Question

19. What is the cell cycle control system? How does it function? What can happen if this system malfunctions?

Answer 1

Ans.

During the process of cell division it is very important that the new daughter cells produced are the exact copy of the parent cell.Error in duplication or the distribution of chromosomes will to lead to mutations that may get passed on to every new cell produced from an abnormal cells.To prevent this continuation of division of such compromised cells,there are certain control mechanism that takes place at the three main cell cycle checkpoint and this mechanism of control is what is called as the cell cycle control system.

Following are the three cell cycle checkpoints and the way it functions for controlling the cell cycle:

1. The G₁ Checkpoint - This checkpoint determines and confirm whether all conditions are in favour of the cell divsion to proceed.External influences like the growth factors play an important role in carrying the cell past G₁ checkpoint.Only if the cell is in appropriate size and has enough energy reserves,it pass the checkpoint.The cell is also checked for any DNA damage at this point and if the cell does not meet all the required conditions,it will not proceed to the S phase.In the event of not meeting the requirements,the cell cycle will get halted and the remedy for the problem is sorted out,or the cell cycle is advanced into G₀ phase (inactive phase) and wait to proceed further until the condition improves.When all the requirement are met,the cell will enter the S phase and DNA replication will start.This checkpoint transistion is signalled by cyclins and cyclin dependent kinases (CDKs),cyclins being the cell signalling molecules which regulates the cell cycel.
2. The G2 Checkpoint - This checkpoint controls the entry of cell cycle into the mitotic phase,if certain conditions are not met with.The G₂ Checkpoint works by ensuring that all the chromosomes are replicated and there is no damage in the replicated DNA.If it detects any problem,the cell cycle is halted and the cell try to complete the DNA replication or repair the damaged DNA.
3. The M Checkpoint - This chekpoint that takes place near the end of the metaphase stage is also called as the spindle checkpoint as its major role is to check whether all the sister chromatids are properly attached to the spindle.Since separation of chromatids during anaphase is an irreversible step,the cell cycle will not proceed unit it confirms a firm anchoring of kinetochores of each pair of sister chromatids to at least two spindle fibers that arise from the opposite poles of the cell.

Apart from the three internally controlled checkpoints,there are two class of intracellular molecules that regulates the cell cycle,that either allows the cell cycle to proceed to next phase,called as positive regulation or halt the cycle,called as negative regulation.These regulatory molecules may act individually or they can work by influencing the activity or the production of other regulator proteins.Thus,the failure of even a single regulator may have almost zero effect on the cell cycle.On the other hand,a non-functioning regular can have adverse effect on the cell cylce,which can possibly be fatal if multiple process are affected.

Effect of malfunctioning cell cycle control system: Continous division of abnormal cells can lead to cancer or tumors,which if not stopped can damage the tissues around them and further spread to other body parts as well,eventually leading to death.While a healthy and normal functioning cell cycle control system prevents this from happening,a malfunctioning control system can cause unwanted and excessive cell division that ultimately will lead to the formation of cancer cells.