FACT
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 inG 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.Figure 4.1: The sequence of events in the cell leading to division of a cell into two daughter cells is
known as the cell cycle and is shown above.
Interphase DUMMY
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 andS phases.
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 itschromosomes. 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 andG 2 phases) accounts for approximately 90% of the cell cycle, with the
other 10% being taken up by mitosis.
Mitotic Phase DUMMY
The mitotic phase (M phase) is composed of two tightly coupled processes: mitosisand
cytokinesis. During mitosis the chromosomes in the cell nucleus separate into two identical
sets in two nuclei. This is followed bycytokinesisin which the cytoplasm, organelles and cell
membrane split into two cells containing roughly equal shares of these cellular components.
We will now describe what takes place during the stages of M-phase, which includes the
four broad phases of mitosis (prophase, metaphase, anaphase, telophase) and the fifth phase
Chapter 4. Cell division 99