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