# What is the cell cycle?

Feb 5, 2018

Cell cycle is a series of events operating towards cell division.

#### Explanation:

The cell cycle is divided into following phases:
1. ${G}_{1}$ phase (Cell prepares for DNA replication)
2. $S$ phase (Replication of DNA is achieved)
3. ${G}_{2}$ phase (Cell prepares for division)
4. $M$ phase (Cell actually divides and makes daughter cells)

Figure 1 A summary figure to describe the cell cycle

Each of the phases is described below in details:

${G}_{1}$ phase
It is the first step of the cells to start preparing for division. It is also called growth phase as cells start preparation for division. In this phase, the cell starts accumulating all the proteins required for its division and grows in size. The duration of this phase in the cell cycle is variable.

${G}_{1}$ phase also acts as the crossroads for the cells where it chooses any one of the following:
1. whether it wants to proceed to cell cycle S phase
2. enters a resting phase ${G}_{0}$
3. Stops replication and enters into differentiation to carry a specific role.

Once the decision is made to proceeds in the cell cycle, cell makes a transition to $S$ phase. If the cell decides to exit the cell cycle and enters in a resting phase the cells enters into ${G}_{0}$ phase. In ${G}_{0}$ phase, cell has the capability to enter into cell cycle later as when it is required.

$S$ phase
It is the second phase in the cell cycle where whole genome of the cell is replicated into two copies. There are proteins which do DNA replication for the cell. IT is a relatively longer phase of the cell cycle as whole genome present in the cell is replicated.
A point to remember here is that cell makes sure that the whole genome is replicated once and only for one time.

During this phase itself the c ell scans the gemone for any abbreations or damages and corrects it , if possible, by staying in $S$ phase. Once the DNA replication is complete, the cell enters into ${G}_{2}$ phase.

${G}_{2}$ phase
In this second Growth phase the cell prepares itself to undergo mitosis in $M$ phase. The cell starts to rearrange its microtubules, which play crucial role in $M$ phase. After the preparation for entering into $M$ phase is complete the cell enters into next phase of the cell cycle.

$M$ phase
It is a very brief part of the cell cycle and in which the cell divides and makes two daughter cells. It is the phase where nuclear division happens (Karyokinesis). It has very strict regulation and very crisp phases.

Mitosis cycle is divided into following phases
1. All of the DNA condenses into Chromosomes and this pahse is called Prophase. Here cells nuclear membrane starts to disintergate.
2. During Metaphase using microtubules the cell aligns all its chromosomes in a common plane called metaphase plate.
3. As cell proceeds in Anaphase, cells separate sister chromatids present in the chromosomes and moves towards two opposite poles.
4. At Telophase, nuclear membrane starts to reappear and ingulf the nuclear material.
5. Finally during Cytokinesis, cell divides and forms daughter cells.