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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Passing on Life’s Information:
DNA Structure and Replication
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.
13.1 What Do Genes Do,
and What Are They Made Of?
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Rise of Molecular Biology
• James Watson and Francis Crick discovered the
chemical structure of DNA in 1953.
• This event ushered in a new era in biology
because it allowed researchers to understand
some of the most fundamental processes in
genetics.
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Rise of Molecular Biology
• In trying to decipher the structure of DNA,
Watson and Crick were performing work in
molecular biology.
• This is the investigation of life at the level of its
individual molecules.
• Molecular biology has grown greatly in
importance since the 1950s.
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13.2 Watson and Crick: The Double Helix
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Watson and Crick
• Watson and Crick met in the early 1950s at
Cambridge University in England and set about
to decipher the structure of DNA.
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Watson and Crick
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Figure 13.1
Rosalind Franklin
• Their research was aided by the work of others,
including Rosalind Franklin, who was using Xray diffraction to learn about DNA’s structure.
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Rosalind Franklin
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Figure 13.2
13.3 The Components of DNA and Their
Arrangement
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The Structure of DNA
• The DNA molecule is composed of building
blocks called nucleotides.
• Each nucleotide consists of:
– One sugar (deoxyribose)
– One phosphate group
– And one of four bases: adenine, guanine, thymine,
or cytosine (A, G, T, or C)
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The Structure of DNA
• The sugar and phosphate groups are linked
together in a chain that forms the “handrails” of
the DNA double helix.
• Bases then extend inward from the handrails,
with base pairs joined to each other in the
middle by hydrogen bonds.
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The Structure of DNA
• In this base pairing, A always pairs with T
across the helix, while G always pairs with C.
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The Structure of DNA
Phosphate
group
Sugar
Bases
deoxyribose
adenine (A)
thymine (T)
guanine (G)
cytosine (C)
Component molecules
1. The DNA molecule is composed
of three types of component
molecules: phosphate groups,
the sugar deoxyribose, and the
bases adenine, thymine, guanine,
and cytosine (A, T, G, and C).
Nucleotides
2. These three molecules link to form
the basic building block of DNA,
the nucleotide. Each nucleotide is
composed of one sugar, one
phosphate group, and one of the
four bases—in this example, A.
Across the strands of the helix, A
always pairs with T, and G with C.
The double helix
3. The sugar from one nucleotide
links with the phosphate from the
next to form the “handrails” of the
double helix. Meanwhile, the
bases form the “stairsteps,” each
base extending across the helix to
link with a complementary base
extending from the other side.
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Figure 13.3
DNA Replication
• DNA is copied by means of each strand of
DNA serving as a template for the synthesis of
a new, complementary strand.
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DNA Replication
• The DNA double helix first divides down the
middle.
• Each A on an original strand then specifies a
place for a T in a new strand, while each G
specifies a place for a C, and so forth.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
DNA Replication
1. DNA to be replicated
2. Strands separate
3. Each strand now serves as
a template for the
synthesis of a separate
DNA molecule as free
nucleotides base-pair with
complementary
nucleotides on the existing
strands.
Order of bases
encodes information
for protein
production.
4. This results in
two identical
strands of DNA.
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Figure 13.4
DNA Replication
• Each double helix produced in replication is a
combination of one parental strand of DNA and
one newly synthesized complementary strand.
• This is how life builds on itself.
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DNA Replication
old
new
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Figure 13.5
DNA Polymerases
• A group of enzymes known as DNA
polymerases is central to DNA replication.
• These enzymes move along the double helix,
bonding together new nucleotides in
complementary DNA strands.
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Protein Production
• DNA can encode the information for the huge
number of proteins used by living things
because the sequence of bases along DNA’s
handrails can be laid out in an extremely varied
manner.
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Protein Production
• A collection of bases in one order encodes the
information for one protein.
• A different sequence of bases encodes the
information for a different protein.
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DNA
PLAY
Animation 13.1: DNA
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13.4 Mutations
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Mutations
• The error rate in DNA replication is very low,
partly because repair enzymes are able to
correct mistakes.
• When such mistakes are made and then not
corrected, the result is a mutation: a permanent
alteration in a cell’s DNA base sequence.
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Mutations
Starting DNA
Incorrect base-pairing
Mutation
Point
mutation
1. In replicating a cell’s DNA,
mistakes are sometimes
made, such that one base can
be paired with another base
that is not complementary to
it (G with T in this case).
2. The next time a cell replicates its DNA, the
replication repair mechanism may “fix” this
error in such a way that a permanent alteration
in the DNA sequence results. The original G
will be replaced, instead of the wrongly added
T. The result is an A-T base pair, whereas the
cell started with a G-C base pair.
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Figure 13.6
Mutations
• Most mutations have no effect on an organism,
but when they do have an effect, it is generally
negative.
• Cancers result from a line of cells that have
undergone types of mutations that cause them
to proliferate wildly.
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Mutations
• Some mutations come about in the body’s
germ-line cells, meaning cells that become eggs
or sperm.
• Such mutations are heritable: they can be
passed on from one generation to another.
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Mutations
• The gene for Huntington disease, which is
expressed in nerve cells, is a heritable, mutated
form of a normal gene.
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Mutations
• Most mutations, however, come about in the
body’s somatic cells, which are cells that do not
give rise to eggs or sperm.
• Dangerous as these mutations may be, they
cannot be passed along to offspring.
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Mutagens
• Mutations can come about through the effects
of mutagens: substances, such as cigarette
smoke or ultraviolet light, that can mutate
DNA.
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Mutations and Evolutionary Adaptation
• Mutations have been important to evolution
because they are the only means through which
completely new genetic information can be
added to a species’ genome.
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Mutations and Evolutionary Adaptation
• The accidental reorderings of DNA sequences
that mutations bring about can, in rare
instances, produce new proteins that are useful
to organisms.
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Mutations
PLAY
Animation 13.2: Mutations
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One-Gene, One-Enzyme Hypothesis
PLAY
Animation 13.3: One-Gene, One-Enzyme Hypothesis
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