Cell cycle can be defined as a ordered and highly regulated sequence of events required for cell division. The cell cycle can be divided into four phase for understanding purpose (as cell cycle is a continuous ongoing process) – Gap 1 (G1), synthesis (S), Gap 2 (G2) and mitosis (M), During these phases a cell grows and divides into daughter cells.
The important questions that comes to mind is that – How does the cell know when to divide? How does the cell decide to move from one phase of cell cycle to next? The answers to these questions can be deciphered by understanding the cell cycle regulation.
The work from Geneticists related to mutations that arrested the cell cycle at various points, allowed us to decipher that cell cycle follows a pathway or chain of orderly events. In this pathway, each step is dependent of the completion of earlier step.
Since cell division is crucial for survival of the organism, it becomes very important to have tight control or regulation on this process, so that the cell could detect and repair of any possible DNA damage and also prevent uncontrolled cell division events.
As mentioned earlier, cell cycle being a sequential process and it is impossible to reverse the cycle. So, It would be detrimental, if a cell with damaged DNA or an altered component of cell division pathway is allowed to divide or progress through the events of cell cycle.
Normal eukaryotic cells have some mechanisms in place termed as cell cycle regulation or control, where it checks for any irregulates in internal or external environment which might compromise regular cell cycle events and leads to formation of abnormal daughter cells. This internal cell cycle regulation will occur at at three main cell-cycle checkpoints.
A checkpoint is a time or stage in the eukaryotic cell cycle where the cells will decide whether or not to continue with the cell division, based on the external and internal factors.
At these checkpoints, upon receiving a positive signal (regulators), the cell can begin the next phase of the cell cycle. On the contrary, If the cells sense any unfavorable conditions (external or internal) they have the ability to halt the progression to the next phase of cell cycle until all the conditions are favorable.
These checkpoints and signals (regulators) allow the cell to complete the present phase before moving forward onto next phase of cell cycle.
There are many checkpoints in the eukaryotic cell cycle. But three are considered very important
These checkpoints are occur at the end of related phase
1) G1 checkpoint : at G1 to S transition
2) G2 checkpoint : at G2/M transition, and
3) During metaphase to anaphase transition
G1 checkpoint :
- At this checkpoint, the cells check if all internal and external factors are favorable for progressing to next phase (S phase) . Some important factors considered by the cells before giving a go ahead for S phase:
- Size: The cells will check if the size of the cell is large enough and has enough proteins required for division.
- Supply of nutrients: Does the cells have enough supply of nutrients required for next steps of cell cycle.
- DNA: Integrity of DNA is checked before replication in S phase
- Type of molecular signals : Depending upon the kind of molecular signal received by the cells. If a positive signal signal in the form of growth factors allows the cell to move to S phase .
- In the event of a negative signal, the cell can prolong the stay in G1 phase or stop the cycle and try to rectify the problem. The other option for the cell is that they can exit cell cycle and go into G0 or resting phase permanently or await further signals for re-entering G1 phase when conditions improve.
- Once the cell crosses this G1 checkpoint and enters S phase, it becomes irreversibly committed to division barring few exceptions such as DNA damage or replication errors.
- This the second important checkpoint before the cell enters mitosis. In G2 to M transition phase, cells will check the integrity of DNA and if all the genomic DNA has undergone replication.
- If the cell encounters any errors or damage in DNA replication process, the cell will pause at this checkpoint. This gives the cell the time required for repairs or completion of DNA replication. In case the DNA is damaged beyond repair, then cell will receive a signal signal for self destruction or called apoptosis.
- Apoptosis makes sure that the damaged DNA is not passed on to daughter cells and this is vital for preventing cancer.
- This checkpoint occurs near the end of the metaphase stage of mitosis.
- The cell will check if all the sister chromatids are correctly attached to the spindle microtubules and are under bipolar tension. Hence M checkpoint is also known as the spindle checkpoint,
- The cycle will pause at this checkpoint until all the chromosomes are attached to two spindle fibers from opposite poles of the cell.
- When all the above aspects are fulfilled, the cell divides and next cell cycle begins.
How to these checkpoints works or molecular mechanisms controlling checkpoints ?
We have just see that cells are regulated by internal (DNA damage etc.) or external factors (growth factors or other molecular signal) at different checkpoints, when it comes to cell division. So these checkpoints of cell cycle provide the cells the window to decide on whether to progress or not to the next phase of cell cycle depending on the many factors.
