Cell Cycle And Its Regulation

 

A cell cycle is a series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division.

The two main events occurs in each cell division. The 1^st is the doubling of genome in synthesis phase also called s phase and the 2^nd is halving of genome during mitosis phase.

The eukaryotic cell cycle complete in two phases –the interphase and M phase.

Interphase is the preparatory phase of cell cycle which leads to doubling the genome.

The interphase is further divided into G1 or post mitotic gap phase, s phase or synthesis phase and G2 or pre-mitotic gap phase.

 

M phase which is a real phase of division is itself composed of two tightly coupled processes: karyokinesis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm divides forming two daughter cells.

The mitotic phase is divided into prophase, metaphase, anaphase and telophase. In M phase the genome becomes half.

Now let’s see in detail the different phase of cell cycle

In G1 phase the different enzymes which is needed for the DNA replication are synthesized as well as ATP and RNA molecule also synthesize for transcription and translation process.

So many thing are synthesized in G1 phase so the size of the cell increases.

In G₁ phase, a cell has three options.

Either

• To continue cell cycle and enter S phase
• Stop cell cycle and enter G₀ phase for undergoing differentiation.
• Become arrested in G₁ phase hence it may enter G₀ phase or re-enter cell cycle.

Cells of liver kidney and neuron after remaining in G1 phase for some time, come out of the cell cycle and enter to G0 phase also known as quiescent phase.

In quiescent phase the cell division stop but the other activity within the cells remain continue.

 

After the completion of G1 phase the cell cycle transits into S phase.

DNA replication is the main events that occur during S phase, some kind of histone protein is also synthesize in S phase.  Usually cell takes 5-6 hrs. to complete s phase.

After s phase the cell inter into G2 phase which is shorter lasting only 3-4 hrs.

In G2 phase the following events occur-

1.     The tubulin protein synthesize which is required for spindle formation.

2.     For plasma membrane formation certain protein are synthesized.

3.     ATP synthesis and RNA synthesis also occur during G2 phase.

The next stage is M phase. M stands for mitosis. This is where the cell actually partitions the two copies of the genetic material into the two daughter cells.

In prophase of mitosis chromatid coiling, spindle formation and disintegration of nuclear membrane occurs.

In metaphase the chromosome oriented at equatorial plane.

In anaphase the sister chromatid separate from each other and moves towards the opposite poles.

And In telophase which is the last phase of mitosis, the chromosome reach the poles and nuclear membrane reforms.

Telophase is followed by cytokinesis or the division of the cytoplasm into two daughter cells.

Check Point in Cell cycle

Activation of each phase of cell cycle is dependent on the proper progression and completion of the previous one.

The deciding point is called check point . The check point ensure the proper progression of cell cycle.

The whole cell cycle is controlled by the three check point which are-

The G1 check point at G1/S transition.

The G2 checkpoint at G2/M transition

And the M checkpoint or spindle checkpoint at the transition from metaphase to anaphase

The G1 checkpoint is the main decision point for a cell where it decide whether the cell will divide or remain suspended.

The G1 check point check all the preparation needed for a cell to enter into s phase.

The next check point is G2 check point before M phase which ensure the Proper DNA replication, and check whether M phase cyclin and cdk complex is activated to initiate mitosis.

The third check point is at the transition of Metaphase and anaphase, here the check point examine whether all the sister chromatids are correctly attached to the spindle microtubule or not.

Regulation of cell cycle

In addition to the check point there are two groups of intracellular molecules that regulates cell cycle. The two groups of proteins, called cyclins and cyclin dependent kinase are responsible for the progress of cell through the various checkpoints.