The Central Dogma
DNA structure
DNA replication
RNA Synthesis
Protein synthesis
Prof. David McConnell
Smurfit Institute of Genetics
The Dawn of Molecular Biology
Crick in the EAGLES pub: “We have discovered
the secret of life”
Watson and Crick: "It has not escaped our
notice that the specific (base) pairing we have
postulated immediately suggests a possible
copying mechanism for the genetic material.”
Nature April 25, 1953
TEMPLATING!
REPLICATION OF INFORMATION!
Models for DNA replication!
1) Semiconservative model:!
Daughter DNA molecules contain one parental !
strand and one newly-replicated strand!
2) Conservative model:!
Parent strands transfer information to an !
intermediate (?), then the intermediate gets copied.!
The parent helix is conserved, the daughter!
helix is completely new!
3) Dispersive model:!
Parent helix is broken into fragments, dispersed,
copied then assembled into two new helices.!
New and old DNA are completely dispersed!
MODELS OF DNA REPLICATION
Testing Models for DNA replication!
Matthew Meselson and Franklin Stahl (1958)!
(a) Hypothesis 1:
(b) Hypothesis 2:
(c) Hypothesis 3:
Semi-conservative
replication
Conservative replication
Dispersive replication
Intermediate molecule
Meselson and Stahl
Semi-conservative replication of DNA
Isotopes of nitrogen (non-radioactive) were used in this experiment
Generations!
0!
HH!
HH!
HL!
1.1!
HL!
1.5!
1.9!
Addition of monomers
Detection of
semiconservative
replication in E. coli
by density-gradient
centrifugation. The
position of a band of
DNA depends on its
content of 14N amd
15N. After 1.0
generation, all the
DNA molecules are
hybrids containing
equal amounts of 14N
and 15N!
0.7!
1.0!
DNA replication
Equilibrium Density
Gradient
Centrifugation!
0.3!
LL + HL!
Substrates are deoxynucleoside triphosphosphates (dNTP’s)
2.5!
3.0!
4.1!
0 and 2.0
mixed!
Substrate
0 and 4.1
mixed!
HL!
LL!
LL!
LH!
Replication as a process
•! Double-stranded DNA unwinds.
The junction of the unwound !
molecules is a replication fork.!
A new strand is formed by pairing !
complementary bases with the!
old strand.!
Two molecules are made. !
Each has one new and one old !
DNA strand. !