Download Chapter 16: THE MOLECULAR BASIS OF INHERITANCE (DNA

BIOLOGY I
Chapter 16:
THE MOLECULAR BASIS OF
INHERITANCE
(DNA Structure and Function)
Evelyn I. Milian
Instructor
BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA Is the Genetic Material
• What is DNA and why is it so important for life?

DNA (deoxyribonucleic acid) is a type of organic
macromolecule; it is a nucleic acid composed of subunits
called nucleotides (we will study its structure in detail).

DNA is the genetic material of nearly all organisms. It is
found in eukaryotic and prokaryotic cells. The “molecular
instructions” in DNA direct the life of each cell in an
organism. DNA stores genetic information regarding the
development, structure, and metabolic activities of a cell.

DNA also enables organisms, or cells within an organism,
to transmit information accurately from one generation to
the next.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Levels of Structure and Function of the Genome
The genome is the sum total of genetic material of a cell. Although most of the
genome exists in the form of chromosomes, genetic material can appear in
nonchromosomal sites as well. For example, bacteria and some fungi contain
tiny extra pieces of DNA called plasmids, and certain organelles of eukaryotes
(mitochondria and chloroplasts) are equipped with their own genetic material.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
History: The Search for the Genetic Material
How Did Scientists Discover that Genes are Made of DNA?
• Early in the twentieth century, scientists knew that the genes are on
the chromosomes, but they did not know the composition of genes.
The identification of the molecules of inheritance was a major
challenge to biologists.
• DNA and proteins were the candidates for the genetic material, but
proteins seemed stronger because of their complexity and variety.
Moreover, little was known about nucleic acids.
• Scientists knew that this genetic material must be:
1. able to store information that pertains to the development, structure,
and metabolic activities of the cell or organism;
2. stable so that it can be replicated with high fidelity during cell division
and be transmitted from generation to generation;
3. able to undergo rare genetic changes called mutations that provide the
genetic variability required for evolution to occur.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
History: The Search for the Genetic Material
• Previous knowledge about nucleic acids:

1869, Johann Friedrich Miescher: He removed nuclei from
pus cells and found a chemical he called nuclein. It was rich
in phosphorus and had no sulfur, properties that distinguished
it from protein. In this way, he helped paving the way for the
identification of DNA as the carrier of inheritance.

Later, other chemists did further research with nuclein and said
that it contained an acidic substance they called nucleic acid.

However, for many years nobody thought about nucleic acids
as the genetic material because their physical and chemical
properties seemed far too uniform to account for the variety of
inherited traits exhibited by every organism.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Search for the Genetic Material:
How Did Scientists Discover that Genes are Made of DNA?
• After several diligent experiments, by the mid-1950s
researchers realized that DNA, not protein, is the
genetic material.
• Experiments with bacteria and viruses that infect
bacteria (phages, or bacteriophages) provided the
first strong evidence that the genetic material is DNA.
• The discovery of how DNA carries life’s blueprints
was one of the greatest achievements of 20th
century biology.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Evidence That DNA Can Transform Bacteria (Transformation)
• 1928: Frederick Griffith was trying to make a vaccine to
prevent bacterial pneumonia. He was working with two
strains (variants) of the Streptococcus pneumoniae
bacterium (pneumococcus), a pathogenic (disease-causing)
strain (S) and a nonpathogenic (harmless) strain (R). The
pathogenic S (smooth) strain had a resistant capsule
not found in the nonpathogenic R (rough) strain.

