2. DNA Replication
⚫A process in which daughter DNAsare synthesized
using the parental DNAsas template.
⚫Transferring thegenetic information to the
descendantgeneration.
3. •DNA replication is semi conservative
Each strand of template DNA is being copied.
•DNA replication is semi discontinuous
The leading strand copies continuously
The lagging strand copies in segments (Okazaki
fragments) which must be joined
•DNA replication is bidirectional
Bidirectional replication involves the formation of
replication forks, which move in oppositedirections
4.
5. DNA Replication
DNA replication includes:
•Initiation – replication begins at an origin of
replication
•Elongation – new strands of DNA aresynthesized by
DNA polymerase
•Termination – replication is terminated differently in
prokaryotes and eukaryotes
6. Prokaryotic DNA Replication
•The chromosomeof a prokaryote isacircular molecule
of DNA.
•Replication begins atone origin of replication and
proceeds in both directionsaround thechromosome.
8. Initiation
•The enzymes involved in DNA replication are closely
associated in one large complex called, the replisome.
•The replisomeconsists of
•the primosome - composed of primaseand
helicase
• DNA polymerase III molecules
• DNA gyrase, topoisomerase, SSB, ligase, initiator
proteins
•The replication fork moves in oppositedirection,
synthesizing both strands simultaneously.
9. DNA Replication in Bacteria
•To begin DNA replication, unwinding enzymes called
DNA helicases cause the two parent DNA strands to
unwind and separate from one another at the origin of
replication to form two "Y"-shaped replication forks.
•These replication forks are theactual siteof DNA
copying.
10. DNA Replication in Bacteria
•Helix destabilizing proteins bind to the single-
stranded regions so the two strands do not rejoin.
•Enzymes called topoisimerases produce breaks in
the DNA and then rejoin them in order to relieve the
stress in the helical moleculeduring replication.
11. Model for the formation of a replication bubble at a replication origin in
E. coli and the initiation of the new DNA strand
12.
13. Model for the “replication machine,” or replisome, the complex of key
replication proteins, with the DNA at the replication fork
14. •Primase binds to the first priming sequence on the leading strand
template and synthesizes a short RNA primer that is
complementary to the DNA template.
DNA Polymerase III uses the primer to initiate DNA synthesis by
adding deoxyribonucleotides to its 3’ end. The leading strand
requires only one priming event, because DNA synthesis is
continuous thereafter, in the 5’ 3’direction.
Elongation
15. DNA Replication in Bacteria
•As the strands continue to unwind in both directions
around the entire DNA molecule, new complementary
strands are produced by the hydrogen bonding of free
DNA nucleotides with those on each parent strand
•As the new nucleotides line up opposite each parent strand
by hydrogen bonding, enzymes called DNA polymerases
join the nucleotides by way of phosphodiester bonds.
17. DNA Replication in Bacteria
•DNA polymerase enzymes are only able to join the
phosphategroupat the 5' carbonof a new nucleotide
to the hydroxyl (OH) group of the 3' carbon of a
nucleotidealready in thechain.
•As a result, DNA can only be synthesized in a 5' to 3'
direction while copying a parent strand running in a 3' to
5' direction.
18. DNA Replication in Bacteria
DNA polymeraseenzymescannot begina new DNAchain from
scratch.
•Itcan onlyattach new nucleotidesonto 3' OH groupof a nucleotide
in a preexisting strand.
•Tostart the synthesisof the leading strand and each DNA fragment
of the lagging strand, an RNA polymerase complex called a
primosomeorprimase is required.
•The primase is capableof joining RNA nucleotideswithoutrequiring
a preexisting strand of nucleic acid - forms what is called an RNA
primer .
20. DNA Replication in Bacteria
•Aftera few nucleotides areadded, primase is replaced by
DNA polymerase.
•DNA polymerasecan now add nucleotides to the 3' end of
theshort RNA primer.
21. DNA Replication in Bacteria
•The two strands areantiparallel –
•one parent strand - theone running 3' to 5' iscalled the
leading strand can be copied directly down its entire
length .
•theotherparentstrand - theone running 5' to 3' is
called the lagging strand must be copied
discontinuously in short fragmentscalled as Okazaki
fragments.
•Okazaki fragments areof around 100-1000
nucleotideseach as the DNA unwinds.
22. •For the lagging strand, DNA synthesis isdiscontinuous
and requires a series of RNA primers. DNA is
synthesised at the 3’ end of each primer, generating an
okazaki fragment thatgrows untill it meets theadjacent
fragments. The RNA primer is then removed by the 5’ to
3’ exonuclease activityof DNA polymerase I and
replaced with DNA by the polymerase activity of the
sameenzyme.
23. DNA ligase links together adjacent Okazaki fragments with
covalent, phosphodiester bonds.
It joins the 5’ phosphate of one DNA molecule to the 3’ OH of
another, using energy in the form of NAD (prokaryotes) or ATP
(eukaryotes). It prefers substrates that are double-stranded, with
only one strand needing ligation, and lacking gaps.
