Chapter 2: Cell division
Knowledge organiser
Make sure you can write a definition for these key terms.
adult stem cell binary fission cell cycle
chromosome clone daughter cells embryonic stem cell
gene meristem mitosis nucleus therapeutic cloning
Key terms
A stem cell is an undifferentiated cell that can develop into one or more types of specialised cell.
There are two types of stem cell in mammals: adult stem cells and embryonic stem cells.
Stem cells can be cloned to produce large numbers of identical cells.
Type of
stemcell
Where are
they found?
What can they
differentiate into? Advantages Disadvantages
adult stem
cells
specific
parts of
the body in
adults and
children – for
example,
bone marrow
can only
differentiate
to form certain
types of cells – for
example, stem cells
in bone marrow can
only differentiate
into types of blood
cell
●
fewer ethical issues – adults can
consent to have their stem cells
removed and used
●
an already established technique
for treating diseases such as
leukaemia
●
relatively safe to use as a
treatment and donors recover
quickly
●
requires a donor, potentially
meaning a long wait time to find
someone suitable
●
can only differentiate into
certain types of specialised
cells, so can be used to treat
fewer diseases
embryonic
stem cells
early human
embryos
(often taken
from spare
embryos
from fertility
clinics)
can differentiate
into any type of
specialised cell
in the body – for
example, a nerve cell
or a muscle cell
●
can treat a wide range of
diseases as can form any
specialised cell
●
may be possible to grow whole
replacement organs
●
usually no donor needed as
they are obtained from spare
embryos from fertility clinics
●
ethical issues as the embryo is
destroyed and each embryo is a
potential human life
●
risk of transferring viral
infections to the patient
●
newer treatment so relatively
under-researched – not yet
clear if they can cure as many
diseases as thought
plant
meristem
meristem
regions in
the roots
and shoots
of plants
can differentiate
into all cell types
– they can be used
to create clones of
whole plants
●
rare species of plants can be
cloned to prevent extinction
●
plants with desirable traits,
such as disease resistance,
can be cloned to produce large
numbers of identical plants
●
fast and low-cost production of
large numbers of plants
●
cloned plants are genetically
identical, so a whole crop is at
risk of being destroyed by a
single disease or genetic defect
The nucleus of a cell contains
chromosomes.
Each chromosome carries a
large number of genes made
of DNA molecules.
A
cell
DNA
gene
nucleus
nucleus
chromosome
each chromosome
carries a large
number of genes
in normal body cells
the chromosomes are
normally found in
pairs
chromosomes are made
of DNA molecules
the nucleus of a cell
contains chromosomes
T
T
A
A
C G
C
G
C
G
T
G
C
G
A T
C
C
G
Chromosomes Stem cells in medicine
Body cells divide to form two identical daughter cells by going through a series of
stages known as the cell cycle.
Cell division by mitosis is important for the growth and repair of cells, for example, the
replacement of skin cells. Mitosis is also used for asexual reproduction.
There are three main stages in the cell cycle:
stage 1
• cell grows bigger
• DNA replicates
(chromosomes are
duplicated)
stage 2 (mitosis)
• a complete set of chromosomes
is pulled to each end of the cell
• the nucleus divides to form
two nuclei
stage 3
cytoplasm and cell membrane
divide to form two identical
daughter cells
• increase in number of
sub-cellular structures,
such as ribosomes
and mitochondria
The cell cycle
Cell division in bacteria is called binary fission. In optimum
temperature and nutrients, bacteria can multiply as often
as every 20 minutes. In a lab, bacteria can be grown in
sterile conditions on an agar gel plate or in a nutrient broth.
The lid of the petri dish must be sealed but not all the
way so that oxygen can still get in. This is so that harmful
bacteria that do not need oxygen aren’t able to grow.
Binary fission
In therapeutic cloning
●
cells from a patient’s own body are used to create a cloned early embryo of themselves
●
stem cells from this embryo can be used for medical treatments and growing
new organs
●
these stem cells have the same genes as the patient, so are less likely to be rejected
when transplanted.
Therapeutic cloning
