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DNA Structure and Function
Miescher Discovered DNA
• 1868
• Johann Miescher investigated the chemical
composition of the nucleus
• Isolated an organic acid that was high in
phosphorus
• He called it nuclein
• We call it DNA (deoxyribonucleic acid)
Mystery of the
Hereditary Material
• Originally believed to be an unknown class of
proteins
• Thinking was
– Heritable traits are diverse
– Molecules encoding traits must be diverse
– Proteins are structurally diverse; so the hereditary
material must be protein
• Wrong!
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
1. Mice injected with
live cells of harmless
strain R.
2. Mice injected with live
cells of killer strain S.
3. Mice injected with
heat-killed S cells.
4. Mice injected with
live R cells plus heatkilled S cells.
Mice live. No live R
cells in their blood.
Mice die. Live S cells in
their blood.
Mice live. No live S cells
in their blood.
Mice die. Live S cells in
their blood.
Figure 13.3
Page 218
Transformation
• What happened in the fourth
experiment?
• The harmless R cells had been
transformed by material from the dead
S cells
• Descendents of the transformed cells
were also pathogenic
Oswald Avery
•
Some substance from the S cells had transformed
the R cells.
a. Both proteins and nucleic acids were candidates.
b. In 1944, Oswald Avery showed that the
substance was DNA.
• Cell extracts treated with protein-digesting enzymes
could still transform bacteria
• Cell extracts treated with DNA-digesting enzymes
lost their transforming ability
• Concluded that DNA, not protein, transforms
bacteria
Hershey & Chase’s
Experiments
• Created labeled bacteriophages
– Radioactive sulfur
– Radioactive phosphorus
• Allowed labeled viruses to infect
bacteria
• Asked: Where are the radioactive labels
after infection?
virus particle
labeled with 35S
Hershey
and
Chase
Results
virus particle
labeled with 32P
bacterial cell (cutaway view)
label
outside cell
Figure 13.5
Page 219
label inside cell
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases
– Adenine, Guanine, Thymine, Cytosine
Nucleotide Bases
ADENINE
(A)
phosphate
group
GUANINE
(G)
purines
deoxyribose
THYMINE
(T)
CYTOSINE
(C)
pyrimadines
Figure 13.6
Page 220
Composition of DNA
• Chargaff showed:
– 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
Rosalind Franklin’s Work
• Was an expert in X-ray crystallography
• Used this technique to examine DNA
fibers
• Concluded that DNA was some sort of
helix
Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen
bonds between bases
• A binds with T and C with G
• Molecule is a double helix
DNA is double stranded and analogous to a ladder. The sides of
the ladder are composed of alternating sugars (deoxyribose) and
phosphate groups that run antiparell to one another. On the left
side (in the picture) the first carbon found on the strand is #5 and
moving on down the last carbon is carbon # 3. This side is said to
be 5'-3'. The opposite side is upside down compared to the other
side. The right hand side, the first carbon found on the strand is
#3 and moving on down the last carbon is carbon # 5. This side is
said to be 3'-5‘.
Watson-Crick
Model
Figure 13.7
Page 221
DNA Structure Helps
Explain How It Duplicates
• DNA is two nucleotide strands held
together by hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand then serves as
template for new strand
DNA
Replication
• Each parent strand
remains intact
• Every DNA
molecule is half
“old” and half “new”
• Semicomservative
new
old
old
new
(Meslson and Stahl)
Figure 13.9
Page 222
Meslson and Stahl Experiment
After 1 replication,
they found that the
“new” strands were
hybrids (contained
both heavy and
light).
After 2 replications,
they found that the
some “new” strands
were hybrids and
some were only
light.
Their results supported the idea of semi-conservative replication.
Base Pairing
during
Replication
Each old strand
serves as the
template for
complementary
new strand
Figure 13.10
Page 223
-Because DNA is such a long
molecule, replication must
occur at the same time.
-micrograph of 3
replication
bubbles
Enzymes in Replication
• Helicase unwinds and separates the two strands opening the
template
• single stranded binding proteins stabilize the unwound parental
strands
• topoisomerase (or gyrase) relieves stress on strands by
breaking, swiveling and rejoining the parental strand at the
start of the replication fork
• Primase makes RNA primers so DNA polymerase has
something to hook onto
• DNA polymerase attaches complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
• -DNA polymerase can only add to the
3' end of a nucleotide. This means
that synthesis can only occur from
the 5’ to 3' direction.
• -DNA polymerase must always have
a nucleotide in front of it to hang the
DNA nucleotide on. Therefore an
RNA primer must be laid down first
and then replaced by DNA
polymerase
A Closer Look at
Strand Assembly
Energy for strand
assembly is
provided by
removal of two
phosphate groups
from free
nucleotides
newly
forming
DNA
strand
one parent
DNA strand
Figure 13.10
Page 223
Continuous and Discontinuous
Assembly
Strands can
only be
assembled in
the 5’ to 3’
direction
Figure 13.10
Page 223
http://www.johnkyrk.com/DNAreplication.html
http://www.fed.cuhk.edu.hk/~johnson/teachin
g/genetics/animations/dna_replication.htm
DNA Repair
• Mistakes can occur during replication
• DNA polymerase can read correct sequence
from complementary strand and, together
with DNA ligase, can repair mistakes in
incorrect strand
• DNA polymerases "proofread” the new bases
for mismatched pairs, which are replaced
with correct bases
Cloning
• Making a genetically identical copy of
an individual
• Researchers have been creating clones
for decades