MEIOSIS
BY:M.SAIDI
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a type of cell division that results in four
daughter cells each with half the number
of chromosomes of the parent cell.
as in the production of gametes and plant
spores.
In animals, its occurs ovary–female-ova
testes-male-spermatozoa
In plants, it occurs in ovary-female-ovules
anthers-male-pollen grains
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Chromosomes are
thread-like structures
present in the nucleus,
which carries genetic
information from one
generation to another.
Each chromosome is
made up of DNA (genes)
tightly coiled many times
around proteins called
histones that support
its structure.
Un-replicated
chromosomes
replicated
chromosomes
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The number of
chromosomes in a cell is a
characteristic of an
organism (for example
humans have 46
chromosomes/23 pairs)
Chromosomes which are
single threads/strand,
become double (two
chromatids joined by a
centromere) as a result of
DNA replication
Single
stranded
chromos
omes
double
stranded
chromosomes
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Haploid is the quality of a cell or organism
having a single set of chromosomes
Diploid is when a cell or organism has paired
or two sets chromosomes, one from each
parent.
For sexual reproduction to take place, a
haploid male gamete will fuse with a haploid
female gamete during fertilization. The result
is a diploid zygote
haploid
Half
n
sperm
+ haploid
Half
n
+ ovum
= diploid
= whole
2n
=zygote
Sex cells (gametes) i.e. sperms and ova are
haploid (n), (23 chromosomes in human)
 somatic cells (body cells) are diploid (2n),(46
/ 23 pairs of chromosomes in human).
The 23 pairs of chromosomes that result in a
zygote are divided as follows:
 22 pairs of autosomes
 1 pair of sex chromosomes / gonosomes
represented by:
XX in females
XY in males
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Humans cells are made of 46 chromosomes,
arranged in pairs, each pair is different from
other by shape, size and genetic composition.
from pair 1 to 22 are called Autosomes,
the 23rd pair is called Gonosome, which has
XX for female and XY for male, X is long and
big over Y
 Occurs
In every living organism, in the
somatic (body) cells to produce
genetically identical cells.
 Two
identical cells are produced with
identical chromosomes equal to the
original cell.
 Mitosis
has only one cycle i.e. NO one
and two
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a type of cell division that results in four
daughter cells each with half the number
of chromosomes of the parent cell.
Meiosis is a continuous process but the
events are divided into different phases.
Events of interphase:
DNA replication takes place
Chromosomes which are single threads,
become double
Each chromosome will now consist of
two
 chromatids joined by a centromere
 DNA Replication
 DNA replication helps to double the
genetic
 material so that it can be shared
equally by the new cells arising from
cell division
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Prophase I
 Metaphase I
 Anaphase I
 Telophase I
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Prophase II
 Metaphase II
 Anaphase II
 Telophase II
 IPMAT
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Meiosis I (first meiotic division): this is called
reduction division where the chromosome
number in the nucleus is halved. The
resulting gametes are haploid.
 PROPHASE 1
 Chromatin network condense
to forms chromosomes.
 Homologous chromosomes are
formed.
 Crossing over takes place
 Spindle threads start formed.
 Nuclear membrane disappears.
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Homologous
chromosomes align
together
One chromatid of each
chromosome overlaps
with a chromatid of its
homologous partner.
crossing-over takes place
at a point called chiasma
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The chromosomes
separate in such that the
each has new genetic
material from both
chromosome is shared.
Homologous
chromosomes
randomly line up
along the equator
of the cell in two
rows.
 Each chromosomes
is attached to the
spindle thread
from each pole.
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The spindle
threads shorten
and pull the
homologous
chromosomes to
the opposite poles.
 Chromosomes of
the homologous
pair separate.
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chromosomes reach at
the poles of the cell.
Cytoplasm and cell
membrane divide
(cytokinesis)
two haploid cells are
formed.
 Nucleolus
reforms
 New nuclear membrane
forms around
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PROPHASE 2
METAPHASE 2
Nucleolus and nuclear
membrane disappears.
New spindle fibres are
formed.
Chromosomes become
visible.
Individual chromosomes
line up at the equator of
each cell, with the
centromeres attached to
the spindle fibres.
ANAPHASE 2
 The spindle fibres
start to contract.