He found that when he killed the pathogenic bacteria with heat
and then mixed the cell remains with living bacteria of the
nonpathogenic strain, some of the living cells became
pathogenic. Furthermore, this new trait of pathogenicity was
inherited by all the descendants of the transformed bacteria.
He concluded that some chemical component of the dead
pathogenic cells caused this heritable change.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Evidence That DNA Can Transform Bacteria:
Bacterial Transformation
• Griffith’s experiments showed that some substance
in the heat-killed S strain changed the living but
harmless R strain into the deadly S strain.
• He called this phenomenon transformation, now
defined as a genetic change due to the assimilation
of external DNA by a cell.
• Griffith’s work set the stage for the search for the identity
of the transforming substance by other scientists during
the following years.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Animation: Griffith’s Experiment – Transformation
• ..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\griffith.html
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Evidence That DNA Can Transform Bacteria
• 1944: Oswald Avery, Colin MacLeod, and
Maclyn McCarty purified the transforming molecule
and published a paper demonstrating that it is DNA.
Their evidence included the following observations:
1.
DNA from S strain bacteria causes R strain bacteria
to be transformed.
2.
Enzymes that degrade proteins (proteinases)
cannot prevent transformation, nor does RNase, an
enzyme that digests RNA (ribonucleic acid).
3.
The molecular weight of the transforming substance
is so great that it must contain about 1,600
nucleotides! Certainly, this is enough for some
genetic variability.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Confirmation of DNA Function:
Evidence That Viral DNA Can Program Cells
• Additional evidence for DNA as the genetic material came from
studies of a virus that infects bacteria, called a bacteriophage or
phage.
• Virus: A noncellular parasitic agent consisting of an inner core of
nucleic acid (DNA or RNA) and an outer coat of protein (capsid).
• To reproduce, a virus must infect a cell and take over the cell’s
metabolic machinery.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Evidence That Viral DNA Can Program Cells
• 1952: Alfred D. Hershey and Martha Chase performed
experiments with phage T2 to determine which of the phage
components—protein or DNA—entered bacterial cells and
directed reproduction of the virus. T2 infects Escherichia coli,
a bacterium that normally lives in the intestines of mammals.
• Hershey and Chase relied on a chemical difference between
DNA and protein to solve the mystery: DNA contains
phosphorus but no sulfur; proteins contain sulfur but no
phosphorus.

They used radioactive phosphorus to label the DNA core of the
phage and radioactive sulfur to label the protein capsid of the
phage. In this way, they were able to trace the component that
entered the bacterial cells, and they determined that it was DNA.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Watson and Crick Model for the Structure of DNA
• 1953: James Watson and Francis Crick reported their
molecular model for DNA: the double helix, for which
they received a Nobel Prize in 1962.
• Their model conformed to X-ray measurements (done by
Rosalind Franklin and Maurice Wilkins) and what was
then known about the biochemistry of DNA.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA, a Nucleic Acid
• Nucleic acids (DNA and RNA) are large organic molecules
composed of subunits called nucleotides. Nucleic acids contain
carbon, hydrogen, oxygen, nitrogen, and phosphorus.
• Nucleic acids store and transmit hereditary information, and are
involved in the synthesis of proteins, molecules with many functions.
• Nucleotides are compounds that contain these molecules:



a phosphate group;
a pentose (five-carbon) sugar: deoxyribose in DNA; ribose in RNA;
a nitrogenous base: one of four possible bases which are: adenine,
cytosine, guanine, and thymine in DNA or uracil in RNA.
Nucleotides are linked
together by covalent
bonds between the
phosphate of one
nucleotide and the sugar
of the next nucleotide.
The numbers 1 to 5 are
the carbons.
Covalent bonds:
sharing of two or
more electrons
(negatively
charged atomic
particles).
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Nucleotides in Nucleic Acids
• Nucleotides contain:
1. A phosphate group.
2. A pentose (five-carbon) sugar: deoxyribose in DNA; ribose in RNA.
3. A nitrogenous base: adenine, guanine, cytosine, and thymine in DNA
or uracil in RNA.
•
A and G are purines, with a double ring.
• T and C are pyrimidines, with a single ring.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA (Deoxyribonucleic Acid)
• 1940s: Erwin Chargaff analyzed the amounts of the four
nucleotides in DNA from diverse organisms. He provided
additional evidence that DNA is the genetic material.
• The result was Chargaff’s rules:
1. The amount of A, T, G, and C in DNA varies from species to species.
2. In each species, the amount of A equals the amount of T and the
amount of G equals the amount of C.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA: The Watson and Crick Model
• The structure of a DNA molecule consists of two long strands of
nucleotides wrapped around each other to form a double helix,
(like a twisted ladder, or spiral) and oriented in antiparallel
(opposite) directions (the sugar-phosphate groups are oriented
in different directions).
• Within each strand, the sugar (deoxyribose) of one nucleotide is
linked to the phosphate of the next nucleotide, forming a sugarphosphate “backbone” on each side of the double helix.
• The two DNA strands are held together by hydrogen bonds
between complementary nitrogenous base pairs (purine
with pyrimidine) (like the rungs or steps of the ladder).

