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Chapter 10
DNA: The Molecule
of Heredity
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc.
10.1 What Is The Structure Of DNA?
 Deoxyribonucleic acid (DNA) is the blueprint
of life.
 DNA carries the information in its molecular
structure, which codes for all the special
features of a given life form.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 Individual traits of an organism are
transmitted from parent to offspring in
discrete units of DNA called genes.
 Genes are located on chromosomes found
within the nucleus of cells.
 What makes all organisms different from
each other is the arrangement and
molecular composition of its genes.
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10.1 What Is The Structure Of DNA?
 DNA is composed of four different subunits,
called nucleotides.
• Each nucleotide has three parts:
• A phosphate group
• A sugar, called deoxyribose
• One of four different nitrogen-containing
bases
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10.1 What Is The Structure Of DNA?
 DNA has four nitrogen-containing bases.
•
•
•
•
Thymine
Cytosine
Adenine
Guanine
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10.1 What Is The Structure Of DNA?
P
–O
CH2
O
H
H
H
O–
O
phosphate
CH2
H
phosphate
H
O
H
OH H
sugar
H
O
H
O
N
H
H
N
H
H
N
N H
OH H
sugar
H
base = guanine
H
O–
O
H
N H
P
N
H
N
H
CH2
N
O
N
H
O
N
CH3
P
–O
–O
H
base = adenine
OH H
sugar
O
N H
N
H
P
H
N
H
O
phosphate
O–
O
O–
O
O
base = thymine
–O
O
phosphate
CH2
H
H
O
N
N
H
H
H
O
base = cytosine
OH H
sugar
Fig. 10-1
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 A DNA molecule contains two nucleotide
strands.
• A DNA molecule consists of two DNA strands
of linked nucleotides.
• Within each strand, the phosphate group of
one nucleotide binds to the sugar group of
another nucleotide.
• The sugar-phosphate bonding produces a
sugar-phosphate backbone to the DNA
molecule.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 All the nucleotides in a single DNA strand
are oriented in the same direction.
• The ends of the two DNA strands are different.
• One strand ends in an unbonded sugar.
• One strand ends in an unbonded
phosphate.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 Hydrogen bonds hold the two DNA strands
together in a double helix.
• The two DNA strands are held together by
hydrogen bonding between the protruding
bases of the separate strands.
• The combined strands of DNA form a ladderlike double helix, with a sugar-phosphate
backbone and nucleotide pairs forming the
rungs.
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 The Watson-Crick model of DNA structure
nucleotide
nucleotide
free
phosphate
phosphate
base
(cytosine)
sugar
free sugar
(a) Hydrogen bonds hold complementary base pairs
together in DNA
(b) Two DNA strands form
a double helix
(c) Four turns of a
DNA double helix
Fig. 10-2
Copyright © 2009 Pearson Education Inc.
10.1 What Is The Structure Of DNA?
 Nucleotide rungs only result in specific pair
combinations.
• Adenine only pairs with thymine.
• Guanine only pairs with cytosine.
• These A–T and G–C pairs are called
complementary base pairs.
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10.2 How Does DNA Encode Information?
 It is NOT the number of different subunits
that code for all the diversity of
characteristics among organisms, but it is
rather the sequence in which they are
arranged along the molecule.
 Within a DNA molecule, the bases can be
arranged in any sequence.
 Each sequence is a unique set of genetic
instructions.
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10.2 How Does DNA Encode Information?
 A stretch of DNA only 10 nucleotides long
can have more than 1 million possible
sequences of the four bases.
 Since a typical organism has millions (e.g.,
a bacterium) or billions (e.g., a plant or
animal) of nucleotides, DNA molecules can
encode an incredible amount of information.
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10.3 How Is DNA Copied?
 Cells reproduce themselves by making two
daughter cells from each parental cell, each
with a complete copy of all the parental
cell’s genetic information.
 During cell reproduction, the parental cell
synthesizes two exact copies of its DNA
through a process called DNA replication.
 One copy goes into each daughter cell.
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10.3 How Is DNA Copied?
 DNA replication produces two DNA double
helices, each with one original strand and
one new strand.
• DNA replication requires three ingredients:
• The parental DNA strands
• Free nucleotides that were synthesized in
the cytoplasm and then imported to the
nucleus
• A variety of enzymes that unwind the
parental DNA double helix and synthesize
new DNA strands
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10.3 How Is DNA Copied?
 The basic features
of DNA replication
1 Parental DNA
double helix
2 The parental DNA
is unwound
3 New DNA strands
are synthesized with
bases complementary
to the parental
strands
free nucleotides
4 Each new double helix is composed
of one parental strand (blue) and one
new strand (red)
Fig. 10-3
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 DNA replication produces two DNA double
helices, each with one original strand and
one new strand (continued).
• The first step involves enzymes called DNA
