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DNA replication stating the enzymes that are involved
1. DNA replication
Learning objectives
Understand the basic rules governing
DNA replication
Introduce proteins that are typically
involved in generalised replication
Reference: Any of the recommended texts
Optional
Nature (2003) vol 421,pp431-435
http://www.bath.ac.uk/bio-sci/cbt/
http://www.dnai.org/lesson/go/2166/1973
2. `It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible
copying mechanism for the genetic
material’
Watson & Crick
Nature (1953)
Original drawing by Francis Crick
3. Four requirements for DNA to
be genetic material
Must carry information
Cracking the genetic code
Must replicate
DNA replication
Must allow for information to change
Mutation
Must govern the expression of the phenotype
Gene function
4. Much of DNA’s sequence-specific information is
accessible only when the double helix is unwound
Proteins read the DNA sequence of nucleotides as
the DNA helix unwinds. Proteins can either bind to a
DNA sequence, or initiate the copying of it.
Some genetic information is accessible even in
intact, double-stranded DNA molecules
Some proteins recognize the base sequence of DNA
without unwinding it.
One example is a restriction enzyme.
DNA stores information in the sequence of
its bases
5. DNA Replication
Process of duplication of the entire genome prior to cell
division
Biological significance
extreme accuracy of DNA replication is necessary in
order to preserve the integrity of the genome in
successive generations
In eukaryotes , replication only occurs during the S
phase of the cell cycle. The slower replication rate in
eukaryotes results in a higher fidelity or accuracy of
replication in eukaryotes
6. Basic rules of replication
A. Semi-conservative
B. Starts at the ‘origin’
C. Can be uni or bidirectional
D. Semi-discontinuous
E. Involves DNA templating
F. RNA primers required
7. A) DNA replication – three possible models
Semiconservative replication – Watson
and Crick model
Conservative replication: The parental
double helix remains intact; both strands
of the daughter double helix are newly
synthesized
Dispersive replication: At completion,
both strands of both double helices
contain both original and newly
synthesized material.
8. Meselson-Stahl experiments confirm
semiconservative replication
Semi-conservative replication An
ingenious experiment by Meselson and
Stahl showed that one strand of each
DNA strand is passed on unchanged to
each of the daughter cells. This
'conserved' strand acts as a template for
the synthesis of a new, complementary
strand by the enzyme DNA polymerase
12. Starts at origin
Initiator proteins identify specific base sequences on
DNA
Prokaryotes – single ori site E.g E.coli - oriC
Eukaryotes – multiple sites of origin (replicator)
E.g. yeast - ARS (autonomously replicating sequences)
Prokaryotes
Eukaryotes
14. Bidirectional replication
Replication forks move in opposite directions
In linear chromosomes, telomeres ensure the
maintenance and accurate replication of
chromosome ends
In circular chromosomes, such as E. coli, there
is only one origin of replication.
In circular chromosomes, unwinding and
replication causes supercoiling, which may
impede replication
Topoisomerase – enzyme that relaxes
supercoils by nicking strands
17. Semi-discontinuous replication
Anti parallel strands replicated simultaneously
Leading strand synthesis continuously in 5’– 3’
Lagging strand synthesis in fragments in 5’-3’
21. Topoisomerases
Helicases
Single strand
binding proteins
Primase
DNA polymerase
Tethering protein
DNA ligase
- Prevents torsion by DNA breaks
- separates 2 strands
- prevent reannealing
of single strands
- RNA primer synthesis
- synthesis of new strand
- stabilises polymerase
- seals nick via phosphodiester
linkage
Core proteins at the replication fork
22. The mechanism of DNA replication
Arthur Kornberg, a Nobel prize winner and
other biochemists deduced steps of
replication
Initiation
Proteins bind to DNA and open up double helix
Prepare DNA for complementary base pairing
Elongation
Proteins connect the correct sequences of
nucleotides into a continuous new strand of DNA
Termination
Proteins release the replication complex
23.
24.
25. Core proteins at the replication fork
Nature (2003) vol 421,pp431-435 Figure in ‘Big’ Alberts too
26. Topoisomerase I
Cause temporary
single strand breaks in
DNA
Allows free rotation of
helix
Monomeric enzymes
(~100kD)
Reversible catenation
(knotting)
Acts by breaking
phosphodiester
linkage via tyrosine
active site
28. Topoisomerase II (DNA gyrase)
~ 375 kD
2 pairs of subunits: A
& B
ATP hydrolysis
mediated double
strand break
Catenates double
stranded circles
Inhibited by antibiotics