The Central Dogma
DNA structure
DNA replication
RNA Synthesis
Protein synthesis
Prof. David McConnell
Smurfit Institute of Genetics
Replication can be Uni- or Bidirectional
DNA Replication
The idea of an origin
•! Since DNA replication is semiconservative, therefore
the helix must be unwound.
•! John Cairns (1963) showed that initial unwinding is
localized to a region of the bacterial circular genome,
called an “origin” or “ori” for short.
UNIDIRECTIONAL REPLICATION
Origin
BIDIRECTIONAL REPLICATION
3’
5’
5’
3’
Origin
5’
3’
Evidence points to bidirectional replication!
John Cairns!
Result of Cairns
Bacterial!
culture!
*T!
*T!
*T!
*T!
in media with low!
concentration of !
3H- thymidine!
Grow cells for several generations!
Small amounts of 3H thymidine!
are incorporated into new DNA!
*T!
*T!
*T!
All DNA is lightly!
labeled with radioactivity!
Grow for brief
period of time!
*T! *T!
Add a high !
concentration!
of 3H- thymidine!
*T!
*T!*T!
*T!
*T!
*T! *T!
*T!
*T! *T!
*T!
*T!
*T!
*T! *T! *T! *T!
*T!
*T! *T! *T! *T! *T!*T!*T!
Dense label at the replication fork!
where new DNA is being made!
Cairns then isolated the chromosomes by lysing the cells very very gently
and placed the lysate on an electron micrograph (EM) grid which he exposed
to X-ray film for two months.!
Label at both replication forks!
3’
5’
T7 DNA replication 1
B
A
A’
Replicating bubble in DNA from bacteriophage T7
Two replication forks heading towards opposite ends of the DNA
Features of DNA Replication
•! DNA replication is semiconservative
–! Each strand of both replication forks is being
copied.
•! DNA replication is bidirectional
–! Bidirectional replication involves two
replication forks, which move in opposite
directions
Replicating DNA from bacteriophage T7. One fork has reached
the ends (A and A’). Second fork (arrow) is heading towards the
opposite end of the DNA (B)