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Danyal Education
Cell Division Notes 1
UNIT14:
CEtT DIUISI()N
Learning Objeclives
State the importance of mitosis
Explain the need for the production of genetically identical cells
Identify the main stages of mitosis and meiosis
Define haploid and diploid
Slate how meiosis andfertilisation can lead to vqriation
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Mitosis is a type of cell division which involves the division of the nucleus
daughter cells.
to
produce two
The daughter cells have the same number and same kind of chromosomes as the parent cell.
The importance of mitosis:
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Produces new cells for growth, repair and replace damaged or dead cells in multicellular
organisms
A form of asexual reproduction in unicellular organisms
Ensures genetical identical of the offspring or new cells to the parent
Preserves the diploid number of chromosomes
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The cell cycle is the order of events that occurs between one cell
division and the next. lt comprises of the following events:
lnterphase
Mitosis
Cytokinesis
cytokinesis
interphase
A cell spends about 90% of the cycle during interphase. lt is the non-dividing period. The cell
carries out regular cellular activities such as synthesising new proteins for growth.
ln interphase, the chromosomes are not visible which appear as long thread-like structures called
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chromatin.
Centrioles in animal cell duplicate at this stage. Plant cells do not have centriole.
During interphase, just before mitosis takes place, the cell will then duplicate the chromatin
threads into two identical threads.
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Mitosis is a process of cell division which results in the production of two daughter cells from
a single parent cell. The daughter cells are identical to one another and to the original parent
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cell.
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chromosome
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parent cell
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replication of DNA
mitosis
two identical
daughter cell
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The daughter cells are diploid (2n) i.e. they have the complete number of chromosomes and are
genetically identical to the parent cell.
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Cytokinesis refers to the process whereby the cytoplasm of daughter cells split off completely
from each other after cell division.
Mitosis in animals
Decription
Name
The chromatin, diffuse in
interphase, condenses into
chromosomes, which are now
visible under a microscope.
Each chromosome has
duplicated and now consists of
two sister chromatids attached
at the centromere.
At the end of prophase, the
nuclear envelope breaks down
into vesicles.
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Stoges of mitosis
microtubules
(spindle fibres)
Prophase
centromere
nuclear envelope
equator
The chromosomes align at the
equator of the spindle.
They are held in place by
microtubules attached to the
mitotic spindle and to part of the
&
Metaphase
pole
centromere.
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Anaphase
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separated
chromosome
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chromosome
uncoiling
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The centromeres divide.
Sister chromatids separate as
the spindle fibres pull them
apart and move toward the
opposite poles. Once separated,
the chromatids are known as
daughter chromosomes.
Daughter chromosomes arrive at
the poles and the microtubules
d isa ppea r.
Telophase
Cytokinesis
a
nuclear
membrane
reforms
The chromosomes uncoil
and lengthen to become thin
chromatin threads again and
the nuclear envelope reappears
around them.
The cytoplasm divides.
The cell membrane pinches
inward ultimately to produce two
daughter cells
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Mitosis in plants
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Centrioles are absent in plant cells.
The cytoplasm does not pinch inwards during cytokinesis.
plate by the fusion of vesicles at the equator of the cell.
lt starts with the formation of cell
A new cell wall is formed between the two daughter nuclei which divides them into two.
cell wall
cell plate
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new cell wall
nucleus
vesicle
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What is mitosis?
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1. (a)
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Where does mitosis take place in humans?
(c)
The diagram below shows the summary of mitosis. Complete the diagram to show how
a parent cell produces two daughter cells.
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(b)
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parent cell
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mitosis
replication of DNA
two identical
daughter cell
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2.
The diagram shows the cell cycle. The cell cycle consists of three stages.
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(a) Name the other two stages shown in the diagram.
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State the main event that takes place during process
(c)
State how process Q is important for the cell cycle.
in mitosis in the correct sequence after process Q.
The diagrams below show the cell at different stages during mitosis. Label the structures A-l
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3.
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(b)
(d) Name all the processes
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How does the mitosis in a plant cell differ from that in an animal cell?
5.
Why is mitosis important to plants and animals? Give two reasons
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4.
MEIOSIS
.
Meiosis involves a reduction in the amount of genetic material.
Meiosis comprises two successive nuclear divisions, meiosis
round of DNA replication.
I and meiosis Il, with only
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meiosis
meiosis
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diploid (2n)
parent cell
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one
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Meiosis is the process by which a single parent diploid cell divides to produce four daughter
haploid cells. Haploid cells contain half the number of chromosomes as the parent nucleus.
Meiosis is the type of cell division by which gametes (eggs and sperm) are produced.
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replication of
chromosomes
meiosis ll
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four haploid (n)
daughter cells
Each nuclear division consists of four phases.
Meiosis
Meiosis ll
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Prophase
Prophase ll
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Metaphase
I
Metaphase ll
Anaphase
I
Anaphase ll
Telophase
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Telophase ll
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During interphase
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The chromosomes are not visible. They have thread-like structures called chromatin.
