Chapter 8
DNA Structure and
Function
Albia Dugger • Miami Dade College
8.1 A Hero Dog’s Golden Clones
• James Symington and his search dog Trakr located the last
living survivor of the 9/11 attack on the World Trade Center
• Trakr later died of a degenerative neurological disease
probably linked to toxic smoke exposure at Ground Zero
• Trakr’s DNA lives on in his clones – genetic copies produced
by inserting his DNA into donor eggs
Symington and Trakr at Ground Zero
Trakr’s Clones
The Cloning Controversy
• Few cloned mammal embryos result in a live birth – many of
the clones that survive have serious health problems
• One problem is, the DNA in adult cells is controlled differently
than the DNA in embryonic cells
• Perfecting methods for cloning animals brings us closer to the
possibility of cloning humans, both technically and ethically
8.2 Eukaryotic Chromosomes
• The DNA in a eukaryotic cell nucleus is organized as one or
more chromosomes that differ in length and shape
• Chromosome
• A structure that consists of DNA and associated proteins
• Carries part or all of a cell’s genetic information
Chromosome Organization
• During most of the cell’s life, each chromosome consists of
one DNA strand.
• When the cell prepares to divide, it duplicates all of its
chromosomes, so that both offspring get a full set
• Each duplicated chromosome has two DNA strands (sister
chromatids) attached to one another at the centromere
centromere
one chromatid
its sister chromatid
a chromosome
(unduplicated)
a chromosome
(duplicated)
p134
Chromosome Structure
• A duplicated, condensed chromosome consists of two long
filaments bunched into a characteristic X shape
• Each filament consists of a coil of DNA wrapped around
“spools” of proteins called histones
• Each DNA-histone spools is a nucleosome, the smallest unit
of chromosomal organization in eukaryotes
• The DNA molecule consists of two strands twisted into a
double helix
Stretched out end to end, the DNA
molecules in a human cell would be
about 2 meters (6.5 feet) long. That is
a lot of DNA to fit into a nucleus that
is less than 10 micrometers in diameter! Proteins structurally organize
the DNA and help it pack tightly into a
small nucleus.
Figure 8-2a p134
DNA molecule
Figure 8-2b p134
1
2
3
4
Figure 8-2c1 p134
5
6
Figure 8-2c2 p134
ANIMATED FIGURE: Chromosome
structural organization
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Chromosome Number
• The total number of chromosomes in a eukaryotic cell
(chromosome number) is characteristic of the species –
human body cells have 46 chromosomes
• Human body cells have two of each type of chromosome –
their chromosome number is diploid (2n)
• A karyotype shows how many chromosomes are in an
individual cell, and reveals major structural abnormalities
A A karyotype. This one shows 22 pairs of autosomes and a pair of X
chromosomes.
Figure 8-3a p135
Autosomes and Sex Chromosomes
• In a diploid organism, one chromosome in a chromosome pair
is inherited from the mother and one from the father
• All except one pair of chromosomes are autosomes – pairs
of chromosomes with the same length, shape, and
centromere location
• Pairs of sex chromosomes differ between females and
males – human females have two X chromosomes (XX);
human males have one X and one Y chromosome (XY)
diploid
reproductive
cell in female
diploid
reproductive
cell in male
eggs
sperm
XX
XY
XX
XY
union of sperm and
egg at fertilization
Stepped Art
Figure 8-3b p135
Take-Home Message:
What are chromosomes?
• A chromosome consists of a molecule of DNA that is
structurally organized by proteins; the organization allows the
DNA to pack tightly
• A eukaryotic cell’s DNA is divided among some characteristic
number of chromosomes, which differ in length and shape
• Members of a pair of sex chromosomes differ between males
and females; chromosomes that are the same in males and
females are called autosomes
8.3 The Discovery of DNA’s Functions
• Investigations that led to our understanding that DNA is the
molecule of inheritance reveal how science advances
Discovery of DNA
• 1800s: Johannes Miescher found DNA (deoxyribonucleic
acid) in nuclei, though it’s function was unknown
Griffith’s Experiments
• Early 1900s: Griffith transferred hereditary material from dead
cells to live cells
• Mice injected with live R cells lived
• Mice injected with live S cells died
• Mike injected with killed S cells lived
• Mice injected with killed S cells and live R cells died; live S
cells were found in their blood
Griffith’s Experiments
1
2
3
4
ANIMATED FIGURE: Griffith's experiment
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Avery and McCarty Find
the Transforming Principle
• 1940: Avery and McCarty separated deadly S cells (from
Griffith’s experiments) into lipid, protein, and nucleic acid
components
• When lipids, proteins, and RNA were destroyed, the
remaining substance, DNA, still transformed R cells to S cells
• Conclusion: DNA is the “transforming principle”
Confirmation of DNA’s Function
• 1950s: Hershey and Chase experimented with
bacteriophages (viruses that infect bacteria)
• Protein parts of viruses, labeled with 35S, stayed outside
the bacteria
• DNA of viruses, labeled with 32P, entered the bacteria
• Conclusion: DNA, not protein, is the material that stores
hereditary information
Bacteriophages
DNA
inside
protein
coat
tail
fiber
hollow
sheath
Virus particle
coat proteins
labeled with 35S
35S
remains
outside cells
DNA being
injected into
bacterium
Virus DNA
labeled with 32P
32P
remains
inside cells
Labeled DNA
being injected
into bacterium
Stepped Art
Figure 8-6 p137
ANIMATED FIGURE: Hershey-Chase
experiments
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Take-Home Message:
What is the molecular basis of inheritance?
