivephosphorus(32P)
RadioactiveDNA
EXPERIMENTRadioactiveprotein
In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2.
Figure 16.4-2
Bacterial cell
Phage
Batch 1:Radioactivesulfur(35S)
Radioactiveprotein
DNA
Batch 2:Radioactivephosphorus(32P)
RadioactiveDNA
Emptyproteinshell
PhageDNA
EXPERIMENT
Figure 16.4-3
Bacterial cell
Phage
Batch 1:Radioactivesulfur(35S)
Radioactiveprotein
DNA
Batch 2:Radioactivephosphorus(32P)
RadioactiveDNA
Emptyproteinshell
PhageDNA
Centrifuge
Centrifuge
Radioactivity(phage protein)in liquid
Pellet (bacterialcells and contents)
PelletRadioactivity(phage DNA)in pellet
EXPERIMENT
Figure 16.7
3.4 nm
1 nm
0.34 nm
Hydrogen bond
(a) Key features ofDNA structure
Space-fillingmodel
(c)(b) Partial chemical structure
3 end
5 end
3 end
5 end
T
T
A
A
G
G
C
C
C
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
G
T
T
T
T
T
T
A
A
A
A
A
A
Figure 16.UN01
Purine purine: too wide
Pyrimidine pyrimidine: too narrow
Purine pyrimidine: widthconsistent with X-ray data
• Erwin Chargaff – measured the amount of each base (ATCG) in segments of DNA from different organisms
• Two findings became known as Chargaff’s rules– The base composition of DNA varies between species– In any species the number of A and T bases are equal
and the number of G and C bases are equal
© 2011 Pearson Education, Inc.
Figure 16.8
Sugar
Sugar
Sugar
Sugar
Adenine (A) Thymine (T)
Guanine (G) Cytosine (C)
Figure 16.9-1
(a) Parent molecule
A
A
A
T
T
T
C
C
G
G
Figure 16.9-2
(a) Parent molecule (b) Separation ofstrands
A
A
A
A
A
A
T
T
T
T
T
T
C
C
C
C
G
G
G
G
Figure 16.9-3
(a) Parent molecule (b) Separation ofstrands
(c)“Daughter” DNA molecules,each consisting of oneparental strand and onenew strand = semi-conservative
A
A
A
A
A
A
A
A
A
A
A
A
T
T
T
T
T
T
T
T
T
T
T
T
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
DNA Replication
• Replication proceeds in both directions from the origins of replication.
• replication fork - a Y-shaped region where new DNA strands are elongating
• Helicases - enzymes that untwist the double helix at the replication forks
• Single-strand binding proteins bind to and stabilize single-stranded DNA
• Topoisomerase – enzyme that corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
© 2011 Pearson Education, Inc.
Figure 16.13
Topoisomerase
Primase
RNAprimer
Helicase
Single-strand bindingproteins
5
3
5
53
3
Synthesizing a New DNA Strand
• Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork by adding to the 3’ end of an existing molecule.
• Most DNA polymerases require an RNA primer (primase) and a DNA template strand to build from.
• The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells.
© 2011 Pearson Education, Inc.
Figure 16.14
New strand Template strand
Sugar
Phosphate Base
Nucleosidetriphosphate
DNApolymerase
Pyrophosphate
5
5
5
5
3
3
3
3
OH
OH
OH
P P i
2 P i
PP
P
A
A
A
A
T T
TT
C
C
C
C
C
C
G
G
G
G
Nucleotides are added as nucleoside triphosphate (i.e. dATP).
Figure 16.15
Leadingstrand
Laggingstrand
Overview
Origin of replication Laggingstrand
Leadingstrand
Primer
Overall directionsof replication
Origin of replication
RNA primer
Sliding clamp
DNA pol IIIParental DNA
35
5
33
5
3
5
3
5
3
5
DNA polymerase can only add to the 3’ end (5 3).
Leading Strand
Lagging Strand
Okasaki Fragments
DNA ligase
Figure 16.15a
Leadingstrand
Laggingstrand
Overview
Origin of replication Laggingstrand
Leadingstrand
Primer
Overall directionsof replication
Figure 16.16b-1
Templatestrand
3
35
5
Figure 16.16b-2
Templatestrand
RNA primerfor fragment 1
3
3
3
3
5
5
5
51
Figure 16.16b-3
Templatestrand
RNA primerfor fragment 1
Okazakifragment 1
3
3
3
3
3
3
5
5
5
5
5
5
1
1
Figure 16.16b-4
Templatestrand
RNA primerfor fragment 1
Okazakifragment 1
RNA primerfor fragment 2
Okazakifragment 2
3
3
3
3
3
3
3
3
5
5
5
5
5
55
5
2
1
1
1
Figure 16.16b-5
Templatestrand
RNA primerfor fragment 1
Okazakifragment 1
RNA primerfor fragment 2
Okazakifragment 2
3
3
3
3
3
3
3
3
3
3
3
5
5
5
5
5
55
55
55
2
21
1
1
1
Figure 16.16b-6
Templatestrand
RNA primerfor fragment 1
Okazakifragment 1
RNA primerfor fragment 2
Okazakifragment 2
Overall direction of replication
3
3
3
3
3
3
3
3
3
3
3
3
5
5
5
5
5
55
55
55
5
2
2
21
1
1
1
1
Figure 16.17
Overview
Leadingstrand
Origin of replication Lagging
strand
LeadingstrandLagging
strand Overall directionsof replicationLeading strand
DNA pol III
DNA pol III Lagging strand
DNA pol I DNA ligase
PrimerPrimase
ParentalDNA
5
5
5
5
5
33
3
333 2 1
4
Overview
Leadingstrand
Origin of replication Lagging
strand
LeadingstrandLagging
strand Overall directionsof replicationLeading strand
Primer
DNA pol III
DNA pol I
Lagging strand
DNA ligase5
5
5
33
3 34
2 1
Figure 16.17b
Figure 16.18
Parental DNA
DNA pol III
Leading strand
Connectingprotein
Helicase
Lagging strandDNA pol III
Laggingstrandtemplate
5
5
5
5
5
5
3 3
33
3
3
DNA Replication Complex
Figure 16.19
Nuclease
DNA polymerase
DNA ligase
5
5
5
5
5
5
5
5
3
3
3
3
3
3
3
3
Proofreading
Nucleotide Excision Repair
Hopefully:
DNA Polymerases edit the new strand as they move along the molecule = mismatch repair
If not:
Nucleotide Excision Repair
Nucleases
