Cambridge International A Level Biology Revision Guide

Chapter 5: The mitotic cell cycle
Mitosis
Mitosis is nuclear division that produces two genetically
identical daughter nuclei, each containing the same
number of chromosomes as the parent nucleus. Mitosis is
part of a precisely controlled process called the cell cycle.
The cell cycle
The cell cycle is the regular sequence of events that takes
place between one cell division and the next. It has three
phases, namely interphase, nuclear division and cell
division. These are shown in Figure 5.6.
S phase: DNA
replication
S
G 2
interphase
Figure 5.6 The mitotic cell cycle. DNA replication takes
place during interphase, the period between cell division
and the next nuclear division: S = synthesis (of DNA);
G = gap; M = mitosis.
G 1
M
nuclear division
by mitosis
cell division
(cytokinesis)
During interphase, the cell grows to its normal size
after cell division and carries out its normal functions,
synthesising many substances, especially proteins, in
the process. At some point during interphase, a signal
may be received that the cell should divide again. If this
happens, the DNA in the nucleus replicates so that each
chromosome consists of two identical chromatids. This
phase of the cell cycle is called the S phase – S stands for
synthesis (of DNA). This is a relatively short phase. The gap
after cell division and before S phase is called the G 1
phase
(G for gap). The gap after S phase and before cell division
is called the G 2
phase. Interphase therefore consists of G 1
,
S and G 2
. During G 1
, cells make the RNA, enzymes and
other proteins needed for growth. At the end of G 1
, the cell
becomes committed to dividing or not dividing.
During G 2
the cell continues to grow and new DNA is
checked and any errors are usually repaired. Preparations
are also made to begin the process of division. For
example, there is a sharp increase in production of the
protein tubulin which is needed to make microtubules for
the mitotic spindle.
Nuclear division follows interphase. This may be
referred to as the M phase (M for mitosis). Growth stops
temporarily during mitosis. After the M phase, when the
nucleus has divided into two, the whole cell divides to
create two genetically identical cells.
The length of the cell cycle is very variable, depending
on environmental conditions and cell type. On average,
root tip cells of onions divide once every 20 hours;
epithelial cells in the human intestine every 10 hours.
In animal cells, cell division involves constriction
of the cytoplasm between the two new nuclei, a process
called cytokinesis. In plant cells, it involves the formation
of a new cell wall between the two new nuclei.
Mitosis
The process of mitosis is best described by annotated
diagrams as shown in Figure 5.7. Although in reality the
process is continuous, it is usual to divide it into four main
stages for convenience, like four snapshots from a film.
The four stages are called prophase, metaphase, anaphase
and telophase.
Most nuclei contain many chromosomes, but the
diagrams in Figure 5.7 show a cell containing only four
chromosomes for convenience. Colours are used to show
whether the chromosomes are from the female or male
parent. An animal cell is used as an example.
The behaviour of chromosomes in plant cells is
identical to that in animal cells. However, plant cells do
not contain centrosomes and, after nuclear division, a
new cell wall must form between the daughter nuclei. It
is chromosome behaviour, though, that is of particular
interest. Figure 5.7 summarises the process of mitosis
diagrammatically. Figure 5.8 (animal) and Figure 5.9
(plant) show photographs of the process as seen with
a light microscope.
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