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` Cell division can be divided into 2 stages:
Cytoplasmic division also known as cytokinesis
Nuclear division Cytoplasmic division
MITOSIS
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` Meiosis is the process ofnuclear division that reducesthe number of chromosomes innew cells to half of the number
of chromosomes in the parentcell.
` In human, meiosis produceshaploid reproductive cells or
gametes (sperm and ovum).
SPERM
OVUM
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` The gametes contain only 23 chromosomesor haploid number of chromosomes (n).
` Sexual reproduction involves the fusion oftwo haploid gametes during fertilisation.This results in the formation of a diploidzygote with 46 chrmosomes.
Diploid
(2n=46)
Male
parent
Diploid
(2n=46)
Female
parent
Diploid(2n=46)
Offspring
Embryo(2n)
Haploid (n=23)
Haploid (n=23)
Diploid
zygote(2n=46)
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` The offspring will inherit traits from both the
parents to ensure the continuation of life.
` Meiosis ensures that the diploid numberis
maintained from one generation to the next.` In multicellular organs like human, meiosis occurs
in the reproductive organs: testis in males &
ovaries in females.
` In plants meiosis occurs in the anthers andovaries of the flowers.
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testis
Ovary
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Ovary
Anther
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MEIOSIS
MEIOSIS I MEIOSIS II
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
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` Meiosis I separates the homologous
chromosomes.
` Meiosis II separates the sister chromatids.
` At the end of meiosis II, four haploid daughtercells are produced, eachgenetically distinct
from the other and from the parent cell.
` In meiosis, although cells undergo two nuclear
divisions, the DNA of each chromosome onlyreplicates once.
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` Interphase (not part ofmeiosis)
` Similar to Interphase ofmitosis
` Organelles are synthesised.
` Chromosomes replicate in S phase of the cell cycle.` Centrioles pairs replicate.
` Energy store is built up to be used later.
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Prophase 1
` Chromosomes begin to condense, becomeshorter, thicker and clearly visible.
` Farthers and mothers chromosomes lie side
by side to form homologous pairs called bivalent
or a tetrad through a process called synapsis.
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` Non-sister chromatids exchange segments of DNA
in a process known as crossing over.
` Chiasmata are points where the exchange of
chromatid material occurs.
` At the end of prophase I, the nucleolus and
nuclear membrane disappear.
` Centrioles pairs move to opposite poles of the
nucleus.
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Metaphase1
Spindle fibres are fully formed.
The homologous chromosomes or tetrads
assemble at the equatorof the spindle.
One chromosome ofeach pairs attached to the
spindle fibre from one pole while its homologue
attached to the fibre of the opposite poles.
The centromere does not divide.
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Anaphase1
` Spindle fibres pull the homologous chromosomesaway.
` Each pole receives only one of eachhomologus
chromosomes.
Telophase1
` Chromosomes arrive at the pole.
` Nuclear membrane and nucleolus reappears tosurround each set of chromosomes.
` Each pole only receives only one of eachhomologus chromosomes.
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` Cytokinesis occurs simultaneously withTelophase
1, resulting in two haploid daughter cells.
` Meiosis II follows immediately after cytokinesis.
` DNA replication does not occur again and thechromosomes remain in a condensed state.
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Prophase II` Chromosomes thicken and shorten.
` Each chromosomes appear as two chromatids,
connected at the centromere.
` Nucleoli and nuclear membrane of the daughter
cells disappear.
` Centriole replicates and both move to
opposite poles.
` Spindle fibres reform.
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Metaphase II
Chromosomes align themselves at the equatorofeach daughter cell.
Chromosomes attach to spindle fibre at its
centromere
Anaphase II
Centromeres divide
Spindle fibres contract to pull the two
chromatids to opposite poles ofeach daughter
cell
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Telophase II
Chromatids reach the poles and become thechromosomes of the new daughter cells
Spindle fibres disappear, nuclear membrane
and nucleoli reform.
Cytokinesis take place and produce fourhaploid daughter cells.
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