Transcript History of DNA

History of DNA
FREDERICK GRIFFITH 1928
• In 1928, the British scientist was studying the way in which
certain types of bacteria cause the disease pneumonia
• When Griffith injected the mice with the disease-causing strain
of bacteria, the mice developed pneumonia and died
• When the mice were injected with the harmless strain they did
not get pneumonia & did not die
• When the mice were injected with a mixture of both harmless
and heat-killed bacteria (from the disease-causing strain) they
developed pneumonia & died
• Griffith’s final conclusion is that one strain of bacteria had
been transformed into another
– They called this process transformation
GRIFFITH’S EXPERIMENT
Heat-killed, diseasecausing bacteria
(smooth colonies)
Harmless bacteria (rough
Disease-causing bacteria
Heat-killed, diseaseControl
colonies)
(smooth colonies)
causing bacteria (smooth (no growth)
colonies)
Dies of pneumonia
Lives
Lives
Harmless bacteria (rough
colonies)
Dies of pneumonia
Live, disease-causing
bacteria (smooth colonies)
FREDERICK GRIFFITH 1928
– CONCLUSION:
• Transformation – somehow, the heat-killed
disease-causing bacteria transferred its diseasecausing ability to the harmless bacteria
AVERY, MCCARTY, MACLEOD 1944
• Repeated Griffith’s work in 1944 to see if they
could discover which molecules were the
transforming factor. Avery and his colleagues
made an extract, or juice, from the heat-killed
bacteria
• They treated the extract with enzymes that
destroy lipids, proteins, and carbohydrates, but
they discovered that transformation still occurred
• When they treated the nucleic acids (DNA) with
enzymes, the transformation did not occur
• This means that DNA is the transforming factor
that carries the genetic code
AVERY, MCCARTY, MACLEOD 1944
– CONCLUSION:
• DNA was the transforming factor
HERSHEY & CHASE 1952
• Studied viruses that infect bacteria (1952) –
bacteriophages
• Wanted to learn whether the protein coat, DNA, or
both parts from the virus enter the bacterium
• Made two batches of the virus and labeled each with
a radioactive isotope
– By adding the radioactive isotopes to the viruses,
they were labeling the virus’ protein and DNA
• Protein – sulfur 35
• DNA – phosphorous 32
HERSHEY & CHASE 1952
• They mixed the radioactive viruses with the bacteria,
and then separated the mixture by placing it in a
centrifuge
• Results showed that DNA enters the bacteria, but the
protein coat stays outside
• This experiment showed conclusively that DNA
carried the genetic code
ROSALIND FRANKLIN & MAURICE
WILKINS 1950’s
• In the 1950’s, used X-ray evidence to help discover the
structure of the DNA molecule
• To do this, she purified large amounts of DNA and stretched the
fibers in a thin glass tube so the strands were parallel
• Next she aimed a narrow X-ray beam on the DNA and recorded
the pattern on film
• Her results gave two important clues about the shape of DNA
– Twisted shape
– Groups of molecules spaced at regular intervals
• Maurice Wilkins was Franklin’s assistant and is a shared
recipient of the Nobel Prize
ROSALIND FRANKLIN
Women in Science: A Controversy
It was Franklin's photograph of the DNA molecule that
sparked a scientific revolution. Wilkins showed Watson
the photo, and, Watson said, "My jaw fell open and my
pulse began to race."
The photo showed, for the first time, the essential
structure of DNA -- the double-helix shape, which also
indicated its method of replication.
ERWIN CHARGOFF 1950’s
• Observed that in any sample of DNA, the
number of adenine molecules was equal to
the number of thymine molecules
– Same was true for guanine and cytosine
• Provided one of the key steps in
developing a structure model of the DNA
molecule
mol % of bases
Ratios
Source
%GC
A
G
C
T
A/T
G/C
PhiX-174
24.0
23.3
21.5
31.2
0.77 ¦
1.08
44.8
Maize
26.8
22.8
17.0 *
27.2
0.99
0.98
46.1
Octopus
33.2
17.6
17.6
31.6
1.05
1.00
35.2
Chicken
28.0
22.0
21.6
28.4
0.99
1.02
43.7
Rat
28.6
21.4
20.5
28.4
1.01
1.00
42.9
Human
29.3
20.7
20.0
30.0
0.98
1.04
40.7
FRANCIS CRICK & JAMES WATSON 1953
• Using Franklin’s and Wilkins’ ideas, Watson &
Crick developed a model showing the strands of
DNA were twisted around each other to form a
double helix
• Their model explains another characteristic
about DNA structure
– The nitrogenous bases on each of the strands
of DNA are positioned exactly opposite each
other
– Used Chargoff’s ideas of how bases bond
together and called it base pairing
FRANCIS CRICK & JAMES WATSON 1953
• Presented their model in
1953 and along with
Maurice Wilkins, won the
Nobel Prize
– Why wasn’t Rosalind
Franklin recognized for
her accomplishments?
• She died in 1958 and
only living scientists
can win
CURRENT AREAS OF RESEARCH
HUMAN GENOME PROJECT
• Website
• Nucleotide base sequencing for all our 30,000+
human genes
• What trait, what gene, what chromosome(s), what it
does
CURRENT AREAS OF RESEARCH
FORENSIC SCIENCE – DNA FINGERPRINTING
• Chemically isolate DNA, cut it into pieces, sort it by
size, denature it by heating or chemically treating, &
finally blot it by fractionation and bake it onto paper
• What about a world-wide DNA database? What are
some potential issues? Benefits?
CURRENT AREAS OF RESEARCH
DNA TESTING FOR BIODIVERSITY, EVOLUTIONARY
TRENDS, & ANIMAL BEHAVIOR
• Needed for genetic variation within/among
populations
• Mating and reproductive strategies
• Obtaining gene pools is essential when designing
programs for management of wildlife species
CURRENT AREAS OF RESEARCH
STEM CELL RESEARCH
• Embryonic stem cell controversy
– George Bush, early 2000’s
• Use of somatic stem cells
– Reverse engineering