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History of DNA
• Early scientists thought protein was
the cell’s hereditary material
because it was more complex than
DNA
• Proteins were composed of 20
different amino acids in long
polypeptide chains
copyright cmassengale
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Frederick Griffith
In 1928, Frederick Griffith carried out
experiments on pneumonia bacteria in
mice.
Discovery: something in heat-killed
virulent bacteria could be transferred to
live, harmless bacteria and make them
virulent.
Griffith’s Experiment
Oswald Avery
Avery continued working with Griffith’s
findings in hope of discovering what
factor in bacteria carried the trait of
virulence.
Isolated proteins, carbohydrates,
nucleic acids and applied them to nonvirulent bacteria. Only nucleic acids
(DNA) caused a change.
The History of DNA
• Alfred Hershey and Martha
Chase - 1952
– radioactive isotope tracer
experiment
– bacterial virus
(bacteriophage T2) infects
a host cell ( bacterium
Escherichia coli)
– found that T2 virus DNA,
not its protein coat, enters
the host cell
– genetic information for
replication of the virus
T 2 grown in
media
containing S35
incorporate S35
into their
proteins
Using S35
T2 attach to bacteria
and inject genetic
material
Bacteria
grown in
normal nonradioactive
media
When centrifuged,
phage protein coats
remain in the
supernatant while
bacteria form a
pellet.
The
supernatant is
radioactive, but
the pellet is not.
Did protein enter the bacteria?
Blending causes phage
protein coat to fall off
Is protein the genetic material?
T2 grown in
P32 containing
media
incorporate P32
into their DNA
Using
T2 attach to bacteria
and inject genetic
material
32
P
Bacteria
grown in
normal nonradioactive
media
When centrifuged,
phage protein coats
remain in the
supernatant while
bacteria form a
pellet
The pellet is
radioactive, but
the supernatant is
not.
Did DNA enter the bacteria?
Blending causes phage
protein coat to fall off
Is DNA the genetic material?
Erwin Chargaff
Chargaff studied DNA
itself, in hopes of
providing some clues
about its structure.
Discovered that there
are always equal
amounts of the bases
Adenine and Thymine,
and equal amounts of
Cytosine and Guanine.
Chargaff proposed that
these bases pair with
one another in some
way.
•Adenine and Thymine
always join together
A
T
• Cytosine and Guanine
always join together
C
G
Wilkins and Franklin
Rosalind Franklin and Maurice Wilkins
worked with X-ray crystalography to
find more clues about the structure of
DNA.
Franklin’s X-ray images suggested a
helical structure.
Franklin and Wilkins
Watson and Crick
James Watson and Francis Crick were
also working on discovering the
structure of DNA.
Applied Chargaff’s rule, assumed that A
always pairs with T, C with G.
Watson was not entirely convinced of
the helical structure that Franklin had
suggested, and his critique of her work
led her to doubt herself.
Watson and Crick
Wilkins consulted with Watson
and Crick. Without Franklin’s
knowledge, he handed them
the data that he and Franklin
had worked on.
Watson immediately
recognized the significance. He
and Crick went to work on a
model of DNA.
The First DNA Model
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
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The Shape of the Molecule
• DNA is a very long
polymer.
• The basic shape is
like a twisted ladder
or zipper.
• This is called a
double helix.
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One Strand of DNA
• The backbone of
the molecule is
alternating
phosphates and
deoxyribose
sugar
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
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One Strand of DNA
nucleotide
• One strand of
DNA is a polymer
of nucleotides.
• One strand of
DNA has many
millions of
nucleotides.
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Four nitrogenous bases
DNA has four different bases:
C
• Thymine T
• Adenine A
• Guanine G
• Cytosine
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Two Kinds of Bases in DNA
• Pyrimidines are
single ring bases.
• Purines are
double ring
bases.
N
N C
O C
C
N C
N
N C
C
C
N
N C
N C
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Thymine and Cytosine are
pyrimidines
• Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
N
C
N
C
cytosine
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Adenine and Guanine are
purines
• Adenine and guanine each have two
rings of carbon and nitrogen atoms.
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
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Two Stranded DNA
• Remember, DNA
has two strands
that fit
together
something like a
zipper.
• The teeth are
the nitrogenous
bases but why
do they stick
together?
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C
N
N
C
N
C
C
C
C
N
N
N
C
C
C
O
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are weak
but there are millions
and millions of them in a
single molecule of DNA.
• The bonds between
cytosine and guanine are
shown here with dotted
lines
N
Hydrogen Bonds
N
O
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Hydrogen Bonds, cont.
• When making
hydrogen bonds,
cytosine always
pairs up with
guanine
• Adenine always
pairs up with
thymine
• Adenine is bonded
to thymine here
N
O
C
C
O
C C
N
C
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DNA
Replication
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Replication Facts
• DNA has to be copied
before a cell divides
• DNA is copied during the S
or synthesis phase of
interphase
• New cells will need identical
DNA strands
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What Is DNA Replication
• DNA Replication is the
process in which the
DNA within a cell makes
an exact copy of itself.
– Why does DNA
replicate?
– During which phase of
the cell cycle does DNA
replicate?
DNA Replication models
The Three Possible DNA
Replication Models
• Conservative- would leave
the original strand intact
and copy it.
• Dispersive-would produce
two DNA molecule with
sections of both old and
new along each strand.
• Semiconservative –would
produce DNA molecule with
both one old strand and
one new strand.
DNA Replication
Replication occurs during Interphase
Replication
fork
Replication
fork
Replication
bubble
Hydrogen
bond
DNA replication is the process where an entire
double-stranded DNA is copied to produce a second,
identical DNA double helix.
DNA Replication
DNA
helicase
• Helicase unwinds the double helix starting at a
replication bubble.
• The two strands separate as the hydrogen bonds
between base pairs are broken.
• Two replication forks form and the DNA is
unwound in opposite directions.
DNA Replication
•Helicase has completed unwinding the DNA
strand.
•Single strand Binding Proteins (SSB) keep the
two strands from re-annealing (coming back
together).
DNA Replication
Leading Strand
Primase
RNA Primer
Lagging Strand
•Primase is an RNA polymerase that makes the
RNA primer.
•These primers “tell” the DNA polymerase where to
start copying the DNA.
DNA Replication
Leading Strand
5’
3’
Direction of Replication
DNA Polymerase
5’
Direction of ReplicationLagging Strand
3’
• The DNA polymerase starts at the 3’ end of the RNA primer of
the leading stand CONTINUOUSLY.
• DNA is copied in 5’ to 3’ direction.
• DNA polymerase copies the lagging strand DIS- continuously.
DNA Replication
Another DNA Polymerase removes the RNA primers
and replaces them with DNA.
DNA Replication
ligase
Finally the gaps in the sugar phosphate backbone
are sealed by DNA ligase
There are now 2 identical double helices of DNA.