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The structure of the DNA double helix was
described by Watson and Crick in 1953. Explain
the structure of the DNA double helix, including its
subunits and the way in which they are bonded
together.
(Total 8 marks)
Please have that paperwork out & your
journal… HW check!
• subunits are nucleotides;
• one base, one deoxyribose and one phosphate in each
nucleotide;
• description / diagram showing base linked to deoxyribose C1
and phosphate to C5;
• four different bases – adenine, cytosine, guanine and thymine;
• nucleotides linked up with sugar-phosphate bonds;
• covalent / phosphodiester bonds;
• two strands (of nucleotides) linked together;
• base to base;
• A to T and G to C;
• hydrogen bonds between bases;
• antiparallel strands;
• double helix drawn or described;
Accept any of the points above if clearly explained in a diagram.
• [8]
11
DNA and Its Role in Heredity
11 DNA and Its Role in Heredity
• 11.1 What Is the Evidence that the
Gene Is DNA?
• 11.2 What Is the Structure of DNA?
• 11.3 How Is DNA Replicated?
• 11.4 How Are Errors in DNA Repaired?
• 11.5 What Are Some Applications of
Our Knowledge of DNA Structure and
Replication?
 Homework Check—
 Journal 7.1
 Quiz Thursday!
11-1:
• Not in IB, but ...
• Experiments on bacteria and
viruses demonstrated that DNA IS
the GENETIC MATERIAL!
11.1 What Is the Evidence that the Gene Is DNA?
By the 1920s: chromosomes consisted of DNA &
proteins.
A new dye stained DNA, provided circumstantial
evidence—DNA’s the genetic material:
It was in the right place
It varied among species
It was present in the right amount
11.1 What Is the Evidence that the Gene Is DNA?
Frederick Griffith, working w/2 strains of
Streptococcus pneumoniae
“transforming principle” from dead
cells of 1 strain produced a heritable
change in the other strain.
Figure 11.1 Genetic Transformation of Nonvirulent Pneumococci
11.1 What Is the Evidence that the Gene Is DNA?
To identify the transforming principle,
Oswald Avery:
Treated samples to destroy different
molecules (RNA, DNA, Protein)
If DNA was destroyed, the transforming
principle was lost.
Figure 11.2 Genetic Transformation by DNA (Part 1)
Figure 11.2 Genetic Transformation by DNA (Part 2)
11.1 What Is the Evidence that the Gene Is DNA?
Hershey-Chase experiment:
• Is DNA or protein the genetic material?
 using bacteriophage T2 virus
• Bacteriophage proteins labeled with 35S
 DNA labeled with 32P
Figure 11.3 Bacteriophage T2: Reproduction Cycle
Bacteriophage T2
Protein coat
DNA
Figure 11.3 Bacteriophage T2: Reproduction Cycle
Bacteriophage T2
Protein
coat
Bacteriophage T2
attaches to the
surface of a
bacterium and
injects its DNA.
DNA
DNA
Viral genes take
over the host’s
machinery and
synthesizes new
viruses.
The bacterium bursts,
releasing about 200
viruses.
Figure 11.4 The Hershey–Chase Experiment (Part 1)
Figure 11.4 The Hershey–Chase Experiment (Part 2)
Figure 11.5 Transfection in Eukaryotic Cells
DNA Origami!
• http://www.dnai.org/teacherguide/pdf/
origami_inst.pdf
• Don’t forget...
www.Msleejichs.wikispaces.com
11-2 SUMMARY:
• DNA: double helix of 2
ANTIPARALLEL polynucleotide
chains
• 2 chains joined by H bonds
between nucleotide bases—pair AT , G-C
11-2 Recap ?s:
• What’s the evidence that Watson &
Crick used to come up with the
double helix model for DNA?
• How does the double helical
STRUCTURE of DNA relate to its
FUNCTION?