• DNA is the material of heredity common to all life on Earth
8.4 The Discovery of DNA’s Structure
• Watson and Crick’s discovery of DNA’s structure was based
on 150 years of research by other scientists
DNA’s Building Blocks
• Nucleotide
• A nucleic acid monomer consisting of a five-carbon sugar
(deoxyribose), three phosphate groups, and one of four
nitrogen-containing bases
• DNA consists of four nucleotide building blocks
• Two pyrimidines: thymine and cytosine
• Two purines: adenine and guanine
Four Kinds of Nucleotides in DNA
adenine (A)
deoxyadenosine triphosphate, a purine
Four Kinds of Nucleotides in DNA
guanine (G)
deoxyguanosine triphosphate, a purine
Four Kinds of Nucleotides in DNA
thymine (T)
deoxythymidine triphosphate, a pyrimidine
Four Kinds of Nucleotides in DNA
cytosine (C)
deoxycytidine triphosphate, a pyrimidine
ANIMATED FIGURE: Subunits of DNA
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Chargaff’s Rules
• The amounts of thymine and adenine in DNA are the same,
and the amounts of cytosine and guanine are the same: A = T
and G = C
• The proportion of adenine and guanine differs among species
Franklin, Watson and Crick
• Rosalind Franklin’s research in x-ray crystallography
revealed the dimensions and shape of the DNA molecule: an
alpha helix
• This was the final piece of information James Watson and
Francis Crick needed to build their model of DNA
Watson and Crick’s DNA Model
• A DNA molecule consists of two nucleotide chains (strands),
running in opposite directions and coiled into a double helix
• Base pairs form on the inside of the helix, held together by
hydrogen bonds (A-T and G-C)
Watson and Crick’s DNA Model
DNA’s Base-Pair Sequence
• Bases in DNA strands can pair in only one way: A always
pairs with T; G always pairs with C
one
base
pair
• The DNA sequence (sequence of bases) is the genetic code
that varies between species and individuals
0.34 nanometer
between each
base pair
2-nanometer
diameter
3.4-nanometer
length of each full
twist of the double
helix
Figure 8-8b p139
ANIMATED FIGURE: DNA close up
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8.5 DNA Replication
• DNA replication is the energy- intensive process by which a
cell copies its DNA
• A cell copies its DNA before it reproduces
• Each of the two DNA strands in the double helix is replicated
• DNA replication requires many enzymes, including DNA
polymerase, and other molecules
DNA Replication
• A cell’s genetic information consists of the order of nucleotide
bases (the DNA sequence) of its chromosomes
• Descendant cells must get an exact copy of that information
• Each chromosome is copied entirely – the two chromosomes
that result are duplicates of the parent molecule
Enzymes of DNA Replication
• DNA helicase breaks hydrogen bonds between DNA strands
• Topoisomerase untwists the double helix
• DNA polymerase joins free nucleotides into a new strand of
DNA
• DNA ligase joins DNA segments on the discontinuous strand
Primers for DNA Polymerase
• There are several types of DNA polymerases
• All types require a primer in order to initiate DNA synthesis
• Primer
• A short, single strand of DNA or RNA that is
complementary to a targeted DNA sequence
Discontinuous Replication
• DNA polymerases attach a free nucleotide only to the 3′ end
of a DNA strand (not the 5′ end)
• Only one of the two new strands of DNA can be synthesized
continuously during DNA replication
• Synthesis of the other strand occurs in segments, in the
direction opposite that of unwinding
• DNA ligase joins segments into a continuous strand of DNA
Semiconservative DNA Replication
• Each strand of a DNA double helix is a template for synthesis
of a complementary strand of DNA
• One template builds DNA continuously; the other builds DNA
discontinuously, in segments
• Each new DNA molecule consist of one old strand and one
new strand (semiconservative replication)
1
initiator proteins
topoisomerase
2
helicase
3
primer
DNA
polymerase
DNA ligase
4
5
6
Figure 8-9 p140
The parent DNA double helix
unwinds in this direction.
