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11DNA and Its Role in Heredity
11 DNA and Its Role in Heredity
• What Is the Evidence that the Gene Is
DNA?
• What Is the Structure of DNA?
• How Is DNA Replicated?
• How Are Errors in DNA Repaired? (If
they are repaired?)
Figure 11.1 Genetic Transformation of Nonvirulent Pneumococci
11.1 What Is the Evidence that the Gene Is DNA?
Identifying the real transforming principle,
Oswald Avery:
Treated samples of the same bacteria
Griffith used, to destroy different
molecules in the bacteria (DNA and
proteins); he hypothesized that 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:
• Confirmed that DNA and not protein is
the genetic material using
bacteriophage T2 virus.
• Bacteriophage proteins were labeled
with radioactive markers as was the
DNA.
• 2 different radioactive markers were
used…..
Figure 11.3 Bacteriophage T2: Reproduction Cycle
Figure 11.4 The Hershey–Chase Experiment (Part 1)
11.2 What Is the Structure of DNA?
Now that it was conclusive that DNA was the inheritance model, the race was on to determine the molecular structure of DNA!
One crucial piece came from X-ray crystallography. (Franklin)
FYI: A purified substance can be made to form crystals; position of atoms is inferred by the pattern of diffraction of X-rays passed through it.
Figure 11.6 X-Ray Crystallography Helped Reveal the Structure of DNA
11.2 What Is the Structure of DNA?
Chemical X-ray also provided clues:
DNA is a polymer of nucleotides:
deoxyribose, a phosphate group, and a
nitrogen-containing base.
The bases:
• Purines: adenine (A), guanine (G)
• Pyrimidines: cytosine (C), thymine (T)
Figure 3.23 Nucleotides Have Three Components
repeat fig 3.23 here
11.2 What Is the Structure of DNA?
1950: Erwin Chargaff found in the DNA
from many different species:
amount of A = amount of T
amount of C = amount of G
Or, the abundance of purines = the
abundance of pyrimidines—Chargaff’s
rule.
Figure 11.7 Chargaff’s Rule
11.2 What Is the Structure of DNA?
Model building: Francis Crick and James
Watson used model building and
combined all the knowledge of DNA to
determine its structure.
A Structure for Deoxyribose Nucleic Acid
J. D. Watson and F. H. C. Crick (1)
April 25, 1953 (2), Nature (3), 171, 737-738
Figure 11.8 DNA Is a Double Helix (A)
Figure 11.8 DNA Is a Double Helix (B)
11.2 What Is the Structure of DNA?
Key features of DNA:
• A double-stranded helix, uniform diameter
• It is antiparallel in charges on either side of the molecules backbone strands
• Monomers (nucleotides) are; pentose (de-oxyribose) sugar, phosphate, nitrogenous base (A,T, G or C)
• Hydrogen bonds between corresponding nucleotides.
11.2 What Is the Structure of DNA?
Complementary base pairing:
• Adenine pairs with thymine by two
hydrogen bonds.
• Cytosine pairs with guanine by three
hydrogen bonds.
• Every base pair consists of one purine
and one pyrimidine.
Figure 11.9 Base Pairing in DNA Is Complementary (Part 1)
11.2 What Is the Structure of DNA?
Antiparallel: The backbone ends on one side with a sugar and the side ends with a phosphate. The opposite side is true so that you have: 3′ sugar (OH) end and a 5′ (P) phosphate match up at one end and alternating all the way down to the end of the other side.
S-P-S-P and on the other side it would be P-S-P-S
WHY have a free 5′ phosphate group; at the other end a free 3′ sugar group?
Sugars are slightly + Phosphates are slightly -(Antiparallel!)
Figure 11.9 Base Pairing in DNA Is Complementary (Part 2)
So how do we get increasing complexity? Two main ways;
1. Endosymbiosis
2. DNA mutation
What and how many prokaryotes that are engulfed gives you the 2
Main types of eukaryotic cells!
The Endosymbiotic Theory
There are two One cell "engulfs" A double membrane prokaryotic cells the other cell can be found inside
http://www.youtube.com/watch?v=nc6ddfWK0sA
11.2 What Is the Structure of DNA?
Functions of DNA:
• Store genetic material—millions of
nucleotides; base sequence stores and
encodes huge amounts of information
• Susceptible to mutation—change in
information
• Possible in 1 of 2 processes, DNA
replication and transcription
11.2 What Is the Structure of DNA?
• Genetic material is precisely replicated
in cell division—by complementary base
pairing.