11.2 What Is the Structure of DNA?
Structure of DNA was determined
using…
Figure 11.6 X-Ray Crystallography Helped Reveal the Structure of DNA
Photo 11.1 X-ray diffraction pattern of DNA.
11.2 What Is the Structure of DNA?
Chemical composition also provided clues:
Bases:
Figure 3.23 Nucleotides Have Three Components
repeat fig 3.23 here
11.2 What Is the Structure of DNA?
1950: Erwin Chargaff
Figure 11.7 Chargaff’s Rule
11.2 What Is the Structure of DNA?
Model building
Linus Pauling—
Figure 11.8 DNA Is a Double Helix (A)
11.2 What Is the Structure of DNA?
X-ray crystallography
Figure 11.8 DNA Is a Double Helix (B)
11.2 What Is the Structure of DNA?
Key features of DNA:
11.2 What Is the Structure of DNA?
Complementary base pairing:
Figure 11.9 Base Pairing in DNA Is Complementary (Part 1)
Figure 11.9 Base Pairing in DNA Is Complementary
Pairs of complementary bases form
hydrogen bonds that hold the two
strands of the DNA double helix together.
Each phosphate group links the 3′
carbon of one sugar to the 5′ carbon
of the next sugar along the
backbone.
3′ end
TA pairs have two
hydrogen bonds.
CG pairs have three
hydrogen bonds.
3′ end
The strands both run in a
5′-to-3′ direction—they 5′ end
are antiparallel.
Photo 11.2 Computer-simulated space-filling model of DNA.
11.2 What Is the Structure of DNA?
Antiparallel strands:
11.2 What Is the Structure of DNA?
Functions of DNA:
11.2 What Is the Structure of DNA?
• Genetic material is precisely replicated
Replication Model
Remember this?!?
Then 7.1
11-3 Recap:
• Meselson and Stahl showed that DNA
replication is semiconservative: each
parent strand serves as a template for a
new strand
• A complex of proteins, most notably
DNA polymerases, is involved
• New DNA is polymerized in one
direction only
• Since the 2 strands are antiparallel, 1
strand is made continuously and the
other is made in Okazaki fragments that
are eventually joined
11-3 Recap ?s:
• How did the Meselson-Stahl expt
work?
• What are 5 enzymes needed for
DNA replication? Role of each?
• How does the leading strand of
DNA differ from the lagging
strand?
11.3 How Is DNA Replicated?
The DNA is a template for
11.3 How Is DNA Replicated?
Three possible replication patterns:
Figure 11.10 Three Models for DNA Replication
11.3 How Is DNA Replicated?
Meselson and Stahl
Figure 11.11 The Meselson–Stahl Experiment (Part 1)
Figure 11.11 The Meselson–Stahl Experiment (Part 2)
11.3 How Is DNA Replicated?
Results of their experiment
Animations!
• Meselson & Stahl Expt
• Replication Part 1
• Part deux
• Leading & Lagging
11.3 How Is DNA Replicated?
Two steps in DNA replication:
Figure 11.12 Each New DNA Strand Grows from Its 5′ End to Its 3′ End (Part 1)
Figure 11.12 Each New DNA Strand Grows from Its 5′ End to Its 3′ End (Part 2)
11.3 How Is DNA Replicated?
DNA replicates in both directions, forming
Figure 11.13 Two Views of DNA Replication
11.3 How Is DNA Replicated?
DNA helicase
Figure 11.14 Replication in Small Circular and Large Linear Chromosomes (A)
11.3 How Is DNA Replicated?
Large linear chromosomes
Figure 11.14 Replication in Small Circular and Large Linear Chromosomes (B)
11.3 How Is DNA Replicated?
DNA polymerases:
Figure 11.15 DNA Polymerase Binds to the Template Strand (Part 1)
Figure 11.15 DNA Polymerase Binds to the Template Strand (Part 2)
11.3 How Is DNA Replicated?
primer required to start DNA
replication—
Figure 11.16 No DNA Forms without a Primer
11.3 How Is DNA Replicated?
Cells have several DNA polymerases!
