how DNA replication takes place its steps ..

1. DNA Replication
2015
AyeshaArshad
5/17/2015

2. DNA replication:
• Copying genetic information for transmission to the next generation.
• Occurs in S phaseof cell cycle.
• Process of DNA duplicating itself.
• Begins with the unwinding of the double helix to expose the bases in each
strand of DNA.
• Each unpaired nucleotide will attract a complementary nucleotide fromthe
medium (will form basepairing via hydrogen bonding).
• Enzymes link the aligned nucleotides by phosphodiester bonds to forma
continuous strand.
– Three possible models:
• Semi conservative replication: Watson and Crick model
• Conservative replication: Theparental double helix remains intact and both
strands of the daughter double helix are newly synthesized.
• Dispersive replication: Atcompletion, both strands of both double helices
contain both original and newly synthesized material.

3. The mechanism of DNA replication:
• Tightly controlled process occurs atspecific times during the cell cycle.
• Itrequires a set of proteins and enzymes and requires energy in the formof
ATP.
• Two basic steps:
– Initiation
– Elongation
• Two basic components:
– Template
– Primer
The mechanism of DNA replication (prokaryotic):
• DNA polymerase: Theenzymethat extends the primer.
– Pol III: Produces new stands of complementary DNA.
– Pol I: Fills in gaps between newly synthesized Okazakisegments
• Additional enzymes/proteins:
– i) DNA helicase:unwinds doublehelix
– ii) Single-strandedbinding proteins: keep helix open
– iii) Primase: creates RNA primers to initiate synthesis
– iv) Ligase: welds together Okazakifragments

4. Origins of Replication:
• Replication proceeds in both directions (bidirectionally) from a single origin of
replication on the prokaryotic circular chromosome.
• Replication proceeds in both directions (bidirectionally) fromhundreds or
thousands of origins of replication on each of the linear eukaryotic chromosomes.
Eukaryotic Origins of Replication:
•Replication Initiation:
•Primaseand the RNA Primer
•Replication Elongation:
•DNA Pol III
•Must have 3’ to add to
•Replication is finished:
•DNA Pol I removes primer
•Fills gap using 3’ends
•DNA Ligase connects frags
•Uses 5’ ends!

5. Replication fork:
The replication fork is a structurethat forms within the nucleus during DNA
replication. Itis created by helicases, which break the hydrogen bonds holding the
two DNA strands together. The resulting structurehas two branching "prongs",
each one made up of a single strand of DNA. These two strands serveas the
template for the leading and lagging strands, which will be created as DNA
polymerasematches complementary nucleotides to the templates; the templates
may be properly referred to as the leading strand template and the lagging strand
template.
DNA is always synthesizedinthe 5' to3' direction. Sincethe leading and lagging
strand templates are oriented in opposite directions at the replication fork, a
major issueis how to achieve synthesis of nascent(new) lagging strand DNA,
whosedirection of synthesis is oppositeto the direction of the growing
replication fork.