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Cell Division
All eukaryotic, multicellular organisms each develop from
a single original cell, i.e. the fertilised egg (zygote)
Repeated divisions of the zygote, by a process of cell division called
mitosis, give rise to all the cells that make up a multicellular organism
The fertilised egg
divides repeatedly
by mitosis
to initially produce a ball of
genetically identical cells
- the early embryo.
The function of mitosis is to produce new cells, each of which receives
a set of chromosomes identical to those of the original cell that gave
rise to them; genetically identical cells are the result of mitosis
Significance of Mitosis
Mitosis is involved in:
DEVELOPMENT – the formation of a multicellular organism from a
single fertilised egg
GROWTH – involves an increase in size through the division of cells by mitosis
CELL REPLACEMENT – many cells within a multicellular organism are
constantly dying and being replaced by mitosis
ASEXUAL REPRODUCTION – asexual reproduction in certain plants and
lower eukaryotic animals involves the process of mitosis
Budding in this species of Hydra is a form
of asexual reproduction; new individuals
are produced as outgrowths of the parent
by the process of mitosis
The epithelial cells lining the intestine are continually being replaced by mitosis
such that the entire lining is replaced every five days
Epithelial cells
The dead layers of cells at the surface of the skin are constantly being lost and
are replaced by the mitotic divisions of the cells in the layer beneath them
Dead cells
Constantly dividing cells
replacing those lost at the
surface
These nerve cells have lost the ability to undergo mitosis – once formed they
do not divide
The Process of Mitosis
maternal in
origin
paternal in
origin
centromere
This diagram represents a parent cell containing
a single pair of chromosomes – the pair of chromosomes are
the same size and shape and are therefore described as being
homologous chromosomes. One member of the pair was
donated by the male at reproduction and the other member
by the female. Each pair of homologous chromosomes thus has
one member that is paternal in origin and another member that
is maternal in origin. Humans possess 23 pairs of chromosomes
in each body cell. Cells in which the chromosomes occur in
homologous pairs are termed DIPLOID (two sets of chromosomes)
Each chromosome replicates itself to form a pair of identical
chromatids called sister chromatids that remain attached to
one another at a region called the centromere
The cell divides into two and each daughter cell receives
one of each of the sister chromatids
The daughter cells are genetically identical
to the parent cell
The function of mitosis is to construct an exact
copy of each chromosome and then to distribute,
through division of the parent cell, an identical
set of chromosomes to each of the
two daughter cells
The Process of Mitosis
Mitosis is a continuous process but for convenience of description is divided
into FOUR MAIN STAGES:
Prophase
Please
Metaphase
Make
Anaphase
Another
Telophase
Two
The following slides describe the process of mitosis in an animal cell containing
two pairs of homologous chromosomes
Prior to mitosis
the cell is in a
stage called
Interphase
During
interphase the
DNA of the
chromosomes
replicates and
new cell
organelles are
manufactured
Although the
chromosomes
have
duplicated, they
are loosely
coiled within
the nucleus
and visible only
as granular
material at this
stage
This granular
chromosome
material is
known as
chromatin
Interphase
Nucleolus
Two pairs of
homologous
chromosomes –
visible only as
granular
chromatin at
this stage
Nuclear
membrane
The centrioles replicate (animal cells only)
Transmission electron
micrograph showing
the granular chromatin
of an interphase nucleus
Interphase prepares the
cell for division and the
cell now enters the first
phase of mitosis known
as prophase
During early prophase the
chromosomes spiralise and
condense – they become
shorter and thicker and
visible as clear threads
The nucleolus
shrinks and
the centrioles
move to
opposite poles
of the cell
Spindle fibres
begin to
form in the
regions of
the centrioles
By late
prophase, the
chromosomes
have condensed
further and each
chromosome can
be seen to
consist of two
chromatids
joined at the
centromere
Early Prophase
The nucleolus
shrinks and
the centrioles
move to
opposite poles
of the cell
Spindle fibres
begin to
form in the
regions of
the centrioles
By late
prophase, the
chromosomes
have condensed
further and each
chromosome can
be seen to
consist of two
chromatids
joined at the
centromere
Late Prophase
Sister chromatids
Centromere
Each pair of
sister chromatids
is a pair of
duplicated
chromosomes
resulting from
the replication
of DNA that
occurred during
interphase
At the end of
prophase the
nuclear
membrane
begins to
disintegrate
and the cell
enters the
next phase
known as
metaphase
During metaphase, the spindle fibres grow across the cell and the
replicated chromosomes (pairs of chromatids) line up independently along
