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The cell cycle
13
...
The cell
cycle is a complex process composed of an interphase followed by mitosis
and cytokinesis
...
3
...
13
...
This is the longest and most active stage of a cell’s lifetime
during which the cell grows then carries out preparation for mitosis
(cell division) (G1 – first growth phase)
...
The cell’s DNA is shared between two daughter cells
(k Fig
...
8)
...
The cell membrane constricts around its middle and
pinches off the two new daughter cells which contain identical genetic
information
...
A cell that is cultured in laboratory conditions can
undergo about 80 cell cycles before it dies, but in physiological contexts,
the cell’s function and environment determine how often it divides
...
a period of quiescence
G1/S CHECKPOINT
preparation for DNA
replication can
pause, continue or
be directed toward
cell death (apoptosis)
G0
differentiation
G1
apoptosis (death)
cytokinesis
metabolism
and growth
phase
M
synthesis of
DNA and
replication of
chromosomes
MITOSIS
S
preparation
for mitosis
G2
G2/S CHECKPOINT
Fig
...
4 Cell cycle
...
3
...
These are decision points in the cell cycle where its internal state is
examined
...
The checking processes exist as a safeguard because each cell must be able
to deal with DNA damage in a way that ensures that mutations – changes in
DNA sequence – are not passed on to their daughter cells
...
• If conditions at G1 are favourable, including cell size, nutrient availability,
growth factors and the DNA has integrity (is undamaged), the cell
progresses to the S phase, which is the next stage of the cycle
...
g
...
S phase
G
During the S phase, the cell’s DNA is replicated (copied) and the cell
becomes irreversibly committed to producing daughter cells
...
This
enables DNA to pass from one cell to its daughter cells and from one
generation to the next (k Fig
...
5)
...
13
...
Control by enzymes
The process of DNA replication is controlled by enzymes, e
...
polymerase
and telomerase
...
If there is an
error, DNA polymerase is able to cut out the wrong nucleotide and replace it
with a correct one (k Box 13
...
Telomerase is the enzyme which counteracts the shortening of telomeres
and may even lengthen them
...
It consists of a sequence of bases – TTAGGG –
that is usually repeated about 3000 times
...
3)
...
If the cell can repair any damaged DNA,
it will be allowed to progress to cell division by mitosis
...
These examination and
repair steps are important, because if irreparable damage to DNA is detected
at either checkpoint, the cell should restrict division and arrest the cycle
(k Box 13
...
Apoptosis
Apoptosis (programmed cell death) is a sequence of events that eliminates
cells without releasing harmful substances that destroy surrounding tissues
...
About one million
cells are produced each second, and about the same number are removed by
apoptosis
...
Cancer develops
when damaged cells contain mutated genes that either accelerate cell
division or prevent apoptosis
...
10)
...
2 In the laboratory, sections
of DNA can be made using a
polymerase chain reaction (PCR),
which is key to DNA sequencing,
genetic recombination and cloning
investigations
...
3 The amount of the enzyme
telomerase in cells normally falls as
people age – except in germ cells
...
Box 13
...
Many of the genes and
proteins that influence the phases
of the cell cycle are now identified
...
10
...