But what exactly these factors do to the cells which eventually helps the cell to decide about progressing, halting or resting phases at the various checkpoints of cell division?
The simple answer is that external or internal regulators activate different signaling pathways inside the cell, which eventually activate or repress protein or set of proteins that are important for progression or halting of different cell cycle events.
Cell Cycle regulators:
Depending upon the external and internal cues in the cell, different cell cycle regulators are turned on or off .
These cell cycle regulatory molecules either help in progress of the cell to the next phase (positive regulation) or halt the cycle (negative regulation).
Two groups of proteins – Cyclins and cyclin dependent kinases (CDKs) are two importan positive regulators of cell cycle.
Cyclins are among the most important core cell cycle regulators. In humans there are four different types:
G1 cyclins, G1/ cyclins, S cyclins and M cyclins. As suggested by their names, these particular cyclins helps in transition of respective phases. eg: S cyclins helps in transition from S to G2 phase.
The levels of the these four cyclin proteins varies during the cell cycle, depending on the cues from both internal and external factors. The concentration of particular cyclin increases dramatically depending upon its need at any given phase. Once the function of that cyclin is not required or cell moves to the next stage of the cell cycle, the cyclins that were active in the previous stage are degraded.
Cyclin-Dependent Kinases (CDKs):
- Cyclins regulate the cell cycle only when they are tightly bound to Cdks.
- CDK are enzymes that activate or inactivate other target molecules through phosphorylation, but they become activated only when bound by cyclins and this cyclin-cdk complex also needs to be phosphorylated for their proper function.
- Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these cell cycle regulators.
- Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins.
- Once phosphate group is attached to target protein, it can make the phosphorylated protein ether more active or less active.
- CDks bound by specific cyclins gains the ability to activate a set of target proteins required for successful progression into next phase eg: G1/S cyclins direct the Cdks to phosphorylate S phase targets genes (e.g., promoting DNA replication). Hence the proteins phosphorylated by Cdks are essential in advancing the cell to the next phase.
- CDKs phosphorylate their substrates on serines and threonines, so they are called serine-threonine kinases.
- The levels of CDKs usually doesn’t change much during cell cycle, but it the levels of cyclins which keep changing depending of the cell cycle phase. The levels of cyclin determines which cyclin-CDk complex is formed (eg: G1/ S cyclins levels are at maximum at G1 / S transition phase, S cyclins peak at S-G2 transition and M cyclins during M phase).
- The different cyclins and CDks can only bind at specific phases in the cell cycle and thus regulate different checkpoints.
Molecular mechanism underlying functions of Cyclin-CDKs complex:
- Once the cell receives a pro-mitotic extracellular signal, G1 cyclin-CDK bind to each other and becomes activated after getting phosphorylated.
- Once phosphorylated, the cyclin-CDK complex prepares the cell for S phase by activating the required expression of transcription factors.
- The transcription factors will bind to its targets genes such as S cyclins and of enzymes required for DNA replication in S phase.
- Later S cyclin-CDK complexes and Mitotic cyclin-CDK complexes are phosphorylated in sequence, which controls the activation of target genes required for respective phases and also downregulates the inhibitors of activators.
- Unlike the positive regulators of cell cycle which activate molecules to progress through the cycle, negative regulators halt the cell cycle.
- The best studied negative cell cycle regulators are retinoblastoma protein (Rb), p53, and p21.
- Rb, p53, and p21 act primarily at the G1 checkpoint.
- If cell encounters damaged DNA, P53 halts cell cycle and tries to repair by recruiting DNA repair enzymes.
- If the damage is beyond repair, P53 initiates apoptosis to prevent damaged DNA from getting transferred to daughter cells.
- The high levels of p53 also increases the levels of p21, which eventually halt cell cycle by binding to cyclin-CDK complex.
- High levels of p53 and p21 accumulate in stressed cell, ensures that the cell doesn’t progress into the S phase.
- Rb halts the cell cycle by binding directly to transcription factor E2F.
- When Rb is bound to E2F, production of proteins required for the G1/S transition is blocked.
- When cell receives positive signal for growth and cell cycle progression, Rbis phosphorylated.
- Once phosphorylated Rb is inactivated and releases the transcription factor, which is now free to turn on the genes requires for advancing of cell cycle.
- For smooth progression of cell cycle through all its checkpoints, all positive regulators must be “turned on” and all negative regulators must be “turned off.”
Image Credit : Biology Openstax