initiation of replication of dna

1. INITIATION OF REPLICATION IN
EUKARYOTES
MADE BY:- MANISHA
1373

2. REPLICATION
It is a process in which the DNA copies
itself to produce identical daughter
molecules of DNA.
It occurs only once in each cell.
It occurs very quick, accurate and at
correct time.
Replication of DNA occurs based on the
Chargaff’s Rule that is
Cytosine – Guanine ( 3 H bonds)
Adenine – Thymine (2 H bonds)

3. SEMI-CONSERVATION OF REPLICATION OF DNA

4. MESELSON AND STAHL EXPERIMENT

5. SIMPLE PROCESS
G 1
S
G 2
Replication
Initiated
• DNA synthesis
occurs
• Two daughter
copies are produced
• Repair mechanisms
occur
Finally, one copy of the genomes is
segregated into each daughter cell at
the mitosis or M phase.
These daughter copies each contain one
strand from the parental duplex DNA and
one nascent anti-parallel strand.
This process is conserved from
prokaryotes to eukaryotes and the
mechanism is called the semi-
conservative mode of replication.

6. Complex Process
DNA replication in eukaryotes is divided into three stages
1. Initiation ( Formation of Pre – Replicative Complex)
2. Initiation complex
3. Elongation (Replication fork and Polymerization)
4. Termination

7. Initiation of Replication
It is the first step in eukaryotic replication in which most of the proteins
combine to form Pre – Replicative complex (Pre-RC).
Involved proteins
 Origin Recognition complex (ORC)
 Cell division cycle 6( Cdc 6)
 Chromatin licensing and DNA
Replication factor 1( Cdt 1)
 Minichromosome Maintenance Protein
Complex (Mcm 2-7)

8. STEPS IN INITIATION

10. 1. The activity of Cdt 1 during the cell cycle is regulated by a protein
called Geminin.
2. It also inhibits Cdt 1 activity during the S phase in order to prevent
the re-replication of DNA, Ubiquitination, and proteolysis.

11. FUNCTIONS OF MCM COMPLEX
1. Minichromosomal Maintenance Complex has helicase
activity and inactivation of any of the six proteins will prevent
the progress of the formation of the replication fork.
2. It also has ATPase activity. A mutation at any one of the Mcm
protein complexes will reduce the conserved ATP binding site.
3. Mcm complex is a hexamer with Mcm 3, Mcm 7, Mcm 2,
Mcm 6, Mcm 4, and Mcm 5.

12. 1. It is the 2nd stage in DNA replication where the Pre – Replicative
complex is converted into the Initiation complex.
2. Involved proteins
 Cell Division Cycle 45 ( Cdc 45)
 GINS
 Cyclin Dependent Kinase ( CDK)
 Dbf 4 Dependent Kinase (DDK) – Combination of Cdc 7 and
dbf 4
INITIATION COMPLEX

13. ⚫Cdc 45 protein is a compound that is needed for the conversion of Pre –
RC into an initiation complex.
⚫Its binds with chromatin after the beginning of initiation in the late G1
phase by being physically associated with Mcm 5.
⚫The binding of Cdc 45 is based on Clb – Cdc 28 as well as the function
of Cdc 6 and Mcm.
⚫GINS are essential for the interaction of Mcm and Cdc 45 at the Ori-c
site during initiation.
Steps In Initiation Complex

14. ⚫GINS complex is composed of four small proteins namely
 Sld5 (Cdc105)
 Psf1 (Cdc101)
 Psf2 (Cdc102)
 Psf3 (Cdc103)
GINS represents 'go, ichi, ni, san' which means '5, 1, 2, 3' in
Japanese.

15. ⚫At the onset of the S phase, the pre-replicative complex must be
activated by two S phase-specific kinases in order to form an initiation
complex at the origin of replication.
⚫ One kinase is the Cdc7-Dbf4 kinase called Dbf4-dependent kinase
(DDK) and the other is a cyclin-dependent kinase (CDK).
⚫The CDK-dependent phosphorylation of Cdc6 has been
considered to be required for entry into the S phase.
⚫DDK targets the Mcm complex, and its phosphorylation
leads to the possible activation of Mcm helicase activity.