Transcript Presentation

How Scientists Identified DNA
• 1928 - Frederick Griffith did a study on
two strains of bacteria to find which was virulent.
One formed rough colonies in a Petri dish while the
other formed smooth colonies.
Virulent - disease causing bacteria
• To determine which strain was virulent he injected
the two different strains into mice.
• The smooth colonies were virulent and killed Minnie.
• The rough colonies were nonvirulent and caused no
harm to Mickey.
• In the next part of the study, Griffith killed the
virulent bacteria with heat. When this strain
was injected into a mouse it caused no harm.
• He then mixed the heat killed virulent bacteria
with the nonvirulent strain. When this mix was
injected into a mouse, the mouse died
• in the mix there were no nonvirulent bacteria.
Only virulent bacteria was present.
• What happened? Why was the only bacteria
present virulent?
What happened? Why was the
only bacteria present virulent?
• Griffith concluded that the nonvirulent strain
had taken something from the virulent strain.
At the time Griffith did not know what that
something was.
• Transformation is the process in which a
bacterium takes up foreign DNA
• The rough colonies had taken up the smooth
colonies DNA
Oswald Avery wanted to continue Griffith’s
work. Was it DNA or protein that made the
nonvirulent strain of bacteria turn virulent?
Experiment #1 Avery extracted (took out) DNA
from bacteria in the smooth colony and added it to bacteria
in the rough colony. The bacteria that grew from the
mixture formed smooth colonies.
Experiment #2 Avery added protein-destroying
enzymes to the bacteria, but transformation still took place.
Experiment #3 Avery added DNA -destroying
enzymes to the bacteria, and transformation did not take
place.
What could he conclude?
It was clear to Avery that
DNA was the genetic
material in the bacteria!!!
Many of his colleagues
still held on to the belief
that protein was the
genetic material.
• Alfred Hershey and Martha Chase
performed an experiment using viruses that infect
bacteria by attaching to the surface of the cell and
injecting hereditary information like tiny
hypodermic needles.
• This way hundreds of new viruses are produced.
• These viruses have a very simple structure: a
core of DNA surrounded by a protein coat.
Hershey and Chase did
experiments to determine if DNA
held the hereditary information
They did experiments with radioactive labels.
One experiment used virus with radioactive sulfur and
another used a virus with radioactive phosphorus.
They infected two batches of bacteria and gave the virus
time to attach to the bacteria and inject their hereditary
material.
The protein coating detached from the original bacteria, but
not from the new.
The DNA stayed the same in both the new and the original.
•DNA was the
clear winner.
DNA structure
•
•
DNA is composed of subunits called
nucleotides.
A nucleotide is comprised of 3 parts:
– A sugar, called deoxyribose
– A phosphate group
– And a base
•
The sugar and phosphate group are the same in
every nucleotide, but the base can be one of four
types.
• The 2 larger bases, adenine and guanine, are
called purines.
• The 2 smaller bases, cytosine and thymine,
are called pyrimidines.
Erwin Chargaff made a stunning
discovery – later known as Chargaff’s rules
• The amount of adenine always equals the
amount of thymine. A = T
• The amount of guanine always equals the
amount of cytosine. G = C
• This suggests a regularity in the composition of
DNA
DNA resembles a tightly coiled
spring, a spiral staircase
• Double helix – a spiral staircase composed
of two strands of nucleotides whose bases face
each other.
• The nucleotides pair up in a regular order:
– Adenine faces thymine
– Guanine faces cytosine
• A sugar phosphate backbone makes the sides of
the Double helix .
Watson and Crick’s model of DNA
• James Watson and Francis
Crick built the first model of a
DNA double helix.
• They followed the clues of
Chargaff’s rules and X-ray
diffraction photograph done by
Rosalind Franklin, a London
chemist.
• Watson and Crick were
awarded the Nobel Prize in
1963 for their development of
this model