replication and cell cycle

Cellular replication is an essential process by which a single-celled organism develops into a multicelluar being. Replication also allows the body to renew itself. The cellular cycle of eukaryotic cells consists of four phases: G1 phase, S phase, G2 phase and M phase. Cells increase in size during G1 phase, which is followed by DNA replication in S phase. Protein synthesis and the production of microtubules occurs during G2 phase, which is then followed by mitosis. Mitosis is the briefest phase of the cell cycle, however is extremely important. During this period, nuclear and cytoplasmic division occurs, concluding with cyotkenesis. When cells are not actively dividing they are considered to be in a state of quiescence, known as the G0 phase. The passage of a cell through the cell cycle is controlled by various cytoplasmic proteins in order to assure quality control.
Eukaryotic DNA replication is regulated to ensure all chromosomes replicate once and only once per cell cycle. Replication begins at many origins scattered along each chromosome. Except for budding yeast, origins are not defined DNA sequences and probably are inherited by epigenetic mechanisms. Initiation at origins occurs throughout the S phase according to a temporal program that is important in regulating gene expression during development. Most replication proteins are conserved in evolution in eukaryotes and archaea, but not in bacteria. However, the mechanism of initiation is conserved and consists of origin recognition, assembly of pre-replication (pre-RC) initiative complexes, helicase activation, and replisome loading. Cell cycle regulation by protein phosphorylation ensures that pre-RC assembly can only occur in G1 phase, whereas helicase activation and loading can only occur in S phase. Checkpoint regulation maintains high fidelity by stabilizing replication forks and preventing cell cycle progression during replication stress or damage.
In all cells studied, DNA replication is regulated by recruiting the replication machinery or “replisome” to sites called origins on the chromosome. The replisome is a molecular machine that replicates the DNA bidirectionally from origins in a semiconservative fashion. The recruitment process is called initiation, whereas subsequent replication of the DNA by the replisome is called elongation. It is initiation and hence the recruitment process that is the site of regulation. When a change in replication rate is needed, adjustments are made to initiation. For example, the rate of DNA replication is almost two orders of magnitude faster in embryos than in somatic cells because more origins are used, resulting in more initiations. This review focuses on the important molecules used in the initiation process in the context of the cell cycle.

1. RELATIONSHIP BETWEEN
REPLICATION AND CELL
CYCLE

2. Content :-
■DNA Replication
■Facts About Initiation
■Cell Cycle
■Initiation Of DNA Replication In Eukaryotes
■Conclusion
■Reference

3. DNA Replication :-
■DNA replication is the process by which a double-stranded DNA molecule is copied
to produce two identical DNA molecules.
■Replication is an essential process because, whenever a cell divides, the two new
daughter cells must contain the same genetic information, or DNA, as the parent
cell.
■DNA replication is a semiconservative method.

4. Facts About The Initiation :-
■We all know that the process of the replication is completed in three steps –Initiation ,
Elongation and Termination.
■Here, the initiation is the very first and the most important step. Because once the
initiation of the dna replication starts it will end to the termination without any stopage.
■So, this process of initiation is quite different while we talk about the eukaryotes and the
prokaryotes.
■In Prokaryotes:-The replication takes place in three steps –Initiation , Elongation and
Termination. So, the whole process of the replication occurs in the S phase of the cell
cycle.
■In Eukaryotes:-The replication takes place in four steps –Pre-initiation , Initiation ,
Elongation and Termination. Where the pre-initiation step takes place in the G1 phase of
the cell cycle while the rest of the steps gets completed in the S phase of the cell cycle.

5. Cell cycle :-
■The cell cycle is an ordered series of events involving cell growth and cell division
that produces two new daughter cells. Cells on the path to cell division proceed
through a series of precisely timed and carefully regu lated stages of growth, DNA
replica tion, and division that produce two genetically identical cells.
■The cell cycle has two major phases: interphase and the mitotic phase. Duringg
interphase, the cell grows and DNA is replicated. During the mitotic phase, the
replicated DNA and cytoplasmic contents are separated and the cell divides

7. ■Initiation process of the DNA Replication In Eukaryotes is completed in
two steps –
1.Pre-initiation
2.Initiation
Initiation Of DNA Replication In
Eukaryotes:-

8. Pre-initiation :-
■In this step our pre-replication complex (pre-RC) forms and the process occurs in the G1 phase of
the cell cycle.
■In the eukaryotes there are several ORI regions.
■ORI region is where our replication starts.
■It is AT rich region. Like in Yeast ARS , it is called Autonomously Replicating Site.
■So, first at the ORI region ORC (Origin of Replication Complex) binds.
■After this two other proteins called CDC6 and CDT1 binds on the other side of the ORC.
■After their addition MCM7 protein (which functions as helicase in eukaryotes) binds to the ORC.
■So, now the complex formed is called as pre-RC (pre-replication complex) and upto this the process
occurs in the G1 phase.
■In the G1 phase the MCM7 will not show its helicase activity , it will show its activity after its
transition from G1 to S phase.

10. Initiation:-
■As S phase is the actual replicative phase so now the initiation will start from the S phase.
■At this stage the MCM-7 becomes active and will show its helicase activity.
■The transition of MCM-7 from inactive to active is done by kinase enzymes which are CDK and DDK
here.
■The CDK will phosphorylate CDC6 protein and mark it for degradation.
■The DDK will phosphorylate MCM-7.
■The phosphorylation of ORC will also leads to its inactivation.
■And the CDT1 us inhibited by Germinin protein.
■The phosphorylation of the MCM-7 leads to the addition of two other proteins named as CDC45
and GINS.
■Now there will be a formation of CMG Assembly (CDC45-MCM7-GINS). This assembly will now
perform the helicase activity.

11. ■Now the CMG Assembly will now moves the DNA and melts the ds-DNA to ssDNA by breaking H-
bonds between two strands inorder to provide room for polymerase enzyme and other enzymes.
■Formation of CMG Assembly also recruits the MCM-10 and CTF4 proteins to the DNA.
■Then these two proteins further recruits RPA , Polymerase-alpha proteins and primase
■Polymerase-alpha and primase are always associated with eachother.
■RPA protein binds to both the strands and prevents reannealing.
■Now the primase will add one primer to the leading strand and many to the lagging strand , as
the lagging strand the DNA synthesis is always discontinuous.
■And after this Polymerase-alpha will show its activity and will add few nucleotides to the primer.
■The polymerase-delta will then add nucleotides to the strands and the process of elongation
continues.

14. Conclusion:-
■In Prokaryotes :-The whole replication process ( Initiation , Elongation and
Termination ) takes place in the S phase of the cell cycle.
■In Eukaryotes :-The Pre-initiation step i.e the formation of pre-replication complex
takes place in the G1 phase of the cell cycle while the rest of the steps ( Initiation ,
Elongation and Termination) takes place in the S phase of the cell cycle.
■This is the basic relationship present their in between the DNA Replication and the
Cell Cycle.

15. Reference :-
■Cell and Molecular Biology (P.K Gupta )
■https://youtu.be/lkLAdgLbm-Y
■https://youtu.be/IwTfWo0Ms4

16. THANKYOU!!