Download DNA PowerPoints, pages 1 & 2

The Genetic Code
•The word genetic refers to heredity.
•A code is a set of symbols or signals used to
pass on information.
•The genetic code is the way in which cells
store information that is passed from
one generation to the next generation.
The Search for the Genetic Code
•Frederick Griffith: He studied bacteria that
caused pneumonia in mice.
•In 1928 he did the following experiment.
Transformation Of Bacteria
Two Strains Of Streptococcus
Rough Strain
(Harmless)
Capsules
Smooth Strain
(Virulent)
Transformation Of Bacteria
The Griffith Experiment
OUCH!
+ Control
- Control
- Control
Experimental
The Search for the Genetic Code
Bacteria injected into Bacterial growth
mouse
pattern
Results
1.Disease-causing Smooth colonies Mouse dies
bacteria
of pneumonia
2. Harmless
bacteria
Rough colonies Mouse lives
3. Heat-killed
disease-causing
bacteria
No growth
Mouse lives
4. Mixed #2 & #3 Smooth colonies Mouse dies
of pneumonia
The Search for the Genetic Code
•Results from #4 show that the heat-killed strain
had passed to the live harmless strain its
disease-causing ability.
•That is, one form of bacteria was transformed
into another form.
•Griffith called this process transformation.
•He hypothesized that a factor was transferred
from the heat killed cells into the live cells
The Search for the Genetic Code
Oswald Avery and others: In 1944 they repeated
Griffith’s experiment to see which molecules
were responsible for the transformation.
•From the heat-killed bacteria they made an
extract or juice.
•Individually they treated the extract with
enzymes that destroy lipids, proteins,
carbohydrates, and RNA.
The Search for the Genetic Code
•The results were that transformation still
occurred and so none of these molecules were
the transferring agent.
•They repeated the experiment with enzymes
that break down DNA (deoxyribonucleic acid).
•The results were that transformation did not
take place.
Enzyme
Transformation?
Lippase
Yes
Protease
Yes
Saccharase
Yes
Nuclease
No
The Search for the Genetic Code
Conclusion: DNA is the transforming material
and is the molecule that stores and transmits
the genetic code from one generation of an
organism to the next.
SEARCH FOR THE GENETIC CODE
CONTINUES
After the Oswald Avery experiment some
scientists still remained skeptical that DNA
was the genetic material.
In 1952 the following experiment was done
by Alfred Hershey and Martha Chase.
The Hershey-Chase Experiment
•They used bacteriophages which are viruses
that infect bacteria (e.g., T2 phage).
•They are composed of a DNA core and a
protein coat.
•They infect bacteria by attaching to the
surface and injecting material.
The Hershey-Chase Experiment
•The injected material is used to make many
copies of the virus.
•Viral copies are produced until they cause
the bacteria to burst, releasing the virus.
•Because the injected material is used to make
copies, it must be the genetic code.
•Important question: What is being injected
from the virus, the protein coat or DNA
core?
The Hershey-Chase Experiment
•They labeled the protein coat of the virus
with radioactive sulfur-35.
•They labeled the DNA core of the virus with
radioactive phosphorus-32.
•They mixed the labeled viruses with the
bacteria, waited for injection, & then separated
the parts using a blender and centrifuge.
The Hershey-Chase Experiment
• Hershey and Chase took advantage of
the fact that T2 phage is made of only
two things: Protein and DNA
H
H
O
H2N C C
CH2
CH2
S
CH3
O
H2N C C
OH
Methionine
CH2
OH
HO P O
O
NH2
OH
Cysteine
SH
Some amino acids contain
sulfur, thus proteins contain
sulfur, but not phosphorous.
OH H
Nucleotides contain phosphorous, thus DNA
contains phosphorous, but not sulfur.
T2 grown in S35
containing media
incorporate S35
into their proteins
Using
T2 attach to bacteria and
inject genetic material
35
S
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.
Blending causes phage
protein coat to fall off
Did protein enter the bacteria?
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.
Blending causes phage
protein coat to fall off
Did DNA enter the bacteria?
Is DNA the genetic material?
The Hershey-Chase Experiment
The Hershey-Chase Experiment
Conclusion:
The Hershey-Chase experiment showed
definitively that DNA is the molecule that
carries the genetic code.
AN IMPORTANT QUESTION REMAINED:
WHAT IS THE STRUCTURE OF DNA?
•In the early 1950’s Rosalind Franklin &
Maurice Wilkins took x-ray pictures of the
DNA molecule.
•These x-ray pictures provided important clues
about the structure of DNA.
•For example, the fibers that make up DNA are
twisted and the molecules in the fibers are
spaced out in regular intervals.
AN IMPORTANT QUESTION REMAINED:
WHAT IS THE STRUCTURE OF DNA?
•Another important piece of information on the
structure of DNA was provided by Erwin
Chargaff.
•He observed that in any sample of DNA, the
number of:
Adenine molecules (A) = (T) Thymine molecules
Cytosine molecules (C) = (G) Guanine molecules