Download 11.1 How Did Scientists Discover That Genes Are Made of DNA?

Chapter 11
DNA: The Molecule Of
Heredity
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 11 At a Glance
 11.1 How Did Scientists Discover That Genes
Are Made of DNA?
 11.2 What Is the Structure of DNA?
 11.3 How Does DNA Encode Information?
 11.4 How Does DNA Replication Ensure
Genetic Constancy During Cell Division?
 11.5 How Do Mutations Occur?
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 By the late 1800s, scientists had a limited
knowledge of genes
– Heritable information is carried in discrete units
called genes
– Genes are parts of structures called
chromosomes
– Chromosomes are made of deoxyribonucleic
acid (DNA) and protein
 They did not know what genes were made of
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 Transformed bacteria revealed the link between
genes and DNA
– In the 1920s, Frederick Griffith worked with two
strains of Streptococcus pneumoniae bacteria
–S-strain caused pneumonia when injected into
mice, killing them
–R-strain did not cause pneumonia when
injected
–When the S-strain was killed and injected into
mice, it did not cause disease
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 Transformed bacteria revealed the link between
genes and DNA (continued)
– Griffith made a sample of heat-killed S-strain and
mixed it with the R-strain
–Injection of the combination into mice caused
pneumonia and death
–This was unexpected, since neither element
alone caused the disease
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 Transformed bacteria revealed the link between
genes and DNA (continued)
– The results of Griffith’s experiments led to
several new deductions about the genetic
material
–Some substance in the heat-killed S-strain
changed the living, harmless R-strain bacteria
into the deadly S-strain, a process Griffith
called transformation
–The substance that caused this transformation
might be the long-sought molecule of heredity
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Transformation in Bacteria
Bacterial strain(s) injected into mouse
Results
Conclusions
(a)
Mouse remains
healthy
Living
R-strain
R-strain does
not cause
pneumonia.
(b)
Mouse contracts
pneumonia and
dies
S-strain causes
pneumonia.
Living
S-strain
(c)
Mouse remains
healthy
Heat-killed
S-strain
Heat-killed Sstrain does not
cause pneumonia.
(d)
Mixture of living
R-strain and
heat-killed
S-strain
Mouse contracts
pneumonia and
dies
A substance from
heat-killed S-strain
can transform the
harmless R-strain
into a deadly
S-strain.
Fig. 11-1
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 The transforming molecule is DNA
– In 1933, J. L. Alloway showed that transformation
occurred just as readily in culture dishes
–This showed that the mouse in Griffith’s
experiments was not involved in the
transformation
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 The transforming molecule is DNA (continued)
– In the 1940s, Oswald Avery, Colin MacLeod, and Maclyn
McCarty isolated DNA from S-strain bacteria, mixed it
with live R-strain bacteria, and produced live S-strain
bacteria
– This suggested that the transforming molecule from
the S-strain was DNA
– To rule out the possibility that a protein contaminant
was actually causing the transformation, Avery treated
samples with protein-destroying enzymes and still
induced transformation
– When DNA-destroying enzymes were added to the
samples, transformation did not occur
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11.1 How Did Scientists Discover That Genes Are
Made of DNA?
 The transforming molecule is DNA (continued)
– Griffith’s experiments can be explained if DNA is the
transforming agent
– Heating S-strain cells killed them but did not completely
destroy their DNA
– When killed S-strain bacteria were mixed with living Rstrain bacteria, fragments of DNA from the dead S-strain
cells became incorporated into the chromosome of the
R-strain bacteria
– If these fragments of DNA contained the genes needed
to cause disease, an R-strain cell would be transformed
into an S-strain cell
– Thus, Avery, MacLeod, and McCarty concluded that
genes are made of DNA
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Molecular Mechanism of Transformation
bacterial
chromosome
DNA fragments are
transported into
the bacterium
A DNA fragment is
incorporated into
the chromosome
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Fig. 11-2
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11.2 What Is the Structure of DNA?
 The secrets of DNA function and, therefore, of
heredity itself are found in the three-dimensional
structure of the DNA molecule
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11.2 What Is the Structure of DNA?

DNA is composed of four nucleotides
– DNA is made of chains of small subunits called
nucleotides
– Each nucleotide has three components
– A phosphate group
– A deoxyribose sugar
– One of four nitrogen-containing bases
1. Thymine (T)
2. Cytosine (C)
3. Adenine (A)
4. Guanine (G)
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11.2 What Is the Structure of DNA?
