Mitosis
• Mitosis is the process of
reproduction where two
identical cells are produced.
Asexual reproduction.
• During mitosis each
chromosome splits and the
genetically identical
chromatid move to opposite
poles to become
chromosomes.
What triggers mitosis?
• External signals,
hormones, internal factors,
growth factors (proteins
able to trigger mitosis):
these proteins have a
receptor on the cell
membrane. When the
protein is attached to the
receptor, it triggers several
events in the cell that end
with the division of the cell.
The Cell Cycle
The process of mitosis is only part of a
continuous cell cycle where most of the
cell's 'lifetime' is spent carrying out its
prescribed role; a phase in the cycle
called interphase.
Interphase is itself divided up into three
stages:
G1
S
G2
First Gap
Synthesis
Second Gap
Synthesis of
DNA to
replicate
chromosomes
G2
S
The
cell
cycle
Second gap as
cell grows and
ensures DNA
replication is
complete
M
Mitosis
G1
First gap as cell monitors its
surroundings, growing and determining
whether to replicate DNA
What are the phases of the cell cycle
and what are the main events during
each phase?





Interphase
Prophase
Metaphase
Anaphase
Telophase
What are the 4 stages of mitosis?
INTERPHASE
• Before we start describing the different phases
of this cell division, we need to remember what
a cell looks like in its normal state, which is
known as the interphase. A normal eukaryotic
cell has 2 pairs of chromosomes (2n
chromosomes). During this phase, DNA is
being replicated. At the end of interphase, the
amount of DNA will be twice the normal
amount. For a transient moment the cell will
have 4n chromosomes.
PROPHASE
• The first stage is prophase, when the chromosomes begin
to condense (shorten and thicken). They become visible
under the microscope.
•
At this stage, each chromosome is double-stranded. This
is because during the S phase of the cell cycle the DNA
has replicated, resulting in two identical copies of the
chromosomes. These two copies, called chromatids, are
held together by a constricted region called the
centromere.
• Late in prophase, the nuclear membrane disappears and a
network of microtubules assemble. This is called the
spindle or spindle fibres, which extends between the two
What happens at prophase?
METAPHASE
• During metaphase (meaning middle
phase), the chromosomes gather in the
central region (equator) of the spindle.
• At the equator, the centromere of each
chromosome attaches to the spindle
fibres.
What happens at metaphase?
ANAPHASE
• In anaphase the spindle fibres
contract (pull tight).
• The centromere of each chromosome
is pulled in two directions and splits.
• The chromatids separate and are
drawn to opposite poles.
• The separated chromatids, which are
single-stranded, are now called
chromosomes.
What happens at anaphase?
ANAPHASE
• The two sister chromatids
separate. Each one migrates
to opposite ends of the cell.
So each daughter cell has an
identical complement of
chromosomes .
• The nuclear membrane has
disappeared at this stage.
• The cell membrane expands
as the cell itself elongates.
The diameter of the cell
decreases at the equator.
TELOPHASE
• In the last stage of mitosis,
telophase, nuclear membranes form
around each group of chromosomes.
What is cytokinesis?
CYTOKINESIS
• This is the last stage of mitosis. It is
the process of splitting the daughter
cells apart.
• A furrow forms and the cell is
pinched in two. Each daughter cell
contains the same number and same
quality of chromosomes.
Mitosis Micrographs
Cell division for somatic growth
and repair.
1. Interphase
2. Prophase
6. Telophase
5. Late Anaphase
3. Metaphase
4. Anaphase
Why do we need meiosis?
• Meiosis is necessary to halve the number
of chromosomes going into the sex cells
• Why halve the chromosomes in gametes?
At fertilisation the male and female sex
cells will provide ½ of the chromosomes
each – so the offspring has genes from
both parents
Meiosis
• Meiosis involves
one duplication of
the chromosomes
but two divisions.