DNA synthesis proceeds
only in the 5′to 3′ direction
because DNA polymerase
catalyzes only one reaction:
the formation of a bond
between the 3′ carbon on
the end of a DNA strand and
the phosphate on a
nucleotide’s 5′ carbon.
Figure 8-10 p141
ANIMATED FIGURE: DNA replication
details
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Take-Home Message:
How is DNA copied?
• DNA replication is an energy-intensive process by which a
cell copies its chromosomes
• Each strand of the double helix serves as a template for
synthesis of a new, complementary strand of DNA
8.6 Mutations: Cause and Effect
• DNA repair mechanisms correct most replication errors
• In science, as in other professions, public recognition for a
discovery does not always include all contributors
DNA Repair Mechanisms
• DNA polymerases proofread DNA sequences during DNA
replication and repair damaged DNA
• When proofreading and repair mechanisms fail, an error
becomes a mutation – a permanent change in the DNA
sequence
DNA Damage
Environmental Causes of Mutations
• Ionizing radiation (gamma rays, x-rays, most UV light)
• Knocks electrons out of atoms
• Breaks chromosomes into pieces that get lost during DNA
replication
• Creates free radicals in tissues
• UV light (320-400 nm)
• Forms pyrimidine dimers that kink the DNA strand
• Causes skin cancer
thymine
dimer
Figure 8-12 p142
Environmental Causes of Mutations
• At least fifty-five carcinogenic (cancer-causing) chemicals in
tobacco smoke transfer small hydrocarbon groups to the
nucleotide bases in DNA
• Many environmental pollutants are converted by the body to
other compounds that bind irreversibly to DNA, causing
replication errors that lead to mutation
The Short Story of Rosalind Franklin
• In science, as in other professions, public recognition does
not always include everyone who contributed to a discovery
• Rosalind Franklin was first to discover the molecular structure
of DNA, but did not share in the Nobel prize which was given
to Watson, Crick, and Wilkins
• Franklin died of cancer at age 37probably caused by
extensive exposure to x-rays during her work
Rosalind Franklin and Her
X-Ray Diffraction Image of DNA
Take-Home Message:
What are mutations?
• Permanent changes in a DNA sequence are mutations
• DNA damage by environmental agents such as UV light and
chemicals can result in mutations, because damaged DNA is
not replicated very well
• Proofreading and repair mechanisms usually maintain the
integrity of a cell’s genetic information by fixing damaged
DNA or correcting mispaired bases
8.7 Animal Cloning
• Various reproductive interventions produce genetically
identical individuals
Cloning
• Clones
• Exact copies of a molecule, cell, or individual
• Occur in nature by asexual reproduction or embryo
splitting (identical twins)
• Reproductive cloning technologies produce an exact copy
(clone) of an individual
Reproductive Cloning Technologies
• Somatic cell nuclear transfer (SCNT)
• Nuclear DNA of an adult is transferred to an enucleated
egg
• Egg cytoplasm reprograms differentiated (adult) DNA to
act like undifferentiated (egg) DNA
• The hybrid cell develops into an embryo that is genetically
identical to the donor individual
Somatic Cell Nuclear Transfer (SCNT)
Figure 8-14a p144
Figure 8-14b p144
Figure 8-14c p144
Figure 8-14d p144
Figure 8-14e p144
Figure 8-14f p144
ANIMATED FIGURE: How Dolly was
created
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A Clone Produced by SCNT
Therapeutic Cloning
• Therapeutic cloning uses SCNT to produce human embryos
for research purposes
• Researchers harvest undifferentiated (stem) cells from the
cloned human embryos
• Such research may ultimately lead to treatments for people
who suffer from fatal diseases
Take-Home Message:
What is cloning?
• Reproductive cloning technologies produce clones:
genetically identical individuals
• The DNA inside a living cell contains all the information
necessary to build a new individual
• Somatic cell nuclear transfer (SCNT) is a reproductive cloning
technology in which nuclear DNA of an adult donor is
transferred to an egg with no nucleus; the hybrid cell develops
into an embryo that is genetically identical to the adult donor
• Therapeutic cloning uses SCNT to produce human embryos
for research