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DNA: The Genetic Material

Chapter 14

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CH 14 Outline

• 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.

• “A genetic material must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way.”

• -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

• 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

• Purines - large bases– adenine and guanine

• Pyrimidines - small bases– cytosine and thymine

Chargaff’s ruleA = T and G = C

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Nucleotides

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Three-Dimensional Structure of DNA

• X-ray diffraction suggested DNA had helical shape with a 2 nanometer diameter.

– Watson and Crick deduced DNA is an inter-twined double helix.

complementary base-pairingpurines 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 daughterhelix 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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(a) Hypothesis 1:

Semi-conservative replication

(b) Hypothesis 2:Conservative replication

Intermediate molecule

(c) Hypothesis 3:Dispersive replication

MODELS OF DNA REPLICATION

14REPLICATION OF INFORMATION

TEMPLATING

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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

• 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

• Double-stranded DNA unwinds.

The junction of the unwound

molecules is a replication fork.

A new strand is formed by pairing complementary bases with theold strand.

Two molecules are made.

Each has one new and one old

DNA strand.

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Origin

5’3’

3’5’

UNIDIRECTIONAL REPLICATION

Origin

5’3’

3’5’

BIDIRECTIONAL REPLICATION

Replication can be Uni- or Bidirectional

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Semi-Conservative Replication

• Each chain in the helix is a complimentary mirror image of the other.

– double helix unzips and undergoes semi-conservative replication

each strand original duplex becomes one strand of another duplex

confirmed by Meselson-Stahl experiment

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Replication Process

• 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

endonucleasesexonucleases

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DNA Replication

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Replication Process

• 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

• 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

• 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

• Replisome is a macromolecular protein machine (replication organelle).

– fast, accurate replication of DNA during cell division

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Stages of Replication

• 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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OverviewFeatures of DNA Replication

• 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