• In the first meiotic
division
homologous
chromosomes align
and crossing over
occurs. A spindle
forms but
centromeres do not
• For each pair of
homologous
chromosomes one
chromosome
consisting of two
chromatids goes to
each pole. Therefore
there is a haploid
set of chromsomes
at each pole
Meiosis
• Cellular division that
reduces the number
of chromosomes in
the parent cell by
half to produce four
haploid daughter
cells.
• The product of
meiosis is gamates
or spores.
Non-Conservative
Multiplication :
• the daughter cells
are different
Interphase
Meiosis I
The first division of meiosis is
called a ‘reduction’ division
2N
Prophase 1
2N
because it reduces (halves) the
number of chromosomes.
Metaphase 1
One chromosome from each
DNA replication
2N
Synapsis
and
crossing
over
Chromosomes
line up on the
equator
homologous pair is donated to
each intermediate cell.
Anaphase 1
Telophase 1
1N
Intermediate cell
Intermediate cell
Interphase
• The step preceding
the first cell division
is the 'interphase'.
The DNA is
replicated into two
identical copies, just
as in mitosis. During
interphase, the DNA
is not visible.
• During 'prophase', which is quite
different to the mitotic prophase,
the chromosomes become visible.
They look very long, as they are not
yet totally condensed. Their ends
are linked to one pole of the
nucleus. The chromosomes that
belong to the same pair come next
to each other. The chromosomes
become thicker and shorter, as they
condense more and more. They are
linked to each other in pairs. At this
stage, the 2 chromatids of each
chromosome become visible. The
chromosomes then start to move
away from each other, but remain
linked at points called chiasmata. At
this stage, some genetic material
can
be
exchanged
between
chromosomes : it is now that
crossing-over may take place.
Crossing over
• During prophase 1,
with homologous
pairs, they may touch
at points called
chiasma. At chiasmata
the pairs may
exchange
chromosome
segments. This results
in RECOMBINATION.
Metaphase I
• the pairs of
chromosomes are
situated at the equator
of the cell. At this stage,
the nuclear membrane
has disappeared. The
centromeres are
orientated toward the
poles of the cell, and
the chromosomes are
fully condensed.
Anaphase I
• the chromosomes (two
chromatids each)
migrate toward
opposite poles. There is
a separation of
chromosomes, and not
of chromatids as in
mitosis. Each pole will
receive one set of
chromosomes of 2
chromatids.
Anaphase 1
Telophase
• The telophase is very short, and
often mistaken with prophase II.
There is no time for the formation of
a new cell membrane, nor for
duplication of DNA. The next step
starts straight away.
Telophase 1
NOTE: Half the full chromosome
complement shown
Meiosis II
1N
Intermediate cell
Prophase 2
The second division of
meiosis is called a ‘mitotic’
1N
division, because it is
similar to mitosis.
Metaphase 2
Sister chomatids of each
chromosome are pulled
apart and are donated to
Anaphase 2
each gamete cell.
Telophase 2
Gamete (egg or sperm)
1N
Gamete (egg or sperm)
Cell division II
• The second division conserves the
number of chromosomes but divides the
chromatids.
Prophase 2
• it is very short.
Everything is ready.
The two centrioles
migrate away from
each other, and a
network of
microtubules forms
in each daughter
cell.
Metaphase II
• the chromosomes
are situated on the
equator. We
therefore see two
equators
Metaphase 2
Anaphase II :
• the chromatids,
although present at
the first cell
division, only
separate now. They
migrate toward the
two opposite poles
of the cell.
Anaphase 2
Telophase II:
• the reconstitution of
four daughter cells
starts and finishes
Telophase 2
Meiosis
First Division
• Interphase
• Prophase I
• Metaphase I
• Anaphase I
• Telophase
Second Division
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Meiosis
• In the testis of a mammal , or the anthers of a
flower, or any organ containing germ cells, you
can observe a type of cell division called meiosis.
