DNA replication is the process by which a cell copies its DNA in preparation for cell division. It occurs during the S phase of the cell cycle and involves unwinding the double helix and using each strand as a template to produce two identical copies of the DNA. The process is semiconservative, meaning each new DNA molecule contains one old and one new strand. Key enzymes involved include DNA polymerase, helicase, primase, and ligase. Replication initiates at origins of replication and proceeds bidirectionally.

1. DNA REPLICATION
KRVS Chaitanya

2. Brief Intro

4. ◦Copying genetic information for transmission to the next
generation
◦Occurs in S phase of 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 from the medium will form base pairing via
hydrogen bonding.
◦ Enzymes link the aligned nucleotides by bonds to form a
continuous strand.

5. ◦Semiconservative –- expt one strand from parent in each new
strand
◦Conservative-– both strands from parent and other is all new
strands

6. ◦Complementary base pairing produces replication––
◦Double helix unwinds Each strand acts as template
semiconservative
◦Complementary base pairing ensures that T signals
addition of A on new strand, and G signals addition
of CT wo daughter helices produced after
replication

8. semiconservative three possible models •
1. Semiconservative replication Watson and Crick model
2.Conservative replication:–– The parental double helix
remains intact;replication both strands of the daughter
double helix are newly synthesized
3.Dispersive replication:At completion, both strands of
both double helices contain both original and newly
synthesized material.

10. The mechanism of DNA replication
Tightly controlled process,
1. occurs at specific times during the cell cycle.
Requires:
1. a set of proteins and enzymes.
2. requires energy in the form of Two basic
steps:InitiationElongation.
Two basic components:
1. Template
2. primer.

11. The mechanism of DNA replication (prokaryotic)
DNA polymerase–– the enzyme that extends the primer;
1. Pol III - produces new stands of complementary DNA
2. Pol I-- fills in gaps between newly synthesized Okazaki
segments
Additional enzymes/proteins)
1. DNA helicase :: unwinds double helix
2. Single-stranded binding proteins :: keep helix open
3. Primase :: creates RNA primers to initiate synthesis
4. Ligase :: welds together Okazaki fragments

13. Origins of Replication
1. Replication proceeds in both directions (bidirectionally)
from a single origin of replication on the prokaryotic
circular chromosome
2. Replication ( proceeds in both directions bidirectionally )
from hundreds or thousands of origins of replication on
each of the linear eukaryotic chromosomes.

16. Replication Initiation
◦DNA origin of replication
◦Initiator proteins Bind
◦Recruits DNA helicase
◦Opening of DNA strands

18. Replication Termination
The ends of chromosomes (telomeres) cannot be replicated on the lagging strand
because there is no primer available.
◦ Telomerases–form a sort of single stranded cap around the chromosome ends to
protect them from being degraded
◦ chromosome ends are progressively shortened with each round of replication. “
◦ old cells with shortened telomeres undergo apoptosis
◦ Protective for normal cells
◦ Kill the old and possibly mutated
◦ Telomerase is over expressed in cancer cells
◦ Hypothesis is that cancer cells do not undergo apoptosis because their
telomeres do not shorten over time.

19. The problem of replicating completely a linear
chromosome in eukaryotes

20. Replicating the Ends of Chromosomes
1. Telomerase adds an RNA primer complementary to
telomere sequences
--chromosomal replication proceeds by adding to the
3 end of the primer
2. Fills the gap left behind by replication
3. Telomerase enzyme can also add DNA basepairs to
TEMPLATE DNA’
--complementary to the RNA primer

22. Replication at the chromosomal level
1.Replication is bidirectional.
2.For circular DNA(and linear chromosomes)
--the unwinding at the replication forks causes supercoiling
3.DNA topoisomerases .
◦Enzymes that help relax the DNA by nicking the strands
◦Releasing the twists
◦Then rejoining the DNA ends.
◦Example is DNA gyrase

25. Assembling Newly Replicated DNA into
Nucleosomes
1. When eukaryotic DNA is replicated, it complexes
with histones .
2. This requires synthesis of histone proteins and assembly
of new nucleosomes
3. Transcription of histone . and genes is initiated near
the end of G1 phase, and translation of histone
proteins occurs throughout S phase.

26. The Assembly of Replication Nucleosomes after

27. Thank You