 DNA is composed of four nucleotides
(continued)
– DNA is a double helix of two nucleotide strands
– Hydrogen bonds between complementary bases
hold two DNA strands together
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DNA Nucleotides
phosphate
phosphate
sugar
base = adenine
sugar
base =•guanine
phosphate
phosphate
base = thymine
sugar
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base = cytosine
sugar
Fig. 11-3
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11.2 What Is the Structure of DNA?
 DNA is composed of four nucleotides
(continued)
– In the 1940s, biochemist E. Chargaff determined
that the amount of A in a DNA molecule equaled
the amount of T, and the amount of C equaled
the amount of G
–This finding was called “Chargaff’s rule”
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11.2 What Is the Structure of DNA?
 DNA is a double helix
– In the 1940s, several other scientists
investigated the structure of DNA
– Rosalind Franklin and Maurice Wilkins studied
the structure of DNA crystals using X-ray
diffraction
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11.2 What Is the Structure of DNA?
 DNA is a double helix (continued)
– From X-ray diffraction patterns, they deduced
several qualities of DNA
–It is long and thin, and has a uniform diameter
of 2 nanometers
–It is helical; that is, twisted like a corkscrew
and consisting of repeating subunits
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X-ray Diffraction Studies of DNA
Fig. 11-4
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11.2 What Is the Structure of DNA?
 DNA is a double helix (continued)
– James Watson and Francis Crick combined the
X-ray data with bonding theory to deduce the
structure of DNA
–DNA is made of two strands of nucleotides
–Within each DNA strand, the phosphate group
of one nucleotide bonds to the sugar of the
next nucleotide in the same strand
–The deoxyribose and phosphate portions
make up the sugar-phosphate backbone
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11.2 What Is the Structure of DNA?
 James Watson and Francis Crick combined the
X-ray data with bonding theory to deduce DNA
structure (continued)
–The nucleotide bases protrude from this
backbone
–All the nucleotides within a single DNA strand
are oriented in the same direction and thus
have an unbonded sugar at one end and an
unbonded phosphate at the other end
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The Discovery of DNA
Fig. E11-3
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
 The bases protrude inward toward each other
from the sugar-phosphate backbone like rungs
on a ladder
 Hydrogen bonds hold the base pairs together
 The ladder-like structure is twisted into a
double helix
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– The two strands in a DNA double helix are said
to be antiparallel; that is, they are oriented in
opposite directions
–From one end of the DNA molecule, if one
strand starts with the free sugar and ends with
the free phosphate, the other stand starts with
the free phosphate and ends with the free
sugar
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases hold
two DNA strands together in a double helix (continued)
– Because of their structures and the way they face each
other, adenine (A) bonds only with thymine (T) and
guanine (G) bonds only with cytosine (C)
– Bases that bond with each other are called
complementary base pairs
– Thus, if one strand has the base sequence
CGTTTAGCCC, the other strand must have the
sequence GCAAATCGGG
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– Complementary base pairing explains Chargaff’s
rule that for a given molecule of DNA, adenine
equals thymine and guanine equals cytosine
–Since every adenine, for example, is paired
with a thymine, no matter how many adenines
are in the DNA molecule, there will be an
equal number of thymines
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– Adenine and guanine are large molecules;
thymine and cytosine are relatively smaller
– Because base pairing always places a large
molecule with a small one, the diameter of the
double helix remains constant
– In 1953, James Watson and Francis Crick
consolidated all the historical data about DNA
into an accurate model of its structure
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The Watson-Crick Model of DNA Structure
nucleotide
nucleotide
free
phosphate
free
sugar
phosphate
base
(cytosine)
sugar
hydrogen
bonds
free sugar
(a) Hydrogen bonds hold complementary
basepairs together in DNA
free
phosphate
(b) Two DNA strands form a
double helix
(c) Four turns of a
DNA double helix
Fig. 11-5
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11.3 How Does DNA Encode Information?
 How can a molecule with only four simple parts
be the carrier of genetic information?
– The key lies in the sequence, not the number, of
subunits
– Within a DNA strand, the four types of bases can
be arranged in any linear order, and this
sequence is what encodes genetic information
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11.3 How Does DNA Encode Information?
 The genetic code is analogous to languages,
where small sets of letters combine in various
ways to make up many different words
– English has 26 letters
– Hawaiian has 12 letters
– The binary language of computers uses only two
“letters” (0 and 1, or “on” and “off”)
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11.3 How Does DNA Encode Information?