A—T: Adenine (a purine,with a double ring) forms hydrogen
bonds only with thymine (a pyrimidine, with one ring).

G—C: Guanine (a purine) forms hydrogen bonds only with
cytosine (a pyrimidine).
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of a DNA Strand
• Each nucleotide monomer consists of
a nitrogenous base (T, A, C, or G),
the sugar deoxyribose (blue), and a
phosphate group (yellow). The
phosphate group of one nucleotide is
attached to the sugar of the next,
resulting in a “backbone” of
alternating phosphates and sugars
from which the bases project.
• The polynucleotide strand has
directionality, from the 5’ end (with
the phosphate group) to the 3’ end
(with the –OH group). 5’ and 3’ refer
to the numbers assigned to the
carbons in the sugar ring.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Watson and Crick’s
Model of DNA
a.
A space-filling model of DNA.
b.
The two strands of the
molecule are antiparallel—
that is, the sugar-phosphates
are oriented in different
directions: The 5’ end of one
strand is opposite the 3’ end
of the other strand.
c.
Diagram of DNA double
helix shows that the
molecule resembles a
twisted ladder or a spiral.
The bases are joined by
hydrogen bonds.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA, a Nucleic Acid
•
Nucleotides (top) are
composed of a
deoxyribose (in DNA)
sugar molecule linked to
a phosphate group and
to a nitrogenous base.
The two nucleotides
shown here are linked
by hydrogen bonds
between their
complementary bases.
•
The ladderlike form of
DNA’s double helix
(bottom) is made up of
many nucleotides, with
the repeating sugarphosphate combination
forming the backbone
and the complementary
bases the rungs.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA: The Watson and Crick Model
• The Watson and Crick model for DNA fit the mathematical measurements
provided by X-ray data for the spacing between the base pairs (0.34 nm)
and for a complete turn of the double helix (3.4 nm).
• The model also agreed with Chargaff’s rules, which said that the amount of
A equals the amount of T and the amount of G equals the amount of C.
A is hydrogen-bonded to T and G is hydrogen-bonded to C. This socalled complementary base pairing means that a purine is always
bonded to a pyrimidine. Only in this way will the molecule have the width
(2 nm) dictated by its X-ray diffraction pattern, since two pyrimidines together
are too narrow, and two purines together are too wide.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA:
The Watson and Crick Model
•
In a DNA double-helix, the two
sugar-phosphate chains run in
opposite directions. This
orientation permits the
complementary bases to pair.
•
Strong covalent bonds link the
units of each strand, while
weaker hydrogen bonds hold
one strand to the other by the
pairs of nitrogenous bases.
•
Adenine (A) pairs with thymine
(T), and guanine (G) pairs with
cytosine (C).
•
The A-T pair has two hydrogen
bonds, the G-C pair has three.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The Structure of DNA: The Watson and Crick Model
• The two upright strands, composed
of the sugar deoxyribose (D) and
phosphate groups (P), are held
together by hydrogen bonds
between complementary bases.
Adenine (A) always pairs with
thymine (T), and guanine (G)
always pairs with cytosine (C).
Each strand can thus provide the
information needed for the
formation of a new DNA molecule.
The A-T pair has two hydrogen bonds,
the G-C pair has three hydrogen bonds.
• The DNA molecule is twisted into
a double helix. The two sugarphosphate strands run in opposite
directions (antiparallel). Each new
strand grows from the 5’ (“five
prime”) end toward the 3’ end.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
The DNA Double Helix
•
Note: The “ribbons” in this diagram represent the sugar-phosphate backbones of the two DNA
strands. The helix is “right-handed”, curving up to the right.
•
Denaturation of DNA, the separation of the two strands of the double helix, occurs under
extreme (noncellular) conditions of pH (acidity level), salt concentration, and temperature.
•
For example, when a double-stranded DNA molecule is heated, it denatures into two singlestranded molecules. The heat breaks the hydrogen bonds holding the bases together in the
center of the molecule but does not affect the covalent bonds of the backbone.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Animation: DNA Structure: Subunits
•
..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\dna_subunits_adv.html
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA REPLICATION
• DNA replication is the process
of copying a DNA molecule.
Following replication, there is
usually an exact copy of the
DNA double helix.
•
DNA replication must occur
before a cell can divide
(Remember: During the S
phase of the cell cycle).
• As soon as Watson and Crick
developed their double-helix
model, they commented, “It
has not escaped our notice
that the specific pairing we
have postulated immediately
suggests a possible copying
mechanism for the genetic
material.”
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
REPLICATION OF DNA
• In a second paper, Watson and Crick stated their
hypothesis for how DNA replicates:

“Now our model for deoxyribonucleic acid is, in effect, a
pair of templates, each of which is complementary to the
other. We imagine that prior to duplication the hydrogen
bonds are broken, and the two chains unwind and
separate. Each chain then acts as a template for the
formation onto itself of a new companion chain, so that
eventually we shall have two pairs of chains, where we
only had one before. Moreover, the sequence of pairs
of bases will have been duplicated exactly.” *
• * F. H. C. Crick and J. D. Watson, “The Complementary Structure of
Deoxyribonucleic Acid.” Proc. Roy. Soc. (A) 223 (1954): 80.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Proposed Models of DNA
Replication
• Conservative model: The parent
molecule somehow re-forms after
the replication (it is conserved); in
other words, the two parental
strands are rejoined.
• Semiconservative model: Each
of the two daughter molecules of
DNA will have one old strand
derived from the parent molecule,
and one newly synthesized strand.
• Dispersive model: Each strand
of DNA following replication has a
mixture of old and new DNA.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Meselson & Stahl Experiment:
Semiconservative DNA Replication
•
•
•
Semiconservative replication was
experimentally confirmed by Matthew
Meselson and Franklin Stahl in 1958. They
showed that each daughter double helix
contains an old strand and a new strand.
Meselson and Stahl grew bacteria in heavy
nitrogen (15N) medium to label the bases of
DNA, making them more dense; and then
transferred some of the cells to light nitrogen
(14N, the ordinary one) so that the newly
synthesized strands would be light. They
isolated DNA from bacterial cells after one
and two generations and centrifuged it to
separate DNA into bands based on density.
After one division in light nitrogen, DNA
molecules are hybrid with intermediate
density. After two divisions, DNA molecules
separate into two bands—one for light DNA
and one for hybrid DNA, consistent with the
semiconservative model of replication.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Semiconservative
Replication of DNA
• Semiconservative
nature of DNA
replication: During DNA
replication, each parent
strand remains intact.
One new strand (gold) is
assembled on each of
the parent strands
(blue). In other words,
each of the two daughter
molecules of DNA will
have one old strand
derived from the parent
molecule, and one newly
synthesized strand.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
* SUMMARY OF STEPS IN THE REPLICATION OF DNA *
• During DNA replication, each old DNA strand of the parental
molecule (original double helix) serves as a template (mold) for a
new strand in a daughter molecule.
• Replication requires the following steps or stages:
1.
Uncoiling (unwinding). The old strands that make up the parental
DNA molecule are unwound (or separated) and “unzipped” (i.e., the
weak hydrogen bonds between the paired bases are broken).
•
2.
3.
•
A special enzyme called helicase unwinds the molecule.
Synthesis: complementary base pairing and elongation. New
complementary nucleotides are positioned through the process of
complementary base pairing.
Joining and termination. The complementary nucleotides join the
template strands to form new strands. Each daughter DNA molecule
contains an old strand and a new strand (semiconservative
replication).
*** Steps 2 and 3 are carried out by an enzyme complex called
DNA polymerase.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
A Simplified View of DNA Replication: The Basic Concept
• In this simplification, a short segment of DNA has been untwisted into a
structure that resembles a ladder. The rails of the ladder are the sugarphosphate backbones of the two DNA strands; the rungs are the pairs of
nitrogenous bases. Simple shapes symbolize the four kinds of bases.
Dark blue represents DNA strands present in the parent molecule; light
blue represents free nucleotides and newly synthesized DNA.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA replication requires
protein “machinery”
• The replication of DNA by
base pairing appears simple
and straightforward, but it
requires special proteins
and enzymes (catalytic
proteins that accelerate
specific chemical reactions).
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA Replication: Basic Steps
• After the DNA double helix
unwinds (by helicase), each old
strand serves as a template for
the formation of the new strand.
• Complementary nucleotides
available in the cell pair with
those of the old strand and then
are joined together to form a new
strand.
• After replication is complete,
there are two daughter DNA
double helices. Each one is
composed of an old strand and
a new strand.
• Each daughter double helix has
the same sequence of base pairs
as the parental double helix had
before unwinding occurred.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA Replication: A Closer Look (* Study the Figures *)
• DNA replication begins at a site called an origin of replication and
proceeds in both directions from that point (it’s bidirectional). The point
where the two DNA strands separate is called the replication fork.