• Genetic material is expressed as the
phenotype—nucleotide sequence
determines sequence of amino acids in
proteins.
11.3 How Is DNA Replicated?
Kornberg showed that DNA contains information for its own replication. He was not sure which model of replication was accurate at first…..
Three possible replication patterns:
• Semiconservative replication
• Conservative replication
• Dispersive replication
Figure 11.10 Three Models for DNA Replication
11.3 How Is DNA Replicated?
Meselson and Stahl showed that semi-
conservative replication was the
correct model.
They used radioactive labeling to
distinguish parent DNA strands from
new DNA strands. And in the end could
see that the new DNA was made up of
a strand of each radioactive template!
.
11.3 How Is DNA Replicated?
Two steps in DNA replication:
• The double helix is unwound, making
two template strands.
• New nucleotides are added to the new
strand at the 3′ end; joined by
phosphodiester linkages (back bone).
Sequence is determined by
complementary base pairing.
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)
Major players are;
Helicase, Histones (single binding
proteins/SBP), Primase, DNA Polymerase,
Ligase, Telomerase
11.3 How Is DNA Replicated?
A large protein complex—the replication
complex—catalyzes the reactions of
replication.
All chromosomes have a base sequence
called origin of replication (ori).
Replication complex binds to ori at start.
DNA replicates in both directions, forming
two replication forks.
11.3 How Is DNA Replicated?
DNA helicase uses energy from ATP
hydrolysis to unwind the DNA and
suspend the H bonds.
Single-strand binding proteins keep
the strands from getting back together.
AKA HISTONES.
11.3 How Is DNA Replicated?
Small, circular chromosomes have a
single origin of replication.
Large linear chromosomes have many
origins of replication.
DNA is replicated simultaneously at the
origins
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?
A primer is required to start DNA
replication—a short single strand of
RNA.
Primer is synthesized by RNA primase.
Then DNA polymerase begins adding
nucleotides to the 3′ parental template
as DNA polymerase runs in a 5-3
direction.
Figure 11.16 No DNA Forms without a Primer
Figure 11.17 Many Proteins Collaborate in the Replication Complex
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#
11.3 How Is DNA Replicated?
At the replication fork:
The leading strand is pointing in the “right”
direction for replication because DNA
polymerase works in a 5’-3’ direction.
The lagging strand is in the “wrong” direction.
Synthesis of the lagging strand occurs in small,
discontinuous stretches produces—Okazaki
fragments.
Figure 11.18 The Two New Strands Form in Different Ways
11.3 How Is DNA Replicated?
The final phosphodiester linkages
between fragments is catalyzed by DNA
ligase.
Figure 11.19 The Lagging Strand Story (Part 1)
11.3 How Is DNA Replicated?
Eukaryote chromosomes have repetitive
sequences at the ends called telomeres.
These sections do not get copied during
replication, thus shortening the DNA each
round of division. This allows for cells to
undergo apoptosis.
Figure 11.21 Telomeres and Telomerase
11.3 How Is DNA Replicated?
Chromosomes can lose 50–200 base
pairs (telomeres) with each replication.
After 20–30 divisions, the cell goes
through pre-programmed death.
11.3 How Is DNA Replicated?
Some cells—bone marrow stem cells,
gamete-producing cells—have
telomerase that catalyzes the addition
of telomeres.
90% of human cancer cells have
telomerase; normal cells do not.
11.4 How Are Errors in DNA Repaired?
DNA polymerases make mistakes in
replication, and DNA can be damaged
in living cells.
Repair mechanisms:
• Proofreading- DNA polymerase
• Excision repair-Ligase
• Mismatch repair-DNA polymerase
Figure 11.22 DNA Repair Mechanisms (A)
Figure 11.22 DNA Repair Mechanisms (B)
Figure 11.22 DNA Repair Mechanisms (C)
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.
• Must have heat tolerant polymerase…why?
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.
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.
Figure 11.23 The Polymerase Chain Reaction
Why do we want to copy DNA?
