Smooth Strain Rough Strain - Nassau Community College
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4
Smooth Strain
! Mice were injected with the live S strain.
! All of the mice developed pneumonia and died
Rough Strain
!Mice injected with a sample of the Roughstrain of bacteria did not get pneumoniaand lived.
Heat-killed Smooth Strain
! Smooth strain is heated until cells die.
! Mice injected with test sample did not get pneumonia
and lived.
The Transforming Factor
! Griffith combined the live R
strain and heat-killed S
strain.
! Mice injected with the
mixture contracted
pneumonia and died.
! Griffith proposed the
presence of a factor within
dead cells that can be used
to Transform live cells.
5
Transformation revisited
Why do it again?
rediscovered!!!!!
1944
Avery, McCarty & MacLeod
! Worked at Cold Spring Harbor, Long Island. Facility
was built by the US Army
! They continued Griffith’s work -
Using Enzyme Extracts.! Used enzymes to digest away selected components
!Proteases eliminated ???? Do mice live?
!Rnase eliminated ????? Do the mice live?
!Dnase eliminated ???? Do the mice live?
1944Page 191
Avery, McCarty &MacLeod
! Once more they used
pneumococcus.
! S strain causes
pneumonia and kills
mice.
! R strain does not. Mice
live.
! Heat-killed S - mice live.
! Mixing Live R and Heat-
killed S strains kills mice
! They improved the
experiment by adding
enzymes to the heat-killed S
strain before mixing it with
the live R strain.
! Adding Proteases - mice die.
! Adding Rnase - mice die.
! Adding Dnase - mice live
! DNA is the CAUSE of the
TRANSFORMATION
Purine : Pyrimidine
A : TG : C
Base Pairs
6
The Rule????
Who discovered it?
Discovered!!!!!
1946
Erwin Chargaff
! Columbia University Professor
! Isolated chromatin from a variety of
life forms:
! Horses, dogs, fish
! Trees, flowers, vines, etc.
!Chargaff’s Rule! The amount of A~T
! The amount of G~C
! 1:1 ratio between purines and
pyrimidines
1946Page 197
The DNA moleculeis shaped like
a right-handed, spiralstaircase.
The Double Helix
The Double Helix
Who discovered it?
Discovered!!!!!
1951
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