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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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