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A Handbook of Biology

MËÏØSÏS

Meiosis is also called

reductional division

as the number of

chromosomes are

reduced to half while

dividing a diploid cell

into haploid daughter

cells.

It leads to the haploid

phase in the life cycle

of sexually reproducing

organisms. Fertilisation

restores diploid phase.

It is the division of

diploid germ cells that

reduces the chromosome

number by half forming

haploid daughter cells

(gametes). It occurs

during gametogenesis to

produce haploid gametes

for sexual reproduction in

plants and animals.

KËÝ FËÅTÜRËS ØF MËÏØSÏS

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It involves two sequential cycles (meiosis I & meiosis II) but only a single

cycle of DNA replication.

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It involves pairing of homologous chromosomes and recombination

between them.

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Meiosis I begins after replication of parental chromosomes to form

identical sister chromatids at the S phase.

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4 haploid cells are formed at the end of meiosis II.

Meiosis I

Meiosis II

Prophase I

Prophase II

Metaphase I

Metaphase II

Anaphase I

Anaphase II

Telophase I

Telophase II

MËÏØSÏS Ï

PRØPHÅSË Ï:

It is typically longer and more complex than prophase of mitosis.

It includes 5 phases based on chromosomal behaviour: Leptotene, Zygotene,

Pachytene, Diplotene and Diakinesis.

Leptotene (Leptonema): Chromatin fibres become long slender chromosomes.

Nucleus enlarges Hence the chromosomes the gradually visible under the light

microscope , the compaction of chromosomes continues throughout leptotene.