Figure 11.17 Many Proteins Collaborate in the Replication Complex
11.3 How Is DNA Replicated?
At replication fork:
Figure 11.18 The Two New Strands Form in Different Ways
11.3 How Is DNA Replicated?
Okazaki fragments
Figure 11.19 The Lagging Strand Story (Part 1)
Figure 11.19 The Lagging Strand Story (Part 2)
11.3 How Is DNA Replicated?
Telomeres
Figure 11.21 Telomeres and Telomerase
11.3 How Is DNA Replicated?
Human chromosome telomeres (TTAGGG) are
repeated about 2500 times.
Chromosomes can lose 50–200 base pairs
with each replication. After 20–30 divisions,
the cell dies.
11.3 How Is DNA Replicated?
Some cells—bone marrow stem cells, gameteproducing cells—have telomerase that
catalyzes the addition of telomeres.
90% of human cancer cells have telomerase;
normal cells do not. Some anticancer drugs
target telomerase.
IB Review
Quiz FRI!
11-4 Recap:
• DNA replication ain’t perfect!
• DNA can also be damaged or
naturally altered
• Repair mechanisms exist that
detect and repair mismatched or
damaged DNA
11.4 How Are Errors in DNA Repaired?
DNA polymerases make mistakes
11.4 How Are Errors in DNA Repaired?
DNA polymerase
Figure 11.22 DNA Repair Mechanisms (A)
11.4 How Are Errors in DNA Repaired?
The newly replicated DNA is scanned for mistakes by
other proteins.
Mismatch repair mechanism detects mismatched
bases—the new strand has not yet been modified
(e.g., methylated in prokaryotes) so it can be
recognized.
If mismatch repair fails, the DNA is altered.
Figure 11.22 DNA Repair Mechanisms (B)
11.4 How Are Errors in DNA Repaired?
DNA can be damaged by radiation, toxic chemicals,
and random spontaneous chemical reactions.
Excision repair: enzymes constantly scan DNA for
mispaired bases, chemically modified bases, and
extra bases—unpaired loops.
Figure 11.22 DNA Repair Mechanisms (C)
• IB Finished with Chapter 11!
Quiz tomorrow (3.3, 3.4) & 7.1, 7.2
11-5 Recap:
• Knowledge of the mechanisms of
DNA replication led to
development of techniques for
making multiple copies of DNA
sequences of DNA molecules!
11-5 Recap ?s:
• What do primers do in PCR?
• Why are dideoxyribonucleosides
used in DNA sequencing?
11.5 What Are Some Applications of Our Knowledge of DNA Structure and
Replication?
Copies of DNA sequences can be made
by the polymerase chain reaction
(PCR) technique.
PCR is a cyclical process:
• DNA fragments are denatured by
heating.
• A primer, plus nucleosides and DNA
polymerase are added.
• New DNA strands are synthesized.
Figure 11.23 The Polymerase Chain Reaction
11.5 What Are Some Applications of Our Knowledge of DNA Structure and
Replication?
PCR results in many copies of the DNA
fragment—referred to as amplifying the
sequence.
Primers are 15–20 bases, made in the
laboratory. The base sequence at the 3′
end of the DNA fragment must be known.
11.5 What Are Some Applications of Our Knowledge of DNA Structure and
Replication?
DNA polymerase that does not denature
at high temperatures (90°C) was taken
from a hot springs bacterium, Thermus
aquaticus.
11.5 What Are Some Applications of Our Knowledge of DNA Structure and
Replication?
DNA sequencing determines the base
sequence of DNA molecules.
Relies on altered nucleosides with
fluorescent tags that emit different
colors of light.
DNA fragments are then denatured and
separated by electrophoresis.
Figure 11.24 Sequencing DNA (Part 1)
Figure 11.24 Sequencing DNA (Part 2)
• Replication Activity
• Sequencing Activity