Lecture 5

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25 Mar 2012
Lecture 5: Introduction to DNA Replication
1. Overview of DNA replication
2. DNA replication in bacteria
Is DNA replication conservative or semi conservative?
o1st option
In a conservative model of replication, both strands are conserved and
wind up in the same cell
The replication event results in daughter strands delocalized in another
o2 nd
Semi conservative replication involves the targeting of 1 parental strand
and 1 daughter strand into each of the resulting daughter cells
In each generation, you are sending forward a parental strand that you
know is correct, that has been proofed. So during the replication events,
any mistakes in the daughter cells are now going to be corrected
What is the direction of DNA replication?
o1st option
Unidirectional growth of a single strand from 2 starting points copying
the parental strand from the 3’ end
o2nd option
Unidirectional growth of 2 strands from 1 starting point
o3rd option
Bidirectional growth from 1 starting point means that where the site of
initiation of replication occurs you wind up with the bidirectional growth
occurring in both directions
Where does DNA replication start?
o1 st
Always start from the same location on DNA
We need a region of the DNA that is easily separated into single strand and
you can maximize the chances by modifying the base configuration – more
A T residues are easier than G C rich
They have to have sufficient sequence diversity in order to be recognized
and bind to initiator complex
o2nd option
Random location
How many origins of replication?
oBacteria are usually single replication
They are located at particular region on circular chromosome 1
replication origin per chromosome
There are specific sense of ques environmental sensors that say ‘it’s time
to grow’
Highly controlled event, because of little growth in bacteria
oEukaryotes are multiple replication
Multiple genomes
The ones in red are non-default and the green parts are the regions that
are usually replicated
If the environment is good, the yeast will replicate in all of the origins
An experiment to identify origins of replication in yeast
oOrigins of bacteria are also found on plasmids
oAs long as you got the origins of replication on that plasmid that’s specific for
that organism, that plasmid will replicate autonomously inside that bacteria
oIn bacteria, you can determine origins of replication by looking for specific
sequences on plasmids, taking components of the chromosome and inserting
into the plasmid and looking to see whether or not it will support, etc.
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