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DNA Structure and Function
Chapter 9
Miescher Discovered DNA
• 1868
• Johann Miescher investigated chemical
composition of nucleus
• Isolated organic acid high in
phosphorus
• He called it nuclein
• We call it DNA (deoxyribonucleic acid)
Griffith Discovers Transformation
• 1928
• Attempting to develop a vaccine
• Isolated two strains of Streptococcus
pneumoniae
– Rough strain was harmless
– Smooth strain was pathogenic
Griffith Discovers Transformation
Griffith's experiment
Transformation
• Harmless R cells were transformed by
material from dead S cells
• Descendents of transformed cells were
also pathogenic
What Is the
Transforming Material?
• Avery found protein-digesting enzymes
did not change results
– extracts still transformed bacteria
• But treated with DNA-digesting
enzymes
– extracts lost transforming ability
• Concluded that DNA, not protein,
transforms bacteria
Bacteriophages
• Viruses that infect
bacteria
• Consist of protein
and DNA
• Inject their
hereditary material
into bacteria
Bacteriophages
Hershey-Chase experiments
Watson-Crick Model
Subunits of DNA
Hershey and Chase’s
Experiments
• Created labeled bacteriophages
– Radioactive sulfur
– Radioactive phosphorus
• Allowed labeled viruses to infect bacteria
• Where were the radioactive labels after
infection?
35S
remains
outside cells
virus particle
labeled with 35S
DNA (blue)
being injected
into bacterium
virus particle
labeled with 32P
35P
remains
inside cells
DNA (blue)
being injected
into bacterium
Fig. 9-2, p.139
Hershey and Chase Results
35S
remains
outside cells
32P
remains
inside cells
2nm diameter overall
Structure of
DNA
In 1953,
Watson and
Crick showed
that DNA is a
double helix
0.34 nm between
each pair of bases
3.4 nm length of each
full twist of helix
Watson-Crick Model
DNA close up
Watson and Crick
Watson-Crick Model
• DNA molecule is a double helix
• Consists of two nucleotide strands that
run in opposite directions
• Strands are held together by hydrogen
bonds between bases
• A binds with T, C binds with G
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• There are four bases:
– Adenine, Guanine, Thymine, Cytosine
Nucleotide Bases
ADENINE
(A)
phosphate
group
GUANINE
(G)
deoxyribose
THYMINE
(T)
CYTOSINE
(C)
Composition of DNA
• Amount of adenine relative to guanine
differs among species
• Amount of adenine always equals
amount of thymine, and amount of
guanine always equals amount of
cytosine
A=T and G=C
DNA Structure
Allows It to Duplicate
• Two nucleotide strands held together by
hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand serves as template
for new strand
Rosalind Franklin’s
Work
• Expert in x-ray
crystallography
• Used technique to
examine DNA fibers
• Concluded that DNA
was some sort of helix
DNA Models
2-nanometer diameter overall
0.34-nanometer distance
between each pair of bases
3.4-nanometer length of
each full twist of the double helix
In all respects shown here, the
Watson–Crick model for DNA
structure is consistent with the
known biochemical and x-ray
diffraction data.
The pattern of base pairing (A
only with T, and G only with C)
is consistent with the known
composition of DNA (A = T, and
G = C).
Fig. 9-6, p.141
DNA
Replication
• Each parent
strand remains
intact
• Every DNA
molecule is half
“old” and half
“new”
new
old
old
new
Base Pairing
during
Replication
Each old strand is
template for new
complementary
strand
Base Pairing during Replication
DNA replication details
Enzymes in Replication
• Enzymes unwind the two strands and
complementary base pairs unzip
• DNA polymerase attaches new
complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
DNA Repair
• Mistakes can occur during replication
• DNA polymerase reads correct
sequence from complementary strand
and, together with DNA ligase, repairs
mistakes in incorrect strand
Clones
• Nuclear transfer from adult cell
• Structural and functional problems
– Most adult DNA inactive
• Potential benefits
– Replacement organs
– Endangered animals
Cloning
• Making a genetically identical copy of
an individual
• Researchers have been creating clones
for decades
• Clones can be created by embryo
splitting (artificial twinning)
Cloning
How Dolly was created
Impacts, Issues Video
Goodbye Dolly
More Clones
• Numerous species been cloned
Mice, pigs, cattle, cats, etc.
• Most cloning attempts are still
unsuccessful
• Many clones have defects
• Clones may vary in their phenotype
More Clones