DNA replication in eukaryotes is a complex process that involves multiple origins of replication, mainly characterized by leading and lagging strands, and occurs in three phases: initiation, elongation, and termination. Key proteins such as DNA polymerases α and δ play critical roles in synthesizing new DNA strands, while factors like the autonomous replication sequence (ARS) guide the process. Unique features such as telomeres and the activity of flap endonuclease-1 (Fen-1) also contribute to the fidelity and completeness of DNA replication.

1. DNA REPLICATION
IN EUKARYOTES

2. ❖WHAT IS DNA REPLICATION.
❖MECHANISM OF DNA REPLICATION IN EUKARYOTES.
❖DNA REPLICATION IN EUKARYOTES.

3. DNA REPLICATION :
▪ The process by which DNA molecule synthesizes it's own
identical copy (DNA TO DNA) is known as replication. The DNA
so formed is exactly similar to the parent DNA.
▪ Replication occurred inside the chromosome .the parent
DNA strand function as a template for the synthesis of new
DNA strand.the new DNA is formed in the semi-
conservative process .
• It has 2 strand ,one is lagging strand and another one
is leading strand .
• Lagging strand discontinuously synthesis and it is also know as
okazaki fragments.And leading strand continues synthesis of
deoxyribonucleotide.

4. ( FIG : Replication fork showing major event in DNA replication.)

5. REPLICATION:-
• Replication in eukaryots is more complex,than in prokaryotic.
• Replication In eukaryots like SACCAROMYCES cerevisiae.
• The origin of replication is known
as autonomous replication sequence (ARS), Which is found in
more than one number.
• Each being 200 base pair in length .
• Eukaryotes replication also comprises three phases, they are:-
➢Initiation.
➢Elongation.
➢Termination.

6. ◦ Contains multiple Origin of Replication called
Autonomously replicating Sequences or ARS.
◦ In Yeast ARS is 200 bp in length containing several sub
domains.
◦ ACS (A-T rich consensus sequence) domain of 11 bp long
is the core component in ARS.
◦ Second domain to 3’ end called Domain B is the site for
binding of initiator protein called ORC (Origin Recognition
Complex)
◦ Sequences 5’ to T rich strand of ARS is called Domain C.
Replication in Eukaryotes (ex. Yeast)
INITIATION:-

7. •The melting at ARS is introduced by DNA binding
Protein called ARS Binding Factor-I or ABF-1, which attaches to
Sub domain B3.
•The opening of DNA also requires replication factor-A or
eukaryotes SSB ,T-antigen protein and topoisomerase -I and II.
•The T-antigen by using it's DNA binding domain and in
presence of ATP,it cause local unwinding .
•Two DNA polymerases are required for replication i.e.
DNA pol α & DNA pol δ.
Replication in Eukaryotes

8. •Both polymerases are required for leading & lagging
strand synthesis.
•The primer synthesized by primase which is directly associated
with DNA polymerases α.
•DNA pol δ has 2 subunits & it depends on PCNA (proliferating
Cell Nuclear Anigen).
Replication in Eukaryotes

9. Replication in Eukaryotes
ELONGATION :-
• DNA pol α sythesizes only a short segment of DNA called i-
DNA & the RNA primer is extended by
DNA pol α upto short length & then DNA pol δtakes over DNA
synthesis on both strands due to high processivity.
• Polymerase α cooperates with Primase enzyme for attachment
of RNA primer of both the strands
• 8 components are involved in replication such as, T-
antigen, Replication Protein-A (Eukaryotic
SSB), Topoisomerase-I, Topoisomerase-II, DNA pol α ,
DNA pol δ ,PCNA or Cyclin, Replication Factor C.

10. •Primase associated with DNA pol α to synthesize
RNA primer. Once synthesiszed primer is extended by DNA
pol α upto 20 NTs & then get replaced by DNA
pol δ (called Switching)
•Loading of DNA pol δ is accompanied by RF-C at 3’
end along with PCNA
Replication in Eukaryotes

12.  Removal of primers are done by FEN-1 (Flap
Endonuclease-1) which is associated with DNA pol
δ at 3’ end & degrade the primer from 5’ end of
adjacent fragment .
 Rnase H degrade RNA part of a base paired RNA-Dna
hybrid, but it can’t cleave the phosphodiester bond
between the last rNT & 1st dNT. So this monophosphate
is cleaved by FEN-1.
 DNA pol α has no 3’-5’ proof reading activity &
therefore DNA synthesis occurs with high error.
 Removal of Primer by FEN-1 is followed by resynthesis
of DNA by DNA pol δ with proof reading activity.
Removal of Primers

13. The 'flap endonuclease' FEN1 cannot initiate primer
degradation
because its activity is blocked by the triphosphate group
present at the 5 end of
the primer.

16. • DNA pol δ causes accurate copy synthesis of template strands.
• And after the DNA polymerase delta add the final deoxyribonucleotide ,in
the gap left by the excised primer was filled by DNA ligase.
• Termination: -
• No sequence equivalent to Ter site is known in eukaryotes. In eukaryotes
DNA polymerase cannot replicate the terminal DNA segment of lagging
strand of linear chromosomes.
• The terminal region of DNA is know as telomer .Telomers have unique
features and and enzymes that facilitates rereplication.
• But in case of linear ds DNA telomerase homologous ter-protein helps to
restore the original site of eukaryotes bt not in case of circular DNA.

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