WHAT IS CELL CYCLE?

Before knowing about cell division we need to know about cell cycle. Cell cycle is very important to study as what happens before cell division and after cell division is a essential need. If in case the cell starts to divide before time then they might produce a large number of cells and that particular situation can be very dangerous for us, as it might cause tumor and can produce cancerous cells.

Cells need to divide very accurately and on time, that's why we need to study cell cycle and further cell divisions and cell division control.

THE CELL CYCLE IS USUALLY CALLED MITOTIC CELL CYCLE

CELL CYCLE

There are 2 kind of signals given during the cell cycle:

1. signals which will inform the cell to grow and divide.
2. signals which will pause or stop the cell division.

WHAT IS THE FIGURE TRYING TO SAY?

There are 2 phases in the cell cycle,

1. INTERPHASE - Cell prepares itself for cell division, checks whether the cell is ready to divide or not, checks the cell cytoplasm to cell nucleus ratio, checks if there is any kind of mutations or DNA damage.
2. M-PHASE - Cell division takes place.

Interphase is divided into 3 phases:

Stating with the G1 OR GAP 1 PHASE: Here the cell is doing preparations for getting divided, by preparing some accessory enzymes which will help in the synthesis or replication of DNA in the next phase. Checks the cell cytoplasm to cell nucleus ratio, as if the cell cytoplasm will grow big in comparison to cell nucleus then obviously the cell will divide to manage the situation.

 S PHASE OR SYNTHESIS PHASE: Here the DNA replication will take place, and by checking the defaults the cell will be moved to the next phase.

G2 PHASE O GAP PHASE: Makes proteins which will guide the next phases of cell cycle. Few RNA's are also synthesized in this phase which will further help in making proteins via translation.

M PHASE O MITOTIC PHASE: Here the actual cell division takes place by division of all the cell organelles, division of nucleus, division of chromosomes etc.

NOTE:

 After GAP 1 OR G1 phase there is a G0 phase where the cells are repaired if any kind of damage is there. Here the cell rest and make itself ready for the next phase.

There are few checkpoints in the cell cycle which either informs the cell to grow and divide or to pause the cell cycle and fix the problem else kill the cell (apoptosis - this is also called programmed cell death, happens when the problem cannot be fixed and if cell division occurs it can be dangerous for the individual).

There are mainly 3 checkpoints in the cell cycle:

1. G1 - S checkpoints: This lies between between G1 and S Phase of the cell cycle. It checks the cytoplasm to nucleus ratio. And see whether the enzymes for the replication of DNA is ready or not.
2. S - G2 Checkpoints: This lies between S and G2 phase of the cell cycle. it checks whether the replication done is proper or not, is there any kind of mutations or kind of DNA damage, if any kind of problems are there then the cell tries to fix them and if they are not able to fix, then apoptosis occurs.
3. SPINDLE checkpoints: This lies between the Metaphase and Anaphase of the M phase, here the checkpoints check whether the chromatids Aline at the center or not that is proper alignment and micotubules assembly.

There are few enzymes or protein which helps checkpoint to do there work properly.

• CYCLIN and CYCLIN DEPENDENT KINASE (CDK): Will give the signal to the cell to divide and move further, giving assurance that there is nothing wrong with the cell. There are 3 kind of cyclin present based on the 3 types of checkpoints. Each cyclin is specific for specific phase of the cell cycle. CDK will be initially inactive and it will be activated by binding upon cyclin and then the combination of cyclin-CDK will go and activate other proteins by phosphorylating (very important as it carries the cell to next level of cell division) them. Old cyclin are degraded when cell enters to the next phase of cell cycle.

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CYCLIN IS MORE OR LESS CONSTANT BUT CDK IS VERY UNSTABLE BECAUSE OLD CYCLIN ARE DEGRADED WHEN CELL ENTERS TO THE NEXT PHASE OF THE CELL CYCLE, AND CDK IS DEPENDENT UPON CYCLIN.

• p53, p21: This will give signal to pause the cell division or to kill the cell. Also known as cell cycle arrest. This will check if there is any DNA damage or mutation during DNA replication. p53 is already present but in inactive form and in a very less amount. CMK ( CYTIDINE MONOPHOSPHATE KINASE) gets the sense of the damage and this activates the p53 by phosphorylation. p53 activates some more proteins after phosphorylation and makes the cell die.