Download Chapter 9 DNA: THE Genetic Material

Chapter 9
DNA: THE Genetic Material
Transformation
Frederick Griffith, a
bacteriologist, prepared
a vaccine against
pneumonia
Vaccine – a substance
that is prepared from
killed or weakened
disease-causing
agents, including
certain bacteria
To protect the body
against future infections
by the disease-causing
agent
Griffith’s Experiments
Griffith worked with 2 strains
of S. pneumoniae
Virulent – (full of poison)
able to cause disease
• 1st strain had a smooth
capsule that protected the
bacterium from body’s
defense systems (S)
• 2nd strain lacked capsule
and didn’t cause disease
(R)
• Mice injected with (S)
strain died; mice injected
with (R) strain lived
Griffith’s Experiments
Griffith injected mice with
dead S bacteria – mice
lived
Griffith injected mice with
heat-killed S bacteria-mice
still lived
Meaning the capsule was
not involved with killing the
mice
He mixed harmless live R
bacteria with the harmless
heat-killed S bacteria-mice
died
Griffith had
discovered what is
now called:
Transformation- a
change in
genotype caused
when cells take up
foreign genetic
material
Avery’s Experiments
In 1944, a series of
experiments proved
that transformation is
stopped by DNA
destroying enzymes.
Almost 100 years
after Mendel’s
experiments, Oswald
Avery & co-workers
demonstrated that
DNA is the material
responsible for
transformation.
Viral Genes and DNA
In 1952, Hershey and
Chase used T2
bacteriophage to prove
that viruses infect
bacteria.
A bacteriophage, also
referred as a phage, is a
virus that infects
bacteria.
When phages infect
bacteria they reproduce
more viruses and burst
to release more viruses.
DNA’s Role Revealed
Alfred Hershey and
Martha Chase
performed The
following experiment:
Step 1 – grew E. coli
labeled with radioactive
sulfur (35S) protein coat.
incorporated the sulfur
Step 2 Labeled phages
used to infect two
separate batches of E.
coli
Step 3 Used centrifuge
to spin the tubes to
separate the bacteria
(heavy) from the viral
parts (lighter).
Concluded that the DNA
of viruses is injected into
the bacterial cells, while
viral proteins coat was
not.
Injected DNA cause
bacterial cells to produce
more viruses.
DNA is the heredity
material in viruses.
The Structure of DNA – Section 2
Watson & Crick
determined that a
DNA molecule is a
double helix – two
strands twisted around
each other
Nucleotides – the
subunits that make up
DNA
3 parts: a phosphate
group, a 5-carbon
sugar, and a nitrogencontaining base
DNA Base Pairing Rule:
Adenine (A) to Thymine
(T) & Guanine (G) to
Cytosine (C)
Adenine (A) and guanine
(G) are classified as
bulky purines –two rings
of carbon & nitrogen
atoms
Thymine (T) and
cytosine (C) are
classified as the smaller
pyrimidines –single ring
C & N atoms
Discovering DNA’s Structure
Chargaff’s 1949
observations –
the amount of
adenine always
equaled the
amount of
thymine; likewise
the amount of
guanine always
equaled the
amount of
cytosine; however
the amount varied
between different
organisms.
Wilkins & Franklin’s Photographs
X-ray diffraction to study
the structures of
molecules
1952 Wilkins Franklin
developed high-quality
X-ray diffraction
photographs of strands
of DNA which suggested
that the DNA resembled
a tightly coiled helix and
was composed of two or
three chains of
nucleotides.
Watson & Crick’s DNA Model
1953 Watson &
Crick to make the
“spiral staircase”
configuration of
DNA.
The model takes
into account the
information from
Chargaff, Wilkins, &
Franklin along with
their knowledge of
chemical bonding.
Pairing Between Bases
Base-pairing rules –
cytosine pairs with
guanine and adenine
with thymine.
These base pairing rules
are supported by
Chargaff's observations.
The strictness of basepairing results in 2
strands containing
complementary base
pairs.
The Replication of DNA – section 3
The complementary
DNA structure serves as
a template, or pattern,
on which the other
strand is built
DNA replication – the
process of making a
copy of DNA, which
occurs during the (S)
phase of the cell cycle
Step 1 – The double
helix needs to unwind
before replication can
begin
Accomplished by
enzymes called DNA
helicases which open
the double helix by
breaking the
hydrogen bonds
between the two
strands. The area
where double helix
separates is called
replication fork.
Step 2 –
Additional proteins
prevent the strands from
assuming their doublehelical shape
Replication forks – areas
where the double helix
separates
Enzymes known as DNA
polymerases add
nucleotides to the
exposed nitrogen bases,
according to the basepairing rules – forming
two double helixes
The process continues until DNA
has been copied & polymerases
are signaled to detach
Nucleotide sequences are
identical in the two DNA
molecules
The enzyme helicase unzips the
DNA
DNA polymerase adds
nucleotides to the exposed
strands
The end results are two identical
strands, each strand with a
parent strand and a daughter
strand
DNA Replication
Checking for errors –
errors sometime occur with
wrong nucleotide added
DNA polymerases
“proofreading role” – can
backtrack and correct
Proofreading reduces errors –
DNA replication about one error
per 1 billion nucleotides
Section 3
The Replication of DNA
• Replication doesn’t begin at one end of DNA
and end at the other.
• The circular molecules found in prokaryotes
have two replication forks.
• Replication forks move away from one another
until meeting at the other side of DNA circle.
• A Eukaryotic cell contains one single long
strand of DNA.
• Each human chromosome is replicated in 100
sections, each with its own starting point.
• Multiple replication forks working at same time.
The entire chromosome replicates in 8 hours.
Chapter 9.3