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DNA replication and transcription
Replication
• What is replication:
– Making a full new copy of the genetic content
of the organism
– Who are the players
• Some of the key features
– Semi-conservative
– Semi-discontinuous
– Highly accurate
• How does it happen?
Who are the players in replication?
• Template DNA
• Enzymes called DNA polymerases (Pol)
• Building blocks are deoxynucleotides
Recall the structure of DNA
• Long strands of
nucleotides joined by
phophodiester linkage
• Has a directionality
• By convention said to run
from 5’ to 3’
– Related to position
on sugar part of
nucleotide
The DNA Double Helix
• Two strands of bonded
nucleotides form sides of twisted
ladder in a ‘double helix’
...
• Nitrogenous base, gets lined up
with complementary base on
other ladder side
...
3’
5’
3’
3’
5’
5’
3’
5’
Semi-discontinuous
• DNA is only synthesized in the 5’ to 3’
direction
• One strand (leading strand) is synthesized as
a single piece
• The other strand (lagging strand) is
synthesized in short pieces
– Okazaki fragments
– Also synthesized in the 5’ to 3’ direction
– Has to wait for template to be exposed
– Joined up later to make complete second strand
Why does lagging strand have to
wait?
• It has to wait for new template to be
exposed at the replication fork
3’
5’
3’
3’
5’
5’
3’
5’
Chemistry of reaction controls
direction
New strand
5 end
Sugar
Template strand
3 end
5 end
3 end
T
A
T
C
G
C
G
G
C
G
C
T
A
A
Base
Phosphate
3 end
DNA polymerase
A
Pyrophosphate 3 end
C
Nucleoside
triphosphate
5 end
C
5 end
Bidirectional replication
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Leading strand
Lagging strand
Overall directions
of replication
Origin of replication
Leading strand synthesis
3
5
RNA primer
5
“Sliding clamp”
3
5
Parental DNA
DNA pol III
3
Direction of
replication fork
5
movement
5
3
5
Fig
...
Make by Primase of Pol III
...
Some other facts
• For most systems it is a bi-directional
process
• Has defined start point called an ‘origin of
replication’, ori
• For higher organisms greater level of
unpacking
Bi-directional replication of the
E
...
coli is replicated bi-directionally
from a single ori, oriC
• Higher organisms can have multiple ori in
chromosome
Fig
...
25 µm
Bubble
Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
DNA fiber
Higher
organisms
have to unpack
their DNA a bit
more before
replication can
take place
Nucleosomes
Solenoid
Metaphase
chromosome
Chromatin
fiber
Chromatid
Fig
...
16-21b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Solenoid
Chromatin fibre
30-nm fiber
Looped domains
(300-nm fiber)
Replicated
chromosome
(1,400 nm)
Metaphase
chromosome
Replication is very accurate
• The main enzyme doing the majority of the work
Pol III is very fast, but not completely accurate
– This would be fatal as mutations would accumulate
• Pol I slower at synthesis but can correct errors,
cuts off mismatched base, and allows a second
attempt
...
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