Biology » Introducing the Cell » The Cell Cycle

# Introducing the Cell Cycle

## The Cell Cycle

The cell cycle is the series of events that takes place in a cell that results in DNA replication and cell division. There are two main stages in the cell cycle. The first stage is interphase during which the cell grows and replicates its DNA. The second phase is the mitotic phase (M-Phase) during which the cell divides and transfers one copy of its DNA to two identical daughter cells. The figure below provides a brief overview of what takes place during each of the key events of the cell cycle.

## Interphase

Interphase is the longest phase of the cell cycle. During this phase the cell grows to its maximum size, performs its normal cellular functions, replicates its DNA, and prepares for cell division. This stage is divided into three parts: $$G_1$$, $$G_2$$ and $$S$$ phases.

### Note:

Some cells no longer need to divide and exit the cell cycle. These cells may exit the cell cycle permanently, such as neurons, or they may exit the cell cycle temporarily. These cells are said to be in $$G_0$$. $$G_0$$ is not a stage of the cell cycle.

### Fact:

In cells without a nucleus (prokaryotic cells e.g. bacteria), there are many copies of the DNA floating around the whole cell. The prokaryotic cell cycle occurs through a process termed binary fission. In cells with a nucleus (eukaryotes) all the DNA is inside the nucleus and so a more complicated cell cycle is required for replication.

$$G_1$$ phase: occurs just after the two daughter cells have split and the cells have only one copy of their DNA. Cells in this stage synthesise proteins and increase in size. Cells can remain in this stage for a long time.

$$S$$ phase: is the stage during which DNA replication occurs. The cell makes an identical copy of each of its chromosomes. Chromosomes are found inside the nucleus of the cell and consist of long strands of DNA that contain the genetic information of the cell.

$$G_2$$ phase: occurs after the DNA had been duplicated in S phase. During this phase the cell may continue to grow and undergo normal cellular functions. Towards the end of this phase the cell will start to replicate its organelles in preparation for mitosis.

Interphase ($$G_1$$, $$S$$ and $$G_2$$ phases) accounts for approximately 90% of the cell cycle, with the other 10% being taken up by mitosis.