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Molecular Basis
Of Inheritance
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
Transformation Of Bacteria
Two Strains Of Streptococcus
Rough Strain
(Harmless)
Capsules
Smooth Strain
(Virulent)
©2000 Timothy G. Standish
Transformation Of Bacteria
The Griffith Experiment
OUCH!
+ Control
- Control
- Control
Experimental
©2000 Timothy G. Standish
Avery, MacLeod and McCarty
1944 Avery, MacLeod and McCarty repeated
Griffith’s 1928 experiment with modifications
designed to discover the “transforming factor”
 Extracts from heat-killed cells were digested
with hydrolytic enzymes specific for different
classes of macro molecules:

Enzyme
Transformation?
Lippase
Yes
Protease
Yes
Saccharase
Yes
Nuclease
No
©2000 Timothy G. Standish
The Hershey-Chase
Experiment
The Hershey-Chase experiment showed
definitively that DNA is the genetic material
 Hershey and Chase took advantage of the fact
that T2 phage is made of only two classes of
macromolecules: Protein and DNA

H
H2N C C
CH2
CH2
S
CH3
H
O
H2N C C
OH
Methionine
CH2
SH
OH
O
OH
Cysteine
Some amino acids
contain sulfur, thus
proteins contain sulfur,
but not phosphorous.
HO P
NH2
O
O
OH
H
Nucleotides contain phosphorous,
thus DNA contains phosphorous,
but not sulfur.
©2000 Timothy G. Standish
S35
T2 grown in
containing media
incorporate S35
into their proteins
Using S35 Bacteria grown in
T2 attach to bacteria and
inject genetic material
normal nonradioactive media
When centrifuged,
phage protein coats
remain in the
supernatant while
bacteria form a pellet
The supernatant is
radioactive, but the
pellet is not.
Blending causes phage
protein coat to fall off
Did protein enter the bacteria?
Is protein the genetic material?
P32
T2 grown in
containing media
incorporate P32
into their DNA
Using P32 Bacteria grown in
T2 attach to bacteria and
inject genetic material
normal nonradioactive media
When centrifuged,
phage protein coats
remain in the
supernatant while
bacteria form a pellet
The pellet is
radioactive, but the
supernatant is not.
Blending causes phage
protein coat to fall off
Did DNA enter the bacteria?
Is DNA the genetic material?
A Nucleotide
Adenosine Mono Phosphate (AMP)
Phosphate
HO
H+
Nucleotide
OH
P
O
Base
N
H
O
5’CH2
4’
NH2
H
N
O
1’
Sugar
3’
OH
2’
H
OH
N
N
Nucleoside
Purines
NH2
Adenine
N
N
N
O
CH3
(DNA)
N
Guanine
NH
N
Thymine
O
NH2
Uracil
(RNA)
NH
N
O
N
N
Pyrimidines
NH
O
N
O
NH2
Cytosine
N
N
O
Base Pairing
Guanine And Cytosine
-
+
+
+
-
Base Pairing
Adenine And Thymine
+ -
Adenine
-
+
Thymine
Base Pairing
Adenine And Cytosine
+
-
-
Base Pairing
Guanine And Thymine
+
+
P
HO
NH2
O
N
O
CH2
OH
N
N
O
H
N
O
CH2
O
HO
P
O
O
N
O
CH2
OH
H
H2O
NH
N
O
OH
P
O
H
O
NH2
N
O
CH2
O
H
O
H
H2O
N
O
CH2
N
O
O
O
CH2
O
P
HO
H
O
OH
HO
P
NH2
OH
P
O
H
O
HO
O
D
N
A
OH
-
-
-
-
-
-
G
-
3.4 nm
1 nm
-
-
Minor
groove
C
G C
T A
A T
-
The Watson - Crick
Model Of DNA
G C
T A
C G
A T
Major
groove
A T
C G
G C
0.34 nm
T A
-
-
-
-
-
-
-
-
-
-
-
©2000 Timothy G. Standish
-
B DNA
Forms of the Double Helix
G
T A
Minor
groove
Z DNA
A T
C G
C
A DNA
3.9 nm
0.8 nm
Minor
groove
G C
T A
C G
A T
Major
groove
A T
G
T A
A T
C G
G C
T A
A T
1.2 nm 2.8 nm
G
C
T A
C
Major
groove
G
T A
G C
C G
C
C
G
A T
A T
G
C
0.26 nm
0.34 nm
10.4 Bp/turn
+34.6o Rotation/Bp
11 Bp/turn
+34.7o Rotation/Bp
12 Bp/turn
-30.0o Rotation/Bp
©2000 Timothy G. Standish
Supercoiling
Open circle
DNA with no
supercoiling
Negatively
(twisting to
the left)
supercoiled
DNA
Opened
negatively
supercoiled
DNA
Opening negatively
supercoiled DNA
may contribute to
strand separation
©2000 Timothy G. Standish
Distribution Of Negative Charge
Prevents
DNA
Annealing
O
-
-O
P
NH2
O
N
O
CH2
OH
N
N
O
H
N
O
CH2
O
-O
P
O
O
N
O
CH2
O
H
NH
N
O
O-
P
O
H
O
NH2
N
O
CH2
O
H
O
N
O
CH2
N
O
O
O
CH2
O
OH
H
O
P
NH2
O-
P
O
-O
O
H
O
P
O-
O-
Salts Allow DNA Annealing
O-O
P
NH2
O
N
O
CH2
OH
N
N
O
H
N
NaCl
O
CH2
O
H
O
H
O
-O
P
NH2
N
NH2
O
N
O
CH2
N
O
OH
H
O
O
Cat ions can cancel
out the negative
charge carried on
the sugar phosphate
backbone.
CH2
O-
P
O
O
CH2
O
N
Na+
O
CH2
NH
H
N
O
O
H
O
O
O
P
O-
P
O
O
O
-O
Cl-
P
O-
O-
Salts Allow DNA Annealing
Na+
ONa+
-O
P
NH2
O
N
O
CH2
OH
N
N
O
H
N
O
CH2
O
Na+
-O
P
O
O
N
O
CH2
N
O
NH2
N
Na+
O
H
NH
O-
P
O
H
O
Na+
O
CH2
O
O
N
O
CH2
N
O
O
O
CH2
O
OH
H
O
P
NH2
H
-O
Na+
O-
P
O
Na+
O
H
O
P
O-
ONa+
Na+
Salts Allow DNA Annealing
Na+
ONa+
-O
P
NH2
O
N
O
CH2
OH
N
N
O
H
N
O
CH2
O
Na+
-O
P
O
O
N
O
CH2
Na+
O
H
NH
N
O
O-
P
O
H
O
NH2
N
O
CH2
O
O
N
O
CH2
N
O
O
O
CH2
O
OH
H
O
P
NH2
H
-O
Na+
O-
P
O
Na+
O
H
O
P
O-
ONa+
Na+
5’AA3’
Base Stacking
3’UU5’
©2000 Timothy G. Standish
Base Stacking
5’AU3’
3’UA5’
©2000 Timothy G. Standish
AZT
3’-Azido-3’-deoxythymidine
O
CH3
Thymine
NH
OH
N
CH2
O
O
Deoxy
ribose
N3
H
©2000 Timothy G. Standish
©2000 Timothy G. Standish