helicases, which pull apart the parental DNA
double helix.
• Next, enzymes called DNA polymerases move
along each separated parental DNA strand,
matching each base on the strand with free
nucleotides.
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10.3 How Is DNA Copied?
 DNA replication keeps, or conserves, one
parental DNA strand and produces one new
daughter strand.
• This process is called semiconservative
replication.
Copyright © 2009 Pearson Education Inc.
10.3 How Is DNA Copied?
 DNA replication is
semiconservative.
One DNA
double helix
DNA replication
Two identical DNA
double helices, each
with one parental
strand (blue) and
one new strand (red)
Fig. 10-4
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 DNA helicase separates the parental DNA
strands by breaking the hydrogen bonds
between complementary bases.
• This activity separates the two strands and
forms a replication bubble where the parental
strands are no longer paired.
• Replication then proceeds.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (1)
replication bubbles
DNA
Fig. 10-5(1)
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10.4 What Are The Mechanisms Of DNA
Replication?
 DNA helicase separates the parental DNA
strands.
• There is a replication fork on each end of the
bubble, where replication is taking place and
the original DNA strand is unzipping.
• The unzipping and replication continues in
both directions until the new strands are
completely formed.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (2)
DNA helicase
DNA helicase
replication forks
Fig. 10-5(2)
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10.4 What Are The Mechanisms Of DNA
Replication?
 DNA polymerase synthesizes new DNA
strands.
• At the replication forks, DNA polymerase
recognizes unpaired nucleotide bases in the
parental strand and matches them up with free
nucleotides.
• It then links up the phosphate of the incoming
nucleotide with the sugar of the previously
added nucleotide, thereby contributing to the
growing molecule backbone.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 DNA helicase and DNA polymerase work
together to copy each strand of separated
parental DNA.
• Polymerase # 1 lands on one strand of DNA
and follows behind the helicase toward the
free phosphate end of the DNA, making a
continuous new DNA strand.
• DNA polymerase # 2 on the other parental
strand moves away from the helicase and
makes only part of the new DNA strand.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (3)
DNA polymerase #1
free sugar
end of the
parental DNA
DNA
polymerase #2
free phosphate
end of the
parental DNA
strand
Fig. 10-5(3)
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 As the helicase continues to unwind more of
the double helix, additional DNA polymerase
(# 3, # 4, etc.) must land on this strand to
synthesize more pieces of DNA.
 Therefore, DNA synthesis on the second
parental strand is discontinuous.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (4)
DNA polymerase #1
continues along the
parental DNA strand
DNA
polymerase #2
leaves
DNA
polymerase #3
Fig. 10-5(4)
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 Multiple DNA polymerases make many
pieces of DNA of varying lengths that need
to be tied together to form a single
continuous DNA polymer.
 DNA ligase joins together the separate
segments of DNA.
Copyright © 2009 Pearson Education Inc.
10.4 What Are The Mechanisms Of DNA
Replication?
 The mechanism of DNA replication,
step (5)
DNA polymerase #3
leaves
DNA
polymerase #4
DNA ligase joins the daughter
DNA strands together
Fig. 10-5(5)
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10.4 What Are The Mechanisms Of DNA
Replication?
 Proofreading produces almost error-free replication of
DNA.
• DNA polymerase is almost 100% perfect in
matching free nucleotides with those on the
original parental strands.
• Once in every 10,000 base pairs, there is an error
in replication.
• Some types of DNA polymerase recognize errors
when they are made and correct them.
• This keeps the total errors in a complete DNA
molecule to one mistake in every billion base pairs.
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10.4 What Are The Mechanisms Of DNA
Replication?
 Mistakes that remain in the DNA nucleotide
sequence are called mutations.
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Mistakes Do Happen
DNA is damaged in a number of ways
 Spontaneous chemical breakdown at
body temperature
 Certain chemicals (some components of
cigarette smoke)
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Mistakes Do Happen

UV light from the sun causes DNA
damage
• DNA damage leads to uncontrollable
cell division and skin cancer
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Types of Mutations



Point mutation - individual nucleotide in
the DNA sequence is changed
Insertion mutation - one or more
nucleotide pairs are inserted into the
DNA double helix
Deletion mutation - one or more
nucleotide pairs are removed from the
double helix
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Types of Mutations


Inversion - piece of DNA is cut out of a
chromosome, turned around, and reinserted into the gap
Translocation - chunk of DNA (often
very large) is removed from one
chromosome and attached to another
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.
Copyright © 2009 Pearson Education Inc.