The chromosomes replicates to form two identical sister chromatids joined at the
centromere.
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The centriole replicates.
Meiosis
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The replication of chromosomes only takes place during interphase. No replication of chromosomes
in meiosis I and meiosis ll.
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Name
Decription
Duplicated chromatin condenses. Each
chromosome consists of two, closely
associated sister chromatids.
Homologous chromosomes; one from
the male parent and one from the
female parent, pair up along their
whole length. This is called synapsis.
Crossing over can occur during this
stage of prophase l.
Centrioles move to the opposite poles
of the cell and each chromosome is
now 2 chromatids joined together at
the centromere..
The chromatids may cross and twist
around each other, causing them to
break and exchange parts. The point
where they cross each other is called
a chiasma (plural: chiasmata)This is
crossing over and it produces genetic
variations in germ cells.
centriole
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Prophase
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a pair of
homologous
chromosomes
Metaphase
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chiasma
Anaphase
Telophase
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Homologous ch romosomes align
themselves at the equator.
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The paired homologous chromosomes
separate from each other. Each
chromosome moves to the opposite
pole of the cell.
Each chromosome contains two
chromatids.
nuclear membrane
Nuclear membrane forms around the
chromosomes at each pole and the
cytoplasm divides.
Two daughter cells are formed. Each
daughter cell contains only haploid
chromosome set.
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Meiosis ll
Decription
Name
DNA does not replicate.
Centrioles move to opposite
poles, spindle fibres appear and
nuclear envelope breaks down.
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Prophase ll
Chromosomes align themselves
at the equator.
Centromeres divide and sister
chromatids migrate separately
to each pole.
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Anaphase
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Metaphase ll
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Telophase ll
Cell division is complete.
Spindle fibres disappear and
nuclear membrane form around
the daughter chromosomes at
each pole.
Four haploid daughter cells are
obtained.
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nuclear membrane
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ln conclusiorr, meiosis differs from mitosis in that there are two cell divisions in meiosis, resulting
in cells with a haploid number of chromosomes.
The importance of meoisis:
Factor
To ensure
Explanation
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trait
inheritance in
offspring
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Meiosis allows the transmission of traits found in the genes
from one generation to the next.
The traits from the parents can be passed on to the offspring
during fertilisation when the gametes from both parents fuse
together.
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Genetic variation in gametes is caused by the random
distribution of the pair of homologous chromosomes from
either the male or the female during meiosis l.
Hence, there is a possible of large number of different
chromosomes combinations during fertilisation.
During meiosis, crossing over of chromosomes allows the
exchange of genetic materials between the pair of homologous
chromosomes. As a result, it increases the chance of genetic
variation among offspring.
To produce
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offspring with
genetic variation
production of
haploid gametes
To produce
offspring with
better chance of
survival
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Meiosis process produces the haploid gametes.
During fertilisation, the fusion of the haploid gametes enables
the restoration of diploid chromosomes in the species.
To ensure the
Meiosis allows the selection of better and stronger traits or
characteristics from one generation to the next.
The favourable characteristics ensure the offsprings to have
a better chance of survival in the changing environmental
conditions.
Mitosis
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DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
Homologous chromosomes do not pair up
Homologous chromosomes
prophase
at
Daughter cells are completely identical to
parent cells
Daughter cells may contain variations from
parent cells
Two daughter cells produced from one
Four daughter cells produced from one
parent cell
parent cell
Daughter cells have same number of
chromosomes (diploid number) as parent
Daughter cells have half the number of
chromosomes (Haploid number) as the
cell
parent cell
0nly one nuclear division involved
Two nuclear divisions involved
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Crossing over may occur
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What is meiosis?
(b) Explain the following
(i) Haploid cell
(ii)
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pair up
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Crossing over does not occur
OREUISION
L. (a)
Meiosis
terms.
Diploid cell
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(c)
The diagram below shows the summary of meiosis. Complete the diagram to show how a
parent cell produces four daughter cells.
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meiosis ll
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parent cell
replication of
chromosomes
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meiosis
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meiosis ll
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four daughter
cells
The diagram shows the cell in one of the stages during meiosis.
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Name the stage shown in the diagram.
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(b)
what has taken place at point X? Explain its significance for this event.
(c)
What is the next stage that would happen to this cell?
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3. (a) List one similarity between anaphase I and ll
(b) List one difference
4.
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between anaphase I and ll
State four differences between mitosis and meiosis
5. (a)
(b)
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Why is meiosis important for organisms that reproduce sexually?
What is the advantage of meiosis for living organisms?
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Topical Practice Questions 14
Section A: Multiple-Choice Questions
L.
Which of the following best describes the process whereby the cytoplasm of daughter cells split off
completely from each other after cell division?
A prophase
2
B anaphase C metaphase D
cytokinesis
Which of the following stages occurs when centromere divides and sister chromatids migrate
separately to each pole during meiosis?
A
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Anaphase
I
B
Anaphase
ll
c
Metaphase
I
D
Metaphase ll
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