The enzyme helicase untwists the double helix at replication forks.
 Single-strand binding proteins stabilize the unpaired DNA strands.
• An RNA primer, a short polynucleotide complementary to the template
strand, is synthesized (by primase) and enters the origin of replication.

The RNA primer is required as a start point for the addition of nucleotides
by DNA polymerase III; it can only add nucleotides to the 3’ end of an
existing polynucleotide strand or RNA primer.
• DNA polymerase then catalyzes the synthesis of the new DNA strand by
adding new nucleotides in the 5’  3’ direction.
• Because of the 5’  3’ direction of DNA polymerase, DNA replication is
continuous in one strand and discontinuous in the other: The leading
strand is synthesized continuously, and the lagging strand is synthesized
in short segments, called Okazaki fragments.

The fragments are joined together by DNA ligase.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
A Closer Look at DNA Replication
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Synthesis of leading and lagging
strands during DNA replication
1. DNA polymerase III elongates
new DNA strands in the 5’  3’
direction (it adds nucleotides to
the 3’ end of an existing strand).
2. One new strand, the leading
strand, can elongate continuously
in the 5’  3’ direction as the
replication fork progresses.
3. The other new strand, the lagging
strand, must grow in an overall 3’
 5’ direction by addition of short
segments, Okazaki fragments,
that grow 5’  3’ (numbered here
in the order they were made).
4. DNA ligase joins Okazaki
fragments by forming a bond
between their free ends. This
results in a continuous strand.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Origins of DNA Replication in
Bacteria and Eukaryotes
•
•
Evelyn I. Milian - Instructor
In the circular chromosome of
E. coli and many other bacteria
(prokaryotes), only one origin of
replication is present. The
parental strands separate at the
origin, forming a replication
bubble with two forks.
Replication proceeds in both
directions until the forks meet
on the other side, resulting in
two daughter DNA molecules.
In each linear chromosome of
eukaryotes, DNA replication
begins when replication bubbles
form at many origins along the
giant DNA molecule. The
bubbles expand as replication
proceeds in both directions.
Eventually the bubbles fuse and
synthesis of the daughter strand
is complete.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Some of the Proteins Involved in the Initiation of DNA Replication
• Helicase unwinds and separates the parental DNA strands.
• Topoisomerase breaks, swivels, and rejoins the parental DNA ahead of
the replication fork, relieving the strain caused by unwinding.
• Single-strand binding proteins stabilize the unwound parental strands.
• Primase synthesizes RNA primers, using the parental DNA as a template.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA REPLICATION: Incorporation of a Nucleotide into a DNA Strand
•
•
DNA polymerase catalyzes the addition of a nucleoside triphosphate to the 3’
end of a growing DNA strand (a nucleoside triphosphate has a nitrogenous
base, a pentose sugar, and three phosphate groups).
When a nucleoside triphosphate bonds to the sugar in a growing DNA strand, it
loses two phosphates. Hydrolysis of the phosphate bonds provides the energy
for the reaction.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
