DNA: The Genetic Material
Chapter 14
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CH 14 Outline
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Chemical Nature of Nucleic Acids
Three-Dimensional Structure of DNA
– Watson and Crick
Replication
– Semi Conservative
– Replication Process
One-Gene/One-Polypeptide Hypothesis
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Replication
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DNA is the Genetic Material
Therefore it must
(1) Replicate faithfully.
(2) Have the coding capacity to generate
proteins and other products for all
cellular functions.
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“A genetic material must carry out two jobs:
duplicate itself and control the development of
the rest of the cell in a specific way.”
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-Francis Crick
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The Dawn of Molecular Biology
April 25, 1953
Watson and Crick: "It has not escaped our
notice that the specific (base) pairing we
have postulated immediately suggests a
possible copying mechanism for the
genetic material."
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Chemical Nature of Nucleic Acids
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DNA made up of nucleic acids
– Each nucleotide is composed of a five
carbon sugar, a phosphate group, and an
organic base.
 nucleotides distinguished by the bases
 reaction between phosphate group of
one nucleotide and hydroxyl group of
another is dehydration synthesis
 phosphodiester bond
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Chemical Nature of Nucleic Acids
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Purines - large bases
– adenine and guanine
Pyrimidines - small bases
– cytosine and thymine
 Chargaff’s rule
 A = T and G = C
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Nucleotides
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Three-Dimensional Structure of DNA
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X-ray diffraction suggested DNA had helical
shape with a 2 nanometer diameter.
– Watson and Crick deduced DNA is an intertwined double helix.
 complementary base-pairing
 purines pairing with pyrimidines
 constant 2 nanometer diameter
 antiparallel configuration
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DNA Double Helix
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Bases paired
Strands antiparallel
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Models for DNA replication
1) Semiconservative model:
Daughter DNA molecules contain one parental
strand and one newly-replicated strand
2) Conservative model:
Parent strands transfer information to an
intermediate (?), then the intermediate gets copied.
The parent helix is conserved, the daughter
helix is completely new
3) Dispersive model:
Parent helix is broken into fragments, dispersed,
copied then assembled into two new helices.
New and old DNA are completely dispersed
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MODELS OF DNA REPLICATION
(a) Hypothesis 1:
(b) Hypothesis 2:
(c) Hypothesis 3:
Semi-conservative
replication
Conservative replication
Dispersive replication
Intermediate molecule
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TEMPLATING
REPLICATION OF INFORMATION
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DNA replication
Nucleotides are successively added using deoxynucleoside triphosphosphates (dNTP’s)
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Key proposal of Watson and Crick: base pairs A : T and G : C
are specific. Base pairing regulates replication.
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DNA Replication
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Since DNA replication is semiconservative,
therefore the helix must be unwound.
John Cairns (1963) showed that initial unwinding
is localized to a region of the bacterial circular
genome, called an “origin” or “ori” for short.
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Replication as a process
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Double-stranded DNA unwinds.
The junction of the unwound
molecules is a replication fork.
A new strand is formed by pairing
complementary bases with the
old strand.
Two molecules are made.
Each has one new and one old
DNA strand.
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Replication can be Uni- or Bidirectional
UNIDIRECTIONAL REPLICATION
Origin
BIDIRECTIONAL REPLICATION
3’
5’
5’
3’
5’
3’
Origin
3’
5’
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Semi-Conservative Replication
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Each chain in the helix is a complimentary
mirror image of the other.
– double helix unzips and undergoes semiconservative replication
 each strand original duplex becomes
one strand of another duplex
 confirmed by Meselson-Stahl
experiment
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Replication Process
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Replication of DNA begins at one or more
sites (replication origin).
– DNA polymerase III and other enzymes
add nucleotides to the growing
complementary DNA strands.
 require a primer
 can only synthesize in one direction
 endonucleases
 exonucleases
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DNA Replication
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Replication Process
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DNA polymerase cannot link the first
nucleotides in a newly synthesized strand.
– RNA polymerase (primase) constructs an
RNA primer.
DNA polymerase adds nucleotides to 3’ end.
– Leading strand replicates toward replication
fork.
– Lagging strand elongates from replication
fork.
 Okazaki fragments
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DNA Synthesis
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Replication Process
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DNA ligase attaches fragment to lagging
strand.
– Because synthesis of the leading strand is
continuous and the lagging strand is
discontinuous, the overall replication of DNA
is referred to as semi-discontinuous.
DNA gyrase removes torsional strain
introduced by opening double helix.
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Replication Process
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Opening DNA double helix
– initiating replication
– unwinding duplex
– stabilizing single strands
– relieving torque
Building a primer
Assembling complementary strands
Removing the primer
Joining Okazaki fragments
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DNA Replication Fork
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Replisome
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Replisome is a macromolecular protein
machine (replication organelle).
– fast, accurate replication of DNA during
cell division
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Stages of Replication
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Initiation
– always occurs at the same site
Elongation
– majority of replication spent in elongation
Termination
– exact details unclear
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Evidence points to bidirectional replication
Label at both replication forks
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Overview
Features of DNA Replication
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DNA replication is semiconservative
– Each strand of both replication forks is
being copied.
DNA replication is bidirectional
– Bidirectional replication involves two
replication forks, which move in opposite
directions
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