the equator of the spindle attaching to the spindle at their centromeres
As the cell
enters anaphase,
the centromeres
divide into
two, separating
the sister
chromatids of
each
chromosome
Metaphase
This electron micrograph shows
a metaphase chromosome about
to enter anaphase
Centromere
Sister chromatids
As the cell enters anaphase, the
divided centromeres repel one
another and the chromatids begin
to move apart
Spindle activity
pulls the
chromatids
apart and the
separated
chromatids
(know called
chromosomes)
move to
opposite poles
of the cell
Early Anaphase
Spindle activity
pulls the
chromatids
apart and the
separated
chromatids
(know called
chromosomes)
move to
opposite poles
of the cell
Anaphase
The chromosomes begin to
uncoil and a nuclear membrane
begins to form around each set
of chromosomes
Telophase
Cytokinesis (division of the
cytoplasm) now begins as the
cell membrane begins to constrict
towards the centre of the cell
The spindle
fibres begin to
disintegrate
A single chromatid from each chromosome has reached the poles of
the spindle - the chromatids are now described as chromosomes
Cytokinesis
As the membrane continues to constrict, the cytoplasm becomes completely
divided forming two genetically identical daughter cells
Each cell now possesses an exact copy of each chromosome that was present in
the nucleus of the original cell
In plant cells, constriction of the membrane at cytokinesis cannot occur due to
the presence of the cell wall
In plant cells, a cell membrane forms in the middle of the dividing cell and this
membrane secretes cell wall material on each side to form
new cell walls for the daughter cells
Summary of Mitosis
Interphase – cell prepares Early Prophase – chromosomes
Late Prophase –
spiralise and condense and
for division; DNA replicates,
chromosomes spiralise and
become visible as threads;
new organelles are
condense further and
manufactured and cell grows; centrioles move to opposite
can now be seen to consist
poles of the cell and spindle
chromosomes present as
of two sister chromatids
fibres begin to form
granular material (chromatin)
joined at the centromere
Metaphase –
Early Anaphase –
nuclear membrane
as the cell
has disintegrated and
enters anaphase,
spindle fibres have
the centromeres
grown across the cell;
divide into
chromosomes line up
two separating
independently along
the sister
the equator of the
chromatids of
spindle attaching to
each chromosome
the fibres via their
centromeres
Summary of Mitosis
Anaphase – spindle activity pulls the
chromatids apart and the separated
chromatids move to opposite
poles of the cell
Cytokinesis – as the membrane
continues to constrict, the
cytoplasm becomes divided
forming two genetically
identical daughter cells. Each
cell now possesses an exact copy
of each chromosome that was
present in the nucleus of
the original cell
Telophase – the chromatids are now
described as chromosomes and they begin
to uncoil. The spindle fibres disintegrate
and the cell begins to constrict along its
central axis. A nuclear membrane begins
to form around each set of chromosomes
Daughter
cells
A
D
B
These photographs are taken
from prepared slides of onion
root tip cells that were
undergoing mitosis:
Identify the photograph
showing Interphase and the
photographs showing the four
stages of mitosis – Prophase,
Metaphase, Anaphase and
Telophase
C
E
Answers
This cell is in
Interphase
The chromosomes
are not visible as
threads but appear
as chromatin
(granular material)
in the nucleoplasm
The nucleolus is
clearly visible
B
During this stage
the cell prepares
for mitosis – DNA
replicates and new
organelles are
manufactured
This cell is in
Prophase
The chromosomes
have spiralised and
condensed – they
are shorter and
thicker and visible
as clear threads
At this stage the
chromosomes can be
seen to have
replicated with each
chromosome now
consisting of two
chromatids
The nucleolus has
shrunk in size
E
Spindle fibres are
beginning to form
close to the nucleus
and the nuclear
membrane
disintegrates
This cell is in
Metaphase
Spindle fibres
have grown
across the
cell
Replicated
chromosomes
attach to the
spindle fibres
by their
centromeres
A
Each replicated
chromosome
lines up
independently
along the
equator
of the cell
This cell is in
Anaphase
The centromeres
of each chromosome
replicate and the
chromatids repel
one another
D
The spindle fibres
contract and pull
the separated
chromatids to
opposite poles of
the cell
This cell is in
Telophase
A single chromatid
from each
chromosome has
reached the poles of
the spindle – the
chromatids are now
described as
chromosomes
The chromosomes
begin to uncoil and
appear as chromatin
once again; a nuclear
membrane forms
around each set of
chromosomes
C
Cytokinesis (division
of the cytoplasm)
follows telophase
In animal cells this
involves constriction
of the cell
membrane along its
central axis and
division of the cell
into daughter cells
In the plant cell shown
in the photograph, this
involves the formation
of a cell membrane in
the middle of the cell
followed by secretion
of cell wall material on
either side of this