• During meiosis, DNA replication (which makes
the cell have twice its usual amount of DNA) is
followed by two cell divisions.
• At the end, one parent cell gives rise to four
haploid gametes (each with one set of
chromosomes).
• The haploid cell contains only one chromosome
of each homologous pair (which is only half of
the normal amount).
• There is no replication on the DNA during the
short interphase between the two cell divisions.
Meiosis is a succession of events:
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
Telophase I
Figure 10.
Effect of Crossing Over
• After crossing over, each
chromosome contains both maternal
and paternal segments
• Creates new allele combinations in
offspring
Meiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st division - pairs split
2nd division – produces
4 gamete cells with ½
the original no. of
chromosomes
Comparing meiosis and mitosis
Mitosis and Meiosis
• These two
processes are very
important.
• Mitosis because it
allows replication
within the body
• Meiosis because
this is how genes
are passed on from
parents to children.
COMPARING MITOSIS & MEIOSIS
•
•
•
•
•
MITOSIS
Occurs in somatic cells
One division during the
process
• IPMAT
Homologous
chromosomes don’t pair
Two daughter cells
formed which are DIPLOID
Daughter cells are
genetically identical





MEIOSIS
Occurs in sex organs of
plants and animals
Two divisions during the
process
 IPMAT/PMAT
Homologous
chromosomes pair
Four daughter cells
formed which are
HAPLOID
Daughter cells are
genetically different
When Meiosis goes wrong
• Meiosis is usually an exact process,
but sometimes errors occur.
• Errors result in gametes with defects,
resulting in serious consequences
for offspring.
Nondisjunction:
When Meiosis Goes Wrong
n+1
n+1
n-1
chromosome
alignments at
nondisjunctio
metaphase I n at anaphase
I
n-1
alignments
at
metaphase
II
anaphase
II
Non-disjunction in Meiosis I
The meiotic spindle normally distributes
chromosomes to daughter cells without
error.
Meiosis I
Sometimes, homologous chromosomes
or sister chromatids fail to separate.
Meiosis II
In these cases, one gamete receives
two of the same type of chromosome
and the other gamete receives no
copy.
This mishap, called non-disjunction,
results in abnormal numbers of
chromosomes in the gametes.
This example shows non-disjunction in
meiosis I; homologous chromosomes fail
to separate properly at anaphase
during meiosis I.
n+1
n+1
n–1
Non-disjunction in Meiosis I
n–1
Non-disjunction in Meiosis II
Non-disjunction can also
occur in meiosis II, when
sister chromatids fail to
separate during anaphase of
meiosis II.
Meiosis I
In cases of non-disjunction, if
either of the aberrant gametes
unites with a normal one at
fertilization, the offspring will
have an abnormal
chromosome number
(e.g. 2N+1 or 2N–1).
Meiosis II
This is known as
aneuploidy and it accounts
for chromosomal defects
such as Down syndrome
(trisomy 21) and Edward
syndrome (trisomy 18).
n+1
n–1
n
Non-disjunction in Meiosis II
n
Meiosis error - fertilisation
Should the gamete with
the chromosome pair be
fertilised then the
offspring will not be
‘normal’.
In humans this often
occurs with the 21st pair
– producing a child with
Downs Syndrome
21 trisomy – Downs Syndrome
Can you see
the extra 21st
chromosome?
Is this person
male or
female?
Polyploidy
• Having more than two sets of
chromosomes (for example 3N, 4N, 6N)
in a genome is called polyploidy.
• Polyploidy can come about through
errors in meiosis.
• In humans, polyploid zygotes do not
survive.
• Polyploidy is more common in plants
than animals because many plants can
survive by asexual reproduction.
Meiosis – division error
Chromosome pair
Meiosis error - fertilisation
Should the gamete with
the chromosome pair be
fertilised then the
offspring will not be
‘normal’.
In humans this often
occurs with the 21st pair
– producing a child with
Downs Syndrome