 The sequence of only four nucleotides can
produce many different combinations
– A 10-nucleotide sequence can code for more
than 1 million different combinations of the four
bases
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 Replication of DNA is a critical event in a cell’s
life
– All cells come from pre-existing cells
– Cells reproduce by dividing in half
– Each of two daughter cells gets an exact copy of
the parent cell’s genetic information
– Duplication of the parent cell DNA is called DNA
replication
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 DNA replication produces two DNA double
helices, each with one original strand and one
new strand
– The fact of complementary base pairing
suggests a model for how DNA replicates
– The ingredients for DNA replication are threefold
–The parental DNA strands
–Free nucleotides
–A variety of enzymes that unwind the parental
DNA double helix and synthesize new DNA
strands
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 DNA replication produces two DNA double helices,
each with one original strand and one new strand
(continued)
– DNA replication begins with enzymes, called DNA
helicases, that pull apart the parental DNA double helix
at the hydrogen bonds between the complementary base
pairs
– A second strand of new DNA is synthesized along each
separated strand by DNA polymerases, which pair free
nucleotides with their complementary nucleotides on
each separated strand
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 DNA replication produces two DNA double
helices, each with one original strand and one
new strand (continued)
– When replication is complete, each parental
strand and the daughter strand that was copied
from it by base pairing wind together into a new
DNA double helix, thus creating two copies of the
original double helix
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Author Animation: DNA Replication
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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 astrands
free nucleotides
4 Each new double helix is composed
of one parental strand (blue) and one
new strand (red)
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Fig. 11-6
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 DNA replication produces two DNA double
helices, each with one original strand and one
new strand (continued)
– Base pairing is the foundation of DNA replication
–An adenine on one strand pairs with a thymine
on the other strand; a cytosine pairs with
guanine
–If a parental strand reads T-A-G, for example,
the new strand reads A-T-C
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11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 DNA replication produces two DNA double
helices, each with one original strand and one
new strand (continued)
– The two resulting DNA molecules have one old
parental strand and one new strand
(semiconservative replication)
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Semiconservative Replication of DNA
One DNA
double helix
DNA replication
Two identical DNA
double helices, each
with one parental
strand (blue) and
one new strand (red)
Fig. 11-7
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11.5 How Do Mutations Occur?
 Accurate replication and proofreading produce
almost error-free DNA
– Infrequent changes in the sequence of bases in
DNA result in defective genes called mutations
–Mutations range from changes in single
nucleotides to movements of large pieces of
chromosomes
–Mutations may have varying effects on
function
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11.5 How Do Mutations Occur?
 Accurate replication and proofreading produce almost
error-free DNA (continued)
– During replication, DNA polymerase mismatches
nucleotides once every 1,000 to 100,000 base pairs, yet
completed DNA strands contain only about one mistake
in every 100 million to 1 billion base pairs
– In humans, this amounts to less than one error per
chromosome per replication
– This reduction in errors is accomplished by DNA repair
enzymes, which “proofread” each new daughter strand
and replace mismatched nucleotides
– Proofreading occurs both during and after replication
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11.5 How Do Mutations Occur?
 Mistakes do happen
– DNA is altered or damaged in a number of ways
–Mistakes are made during normal DNA
replication
–Certain chemicals (some components of
cigarette smoke, for example) increase DNA
errors during and after replication
–Ultraviolet radiation or X-rays also contribute
to incorrect base pairing
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11.5 How Do Mutations Occur?
 Mutations range from changes in single nucleotide
pairs to movements of large pieces of chromosomes
– Point mutations—also called nucleotide
substitutions—involve changes to individual nucleotides
in the DNA sequence
– One type of point mutation occurs when a repair
enzyme finds a mismatch but mistakenly cuts out the
correct base and puts in the complement of the
erroneous base
– Insertion mutations occur when one or more new
nucleotide pairs are inserted into the DNA double helix
– Deletion mutations occur when one or more nucleotide
pairs are removed from the double helix
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11.5 How Do Mutations Occur?
 Types of mutations (continued)
– An inversion occurs when a piece of DNA is cut
out of a chromosome, turned around, and reinserted into the gap
– A translocation occurs when a chunk of DNA
(often very large) is removed from one
chromosome and attached to another
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Mutations
Fig. 11-8a
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Mutations
Fig. 11-8b
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Mutations
Fig. 11-8c
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Mutations
Fig. 11-8d
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Mutations
Fig. 11-8e
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11.5 How Do Mutations Occur?
 Mutations may have varying effects on function
– Mutations are often harmful, and an organism
inheriting them may quickly die
– Some mutations may have no functional effect
– Some mutations may be beneficial and provide
an advantage to the organism in certain
environments
–These advantageous mutations may be
favored by natural selection and are the basis
for evolution of life on Earth
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