membrane
The Cell Cycle
The stages through which a cell passes from one cell division to the next
constitute the cell cycle
The cell cycle is divided into two major phases: the M phase or mitotic phase
and the interphase
The Cell Cycle
INTERPHASE
S
G1
G2
Interphase is divided
into G1, S and G2 phases
MITOSIS (M PHASE)
The Cell Cycle
The M phase
occupies only
a small portion
of the cycle,
lasting from
about 30
minutes to
one hour
Interphase occupies
the largest portion
of the cycle
S
G1
PROPHAS
E
HA
S
TAP
ME
AN
AP
HA
SE
S IS E
E
I N AS
K
TO OPH
Y
C
L
TE
E
M
G2
The M phase consists of the process of
mitosis, when the chromosomes separate,
together with cytokinesis, when the entire
cell is physically divided into two
daughter cells
The Cell Cycle
G1
- First Growth Phase
The cell grows and
new organelles and
proteins are
manufactured
PROPHAS
E
HA
S
TAP
ME
AN
AP
HA
SE
S IS E
E
I N AS
K
TO OPH
Y
C
L
TE
E
M
S
DNA replication
takes place
G2
- Second
Growth Phase
The cell grows
and prepares
for mitosis
During interphase, preparations for
mitosis take place
Variations in the Cell Cycle
G1 is the most variable
of all the phases
The rapidly dividing epithelial
cells lining the human intestine
remain in the G1 phase for
only about 2 hours
S
G1
M
Some cells, such as nerve
and muscle cells never divide;
they may be considered to be
permanently in the G1 phase
and never enter the S phase
G2
Slowly dividing liver cells
may take many months to
move through G1 to the
S phase
Cancer: The Cell Cycle Out of Control
The multiplication of cells is a closely regulated process
Cell division is under genetic control, and it is known that there are
specific genes which code for proteins that ‘switch on’ and
‘switch off’ the process
Cancer is a disease that results from uncontrolled cell divisions
Normal cells become transformed into cancer cells when the genes that
control cell division mutate and become ONCOGENES
Environmental cancer-causing agents, known as carcinogens, play a part
in causing the alteration of DNA structure that leads to oncogene formation
Known carcinogens include ultraviolet radiation, cigarette smoke and X-rays
Scanning Electron Micrograph of Dividing Cancer Cells
Cancer: The Cell Cycle Out of Control
When a normal body cell mutates it may divide to produce a
clone of cells that form a tumour
mutation
normal
body cell
mitosis
mutated
body cell
tumour
Many such tumours are found to be BENIGN and do not spread from
their site of origin – they may nevertheless compress and damage adjacent tissues
Malignant, cancerous tumours may spread from their site of origin
These tumours develop their own blood and lymph supply which can transport
malignant cells from the tumour to other sites in the body
malignant cancer
cells carried to
other body sites
malignant tumour
these cells invade
other body regions
to form secondary
cancers
This is called
metastasis
secondary
tumour
Observing Mitosis in Plant Tissues
Plant roots grow by mitotic division of the cells at the root tip
Onion root tips are an ideal source of material for observing the stages
of mitosis
• A scalpel is used to cut about 4 mm from the tip of
the growing onion root
• Acetic acid is added to the tip on a watch glass and
warmed gently for about 5 minutes; the acid helps to
macerate the cells
• The root tip is transferred to a slide where two or
three drops of aceto-orcein stain are added; this stain
is taken up by the chromosomes and makes them
more visible as they stain red
• The tip is gently broken up with a mounted needle
and the cells are spread across the slide
• A coverslip is placed over the root preparation and
gently squashed
• The slide is examined for stages of mitosis using an
optical microscope
Photomicrograph showing cells from an onion root tip
Note that many of the cells are in interphase
An Introduction to Meiosis
Meiosis is another form of cell division that is associated
with reproduction in many organisms
In humans, meiosis is responsible for the formation
of the reproductive cells or gametes
In humans, these are the egg and sperm cells
Whereas most body cells have a complement of
23 pairs of chromosomes, human gametes possess
only 23 single chromosomes. A gamete’s complement of
23 single chromosomes is constituted by one chromosome
taken from each of the 23 pairs of chromosomes
Within the human ovaries and testes, gametes are
produced by meiosis and this process halves the
chromosome number
Human body cells are DIPLOID as they possess two sets of
chromosomes (23 pairs)
Human gametes are described as being HAPLOID as they
possess only one set of chromosomes (23 chromosomes)
SPERM
If the gametes were diploid then the number of
CELL chromosomes would double at every generation after
fertilisation
EGG
CELL
MITOSIS
MEIOSIS
Diploid body
cell
The nucleus
divides twice
Two diploid daughter cells
Meiosis is important as it ensures
that, when the gametes fuse at
fertilisation, the normal diploid number
of chromosomes is maintained; meiosis
is also an important source of
genetic variation
Four haploid, genetically different
gametes are produced