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INITIAL OBSERVATIONS:
1) Two type of Pneumococcus bacteria:
a) Smooth colonies , cells with capsule
b) Rough colonies, cells without capsule
2) Mouse infected with capsulated bacteria get sick and die
3) Mouse infected with non capsulated bacteria do not get sick & live
4) Mouse infected with killed smooth (capsule) strain do not get sick & live
EXPERIMENTAL FINDING
5) Mouse infected with 3 & 4 (from above) did get sick and die
live bacteria with capsule was found in its blood
GRIFFITH’S TRANSFORMATION EXPERIMENT
R cells (no capsule) produce rough colonies
S cells (capsule) produce smooth colonies
Avery, McLeod & McCarty made extracts from S cells
Capsule material
Proteins
DNA
RNA
Rough Cells are mixed with one of the extracts from S cells
Capsule material
No change
Proteins
No change
RNA
No change
DNA
Transformation into S type
Experiment of Avery, McLeod & McCarty
identifies DNA as the transforming molecule
Protein coat
DNA
PHAGE T2
35S
labeled Protein coat
Label remains outside
32P labeled
DNA
labeled material found
inside infected cell
Hersey-Chase Experiment Demonstrating that DNA
is the genetic material for phage T2
UTILIZATION OF 15N & DIFFERENTIAL CENTRIFUGATION
TO STUDY REPLICATION MECHANISMS OF DNA
E. coli grown in
15N
Upon centrifugation
DNA settles near the bottom
Heavy E. coli allow to grow one generation in 14N
Upon centrifugation
DNA settles in the middle
A second round of replication in 14N
Upon centrifugation
DNA settles near the surface
and in the middle
Demonstration of DNA Semiconservative Replication
by Meselson & Stahl