The two replication forks move in opposite directions sequentially around the circular
chromosome
around(the(circular(chromosome(
unique ori
!
fully !
replicated
!
The Phage
Chromosome
Three forms of
DNA
o
Linear
o
H-bonded
o
Covalently bonded circular
AT-rich and GC-rich regions used to
demonstrate bidirectional replication
Denaturation mapping
Increase temperature or pH to denature
genome
Easier to break A-T bonds than G-C
bonds because G-C have more hydrogen
bonds
By examining the positions of the branch point
(Y-shaped structures) relative to the positions
of the denaturing bubbles in a large number of
Y-shaped replicative intermediates, Schnos and
Inman demonstrated that
both branch points
are replication forks that move in opposite
directions around the circular chromosome
Key Points
DNA replicates by a semiconservative
mechanism: as the two complementary
strands of a parental double helix

unwind and separate, each serves as template for the synthesis of a new complementary
strand
The hydrogen bonding potentials of the bases in template strands specify
complementary base sequences in the nascent DNA strands
Replication is initiated at unique origins and usually proceeds bidirectionally from each
origin
Exercise:
The Escherichia coli chromosome contains approx. 4 x 10
6
nucleotide pairs and replicates
as a single bidirectional replicon in approx. 40 minutes under a wide variety of growth
conditions.
The Supermegabigia coli chromosome contains approx. 8 x 10
7
nucleotide pairs and
replicates at the same rate as E.coli
How long does it take to replicate the Supermeganigia coli?
Is the answer:!
!
a) 80 minutes!
!
b) 120 minutes!
!
c) 800 minutes!
!
d) 2000 minutes!
Because each chromosome replicates bidirectionally,
each replication fork must traverse 2 x 10
6
nucleotide
pairs in E. coli (or half the total nucleotide pairs).!
Same principle applies to “
Supermegabigia#
coli
”
chromosome; it must traverse 4 x 10
7
nucleotide pairs.!
4 x 10
7
nucleotide pairs in “
Supermegabigia#
coli
” !
2 x 10
6
nucleotide pairs in E. coli
!
!
=
20
times more nucleotide pairs in !
“
Supermegabigia#
coli
” !
40 minutes to replicate
all the nucleotide pairs in
E. coli x 20
!
!
= 800 minutes
!
!
DNA Replication in Prokaryotes
Must understand the process and the different proteins that enable this process
Names of proteins involved
How the process works
Know how each protein works
Don’t need to know structures of proteins
Won’t be asked how many subunits

Many different DNA polymerases
E.coli contains at least 5 different DNA polymerases:
o
DNA polymerase I –multiple functions (polymerase and nuclease)
o
DNA polymerase II – replication of damaged DNA
o
DNA polymerase III – multiple functions (polymerase and nuclease)
o
DNA polymerase IV – replication of damaged DNA
o
DNA polymerase V – replication of damaged DNA
(dNMP)
n
+ dNTP
(dNMP)
n+1
+ PP
i
DNA template
DNA dependent
DNA polymerase
DNA replication cannot occur de novo (from nothing)
o
Primer with a free OH group at its 3’ end
o
Template DNA to specify the sequence of the new DNA strand


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- Fall '13
- DNA, RNA Chains