• Review the details for this figure in your book (Campbell & Reece, Biology).
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Enzymes and Other Proteins in Bacterial DNA Replication
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Proofreading and Repairing DNA
• Mistakes in DNA can result in the altered or diminished function
of encoded proteins and thus disrupt normal cell operations.
• During DNA replication, DNA polymerases proofread newly
made DNA, replacing any incorrect nucleotides.
• Mismatched nucleotides sometimes evade proofreading by a
DNA polymerase or arise after DNA synthesis is completed.

Mismatch repair. Special repair enzymes recognize incorrectly
paired nucleotides and remove them. DNA polymerases then fill
in the missing nucleotides.

Nucleotide excision repair. This repair mechanism is commonly
used to repair DNA lesions caused by the sun’s ultraviolet
radiation or by harmful chemicals. It involves three enzymes:
nucleases cut out (excise) and replace damaged stretches of
DNA. DNA polymerase adds the correct nucleotides, and DNA
ligase closes the breaks.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Nucleotide Excision Repair
of DNA Damage
• A team of enzymes detects and
repairs damaged DNA. This figure
shows DNA containing a thymine
dimer, a type of damage often
caused by ultraviolet radiation. A
nuclease enzyme cuts out the
damaged region of DNA and a
DNA polymerase (in bacteria,
DNA pol I) replaces it with a
normal DNA segment. Ligase
completes the process by closing
the remaining break in the sugarphosphate backbone.
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Chromatin Packing in a Eukaryotic Chromosome
•
Diagrams and micrographs (done with a transmission electron microscope)
depicting the progressive levels of DNA coiling and folding. Eukaryotic chromatin
making up a chromosome is composed mostly of DNA, histones, and other proteins. The
histones bind to each other and to the DNA to form nucleosomes, the most basic units of
DNA packing. Histone tails extend outward from each bead-like nucleosome core.
Additional folding leads ultimately to the highly condensed chromatin of the metaphase
chromosome. In interphase cells, most chromatin is less compacted (euchromatin), but
some remains highly condensed (heterochromatin). Histone modifications may influence
the state of chromatin condensation.
Evelyn I. Milian - Instructor
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
Animation: DNA REPLICATION
• ..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\replicating_dna.html
Evelyn I. Milian - Instructor
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
DNA STRUCTURE AND REPLICATION:
Some Websites for Review
• DNA structure interactive tutorial:
http://www.umass.edu/molvis/tutorials/dna/
• DNA replicating song (creative and funny!):
http://www.youtube.com/watch?v=dIZpb93NYlw
• DNA structure: http://www.youtube.com/watch?v=qy8dk5iS1f0
• Molecular visualization of DNA:
http://www.youtube.com/watch?v=4PKjF7OumYo
• DNA replication brief videos:

http://www.youtube.com/watch?v=teV62zrm2P0&feature=related

http://www.youtube.com/watch?v=AGUuX4PGlCc&feature=related

http://www.youtube.com/watch?v=z685FFqmrpo&feature=related
Evelyn I. Milian - Instructor
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BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA)
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