Molecular Cell Biology
 Nucleic acid structures
 DNA structure
 Evidence that DNA is the genetic material
 DNA replication
 Central dogma
 Transcription
 Genetic code
 Translation
 Mutation
1
DNA replication
 For transmission of the genetic information
to the next generation the DNA needs to be
copied
 Watson and Crick noted that the specific
base pairing suggested a possible copying
mechanism for genetic material
 Since the two strands of DNA are
complementary, each strand can act as a
template for building a new strand during
replication
2
a)
Parental
molecule
b) Separation of
parental strands
into templates
c) Formation of new
strands complementary
to template strands
In DNA replication, the parent molecule
unwinds, separates and two new daughter
strands are built based on base-pairing rules
3
 Watson and Crick’s semiconservative
model of replication predicts that when a
double helix replicates, each daughter
molecule will have one old strand (derived
or “conserved” from the parent molecule)
and one newly made strand
 Competing models were the conservative
model (the two parent strands rejoin) and
the dispersive model (each strand is a mix
of old and new)
4
Possible mechanisms of DNA replication
Conservative
Parent DNA
+
Dispersive
+
Semi-conservative
+
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Mechanism of replication for each of these models
6
Mechanism of replication for each of these models
What will the DNA
look like in the
second round of
replication?
7
 In 1958 Matthew Meselson and Franklin
Stahl experimentally proved that DNA was
replicated semiconservatively
• Each strand of DNA
acts as a template for
the synthesis of the
complimentary strand
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Meselson and Stahl Experiment (1958)
 Started their experiment by growing E. coli in
nutrient broth, containing a "heavy" isotope of
nitrogen
• 14N = common N isotope
• 15N = heavy isotope
 After a few generations all the DNA will contain
the heavy isotope (15N-labeled) and the DNA will
be more dense.
 If the cells were then moved to broth containing
the common lighter nitrogen isotope then the
newly synthesized DNA would be less dense
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Meselson and Stahl (1958)
 Molecules of different density can be separated
by density gradient centrifugation.
 Lighter molecules sit higher in the centrifugation
tube.
 Remember DNA is double stranded therefore
there are 3 possible combinations with the two
types of nitrogen
14N / 14N
14N / 15N
15N / 15N
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Meselson and Stahl (1958)
 Bacteria cultured in medium containing 15N.
 Transferred to medium containing 14N.
 DNA centrifuged after first round of replication
11
After the first round of
replication they saw this
14N/15N
One band of
medium density
Which model of replication is supported by
this experiment?
12
13
Messelsohn and Stahl experiment
First round of
replication
Second round of
replication
14N/14N
14N/15N
14N/15N
Which model of replication is supported by
this experiment?
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15
In the Meselson-Stahl DNA replication experiment,
if the cells were first grown for many generations in
15N containing media, and then switched to 14N
containing media, what percent of the DNA had 1
light strand and 1 heavy strand after 1 generation
of growth in 14N containing growth media?
1) 0
2) 25
3) 50
4) 75
5) 100
16
In the Meselson-Stahl DNA replication experiment,
if the cells were first grown for many generations in
15N containing media, and then switched to 14N
containing media, what percent of the DNA had 1
light strand and 1 heavy strand after 3 generations
of growth in 14N containing growth media?
1) 0
2) 25
3) 50
4) 75
5) 100
17
Semiconservative replication will result in
A. Two molecules of DNA containing one
parental strand each
B. Two molecules of DNA one containing only
parental DNA and the other only new DNA
C. Two molecules of DNA containing random
amounts of parental and new DNA
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