Download 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. Figure Figure 1 provides a brief overview of what takes place
during each of the key events of the cell cycle.
Figure 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
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: G1, G2 and S phases.
Interesting 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 in G0. G0 is not a stage of the cell cycle.
Interesting 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.
G1 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.
G2phase: 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 (G1, S and G2 phases) accounts for approximately 90% of the cell cycle, with
the other 10% being taken up by mitosis.
Mitotic Phase
The mitotic phase (M phase) is composed of two tightly coupled
processes: mitosis and cytokinesis. During mitosis the chromosomes in the cell nucleus
separate into two identical sets in two nuclei. This is followed by cytokinesisin 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 of cytokinesis:
1. prophase
2. metaphase
3. anaphase
4. telophase
5. cytokinesis
1. Prophase
During prophase, the chromatin material shortens and thickens into individual
chromosomes which are visible under the light microscope. Each chromosome consist of
two strands or chromatids joined by a centromere (FigureFigure 2 ).
Figure 2: Chromosome structure showing (1)
Chromatid, (2) Centromere, (3) Short and (4)
Long arms of chromosome.
Interesting Fact:
Human cells have 46 chromosomes. (23 from the mother and 23 from the father).
As prophase progresses, the nuclear membrane and nucleolus disintegrates. In animal cells
the centriolesseparate and move to opposite poles. The centrioles give rise to the spindle
fibres which form between the poles. In plant cells there are no centrioles to move to the
poles, so spindle fibres form in the cytoplasm.
Schematic diagram: animal cell
Micrograph: plant cell
Figure 4
Figure 3
Table 1
2. Metaphase
During metaphase, chromosomes line up on the equator of the cell. The chromosomes
appear in a straight line across the middle of the cell. Each chromosome is attached to the
spindle fibres by its centromere.
Interesting Fact:
HINT: The stages of the cell cycle (interphase, prophase, metaphase, anaphase, telophase)
can be remembered by using the mnemonic IPMAT.
Schematic diagram: animal cell
Micrograph: plant cell
Figure 5
Figure 6
Table 2
3. Anaphase
During anaphase the chromatids are pulled to opposite poles of the cell by the shortening of
the spindle fibres. The chromatids are now called daughter chromosomes.
Schematic diagram: animal cell
Micrograph: plant cell
Figure 7
Figure 8
Table 3
Interesting Fact:
In plant cells there are no centrioles to move to the poles, so spindle fibres form in the
cytoplasm.
4. Telophase
During telophase, a nuclear membrane reforms around the daughter chromosomes that
have gathered at each of the poles. The daughter chromosomes uncoil to form chromatin
once again. The nuclear membrane reforms.
Schematic diagram: animal cell
Micrograph: plant cell
Figure 9
Figure 10
Table 4
5. Cytokinesis
The cytoplasm then divides during a process called cytokinesis. Cytokinesis is not a stage
of mitosis but the process of the cytoplasm splitting into two. In an animal cell the cell
membrane constricts. This invagination or in-folding of the cytoplasm divides the cell in two.
In a plant cell a cross wall is formed by the cell plate dividing the cytoplasm in two.
Schematic diagram: animal cell
Micrograph
Figure 11
Figure 12
Figure 13
Table 5
There are now two genetically identical daughter cells which are identical to the parent cell
and to each other.