 The centromeres
split and
homologous
chromosomes/
chromatids are
pulled to the
opposite poles of
each cell.
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The daughter chromosomes
reach the poles
a new nucleus forms.
The cell membrane of each
cell constricts
the cytoplasm divides into two
cells. (cytokinesis)
Four haploid daughter cells
are formed.
Each daughter cell has half
the number of chromosomes
of the original cell.
The daughter cells are
genetically different from each
other.
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Production of haploid gametes
The halving effect of meiosis overcomes the
doubling effect of fertilisation,
Thus maintaining a constant chromosome
number from one generation to the next.
produces genetic variation by way of the
process of recombination.
Why is meiosis important for survival?
It is of great importance, because it creates
genetic diversity in the population. Hence
differences in survival of organisms in
environment
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Is the differences between organisms of
the same species. It may be due to genetic
factors or environmental factors.
 CAUSES
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OF GENETIC VARIATION
1.Crossing-over
2.Random arrangement of
chromosomes during metaphase
3.Random fertilization
4.Random mating
5.Mutation
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1.CROSSING-OVER
When homologous
chromosomes form pairs
during prophase I of
meiosis I, crossing-over can
occur.
leads to new
combinations of maternal
and paternal genetic
material in each new cell
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The random arrangement of chromosomes during
metaphase results in gametes with a unique
combinations of alleles.
at the equator during metaphase, it allows different
combinations of chromosomes/chromatids to go
into each new cell resulting from meiosis, making
them different.
Any sperm cell stand a chance of fertilising
any egg cell/ovum.between different ovum
and different sperm cells formed by meiosis
result in offspring that are different from each
other.
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Mating takes place within species/ organisms
randomly which leads to a different set of
offspring from each mating pair.
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changes the structure of a gene or
chromosome and therefore the organism’s
genotype changes too.
Since the genotype influences the phenotype,
it creates organisms with new, different
characteristics from one generation to the
next.
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Non-disjunction, is the failure of
chromosomes or chromatids to separate
during anaphase.
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Each nucleus should contain 23 chromosomes after
meiosis but if Non-disjunction occurs,
one nucleus contains 22 while the other has
24,When either of these resulting gametes fuses
with a normal gamete(23), the result in 45 or 47
chromosomes instead of 46 (23 + 22 = 45 or 23 +
24 = 47),
this may lead to Down syndrome child
The child will have 45 autosomes, with three
number 21chromosomes instead of the normal pair
and one pair of sex chromosomes.
Women over the age of 40 have a greater chance
of having a child with Down;s Syndrome.
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stocky body
distinct facial features such as a flattened face
with a small, broad nose, small skin
folds in the inner corner of the eyes that appear
to slant upward, ears situated lower
down on the head and a large tongue
broad hands with short fingers and a crease that
runs across the palm
a wide space between the first and second toes.
MEIOSIS I
MEIOSIS II
The chromosomes arrange at the
equator of the cell in homologous
pairs.
Whole chromosomes move to
opposite
poles of the cell.
Two cells form at the end of this
division.
Daughter chromosomes/chromatids
move to opposite poles of the cell.
The chromosome number is halved
during meiosis I.
The chromosome number remains
the same during meiosis II.
Crossing over takes place.
Crossing over does not take place.
Chromosomes line up at the
equator of the cell individually.
Four cells are formed at the end of
this division.
 Both
processes involve division of the nucleus.
 Both involve cell division.(formation of new cells)
 Both the processes occur in the M-phase of the
cell cycle.
 In both cycles, the stages are common –
prophase, metaphase, anaphase and telophase
 Synthesis of DNA occurs in both.
 Cytokinesis takes place
MITOSIS
MEIOSIS
Occurs all cells/somatic (body)
cells
Occurs Reproductive organs
(ovaries and testes)
Production of genetically identical
cells
Production of genetically non identical cells
Results in 2 daughter cells
Results in 4 daughter cells
One nuclear division
Two nuclear divisions
No crossing-over in prophase 1
crossing-over in prophase 1
occurs
Resulting daughter cells are
diploid (2n)
Resulting daughter cells are
haploid (n)
Results in somatic cells
Results in gametes (eg sperms and
ova)
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BY:M.SAIDI
Email:info@thundereduc.com
contact: 0848331738
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