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v¡tal: essential
Over time, cells grow and the size of their cltoplasm increases.
Eventually, cells reach the stage where they divide. This division of cells is
how they reproduce.
reproductive cell: specialised cell
that is able to unite with a
similar cell of the other sex and
produce a new individual
Cell division is the way that an organism produces new cells in order to
grow and repair its body's tissues.
mitotic spindle: structure made of
fibres that chromosomes attach
to during cell division
condensation: thickening and
shortening of chromosomes
during mitosis
(entromere: point that joins the
two halves of a chromosome
together
cett division
6.1. Mitosis
During cell division, two daughter cells are produced from the parent
cell. The daughter cells are identical to each other. It is vital that an exact
copy of the genetic information is transmitted to the daughter cells. To do
this, the nucleus has to divide in a process called mitosis.
The parent cell needs to have two copies of its genetic material to give to
each of the daughter cells. So before mitosis begins, the cell's genetic
information is duplicated. As a consequence, mitosis can provide a copy of
the parent cell's genetic information to each daughter cell.
All
cells,
with the exception of reproductive cells, divide this way.
There are four main stages in mitosis: prophase, metaphase, anaphase
and telophase.
Prophase
s,oind le
Ín the first stage of mitosis, there are four maín
processes.
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forming
Genetic material thickens and shortens to
form chromosomes.
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The nucleolus disappears.
Between thetwo ends of the cells (poles),
protein fibres appear to make a mitotic
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spindle.
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At the end ofthe prophase, the nuclear
membrane disappears.
fragments of
nuclear membráne
Metaphase
These are the main events during the second
stage of mitosis.
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The condensed chromosomes move within
the mitotic spindle that connects the two
centrosomes.
The chromosomes attach themselves to the
spindle fibres with their centromeres.
The chromosomes line up along the middle
of the cell.
spindle pole
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Anaphase
ch
romatids
There are two main stages of the anaphase.
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The spindle ñbres split into two equal
parts. This causes the chromosomes'
centromeres to break. As a result, the two
chromatids that make up each
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chromosome separate into two
unconnected strands.
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The spindle fibres contract and pull the
chromatids to the opposite poles of the
cell. From this point onwards, the
chromatids are considered to be
independent chromosomes.
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centrioles
mid section
Telophase
ln the final stage of mitosis, four main
events occur.
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Once the chromatids have moved to the
poles of the cell, the mitotic spindle
disappears.
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A nuclear membrane surrounds each
of daqghter chromosomes. This
j:roqn
results in the formation of two daughter
nuclei.
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The chromatids change to chromatin.
The nucleolus
reappears.
nucleolus
ch
romatin
If the mitotic process has been normal, the resulting daughter nuclei will
contain the same genetic information. This information is an identical copy
of the genetic information found in the parent nucleus.
Mitosis is usually followed by the dividing of the cytoplasm into two
parts. A plasma membrane forms and separates the two daughter nuclei'
This process of cytoplasmic division is called cytokinesis. There are two
different typ es of cytokinesis.
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split lvb.); separate, break open
chromatid: strand formed when
a
chromosome divides
In animal cells, the cytoplasm stretches, thins out and eventually
separates.
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In plant cells, a wall called a cell plate forms which divides the
cytoplasm in half.
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At the end of cytokinesis, two separate and complete daughter cells are
produced.
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EEI Why do the nuclear membrane and nucleolus disappear and then reappear
during mitosis? Explain your answer in your exercise book.
C¡okinesis in animal cells (A) and plant
cells (B)
cellsfl
6.2. Meiosis
diploid: having a complete set of
chromosomes
haploid: having a single set of
unpaired chromosomes
somatic celh any cell of an
organism that isn't a
reproductive cell
offspring: new individual or
individuals that result from
sexual or asexual reproduction
1
cell (2n)
Sometimes cell division results in cells
that aren't identical to the parent cell. This is
the case during a special type of cell division
called meiosis. During this process, cells
with half the amount of genetic information
are produced.
The cells produced from meiosis are
known as reproductive cells or gametes. The
male and female gametes join together to
form a zygote, which contains a full set of
genetic information . The zygote then divides
repeatedly to create a new and complete
individual.
informationlnJ
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information: n
1st mitosis
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2 cells (n)
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Sperm (male gamete) fertilising
an egg (female gamete)
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informat¡o1f
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2nd mitosis
parents
gametes
*
fertilisatíon
4 cells (n)
d
Diagram of meiosis
new individual
information: n
information: 2n l
Meiosis is the process of cell division when a diploid cell with 2n
chromosomes divides into 4 daughter haploid cells with n chromosomes
If gametes contained the same number of chromosomes as somatic
cells, the specific genetic characteristics of individuals could not be passed
on to their offspring. This would mean that the specific genetic proiile of
each species could not be maintained. The process of meiosis ensures that
the correct number of chromosomes is passed onto the next generation.
As a consequence, there are two consecutive cell divisions in meiosis.
The first division is a special type of cell division that produces cells with
half the number of chromosomes. The second division is the normal
mitosis process.
Meiosis happens at some point during the life cycle of most sexually
reproductive organisms.
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El-l 1¡u. or false?
Discuss these statements with a partner.
a) Only sexually reproductive animals have meiosis occur during their life cycle.
b) Meiosis consists of two consecutive cell divisions because gametes have
twice the number of chromosomes.
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Reduction division
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This first cell division of meiosis produces cells with half the number
ch¡omosomes. It has the same stages as normal mitosis, but with some
special characteristics.
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In the prophase, chromosomes appear, but unlike in mitosis, they are
in pairs. These pairs of homologous chromosomes exchange genetic
material. This is called crossing over.
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In the metaphase, pairs of homologous chromosomes (not individual
chromosomes) become attached to the spindle fibres.
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In the anaphase, complete chromosomes (not chromatids) are pulled
to each spindle pole.
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The telophase in meiosis is the same as for mitosis.
At the end of the first division, there are two daughter haploid nuclei
that have ¡z chromosomes.
homologous: having the same
structure or position
H
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Crossing over
spindle
microtubules
\/
fragments of
the nuclear
/
separation of
homologous chromosomes
that have crossed over
membrane
\/
homologous
chromosomes
prophase
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anaphase
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First division in meiosis
cells
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Second division
The second division process of meiosis is normal mitosis. rn this
division, each chromosome sprits into chromatids. Each
daughter ce:receives one set of chromatids. At the end of second
divisiori, the
cytoplasm divides and creates two separate
ce[s. This is the rast e\-e:-:
the process of meiosis.
metaphase
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prophase il
anaphasetl
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cytokinesis
telophase ll
separation of
sister chromatids
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4 haploid
cells
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,6Second division in meiosis and cytokinesis
After the two
divisions of meiosis, four daughter cels are produced;
each one has half the number of chromosomes-of
the originai parent cell.
In other words, one diproid cen resurts in four haploid
,"i.oau"iire cens.
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What
is
trossing over,and why
is it an important
part of meiosis?
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