Meiosis

Meiosis

MATA KULIAH : Biologi Keperawatan

©2000 Timothy G. Standish

Chromosomes, Chromatids  and Centromeres

A packaged  chromosome

Chromatid

Identical chromatid

Chromosome  arm Centromere

Two identical  chromosomes Replication

Anaphase

Chromosome  arm ©2000 Timothy G. Standish

In The Beginning Two Humans and many other complex multi­celled  organisms incorporate genetic recombination in  their reproduction ● Reproduction in which there is a re­mixing of the  genetic material is called sexual reproduction ● Two cells, a sperm and an egg, unite to form a  zygote, the single cell from which the organism  develops ● Meiosis is the process of producing sperm and  eggs (gametes) ●

©2000 Timothy G. Standish

Fertilization Results In A  Diploid Zygote Egg 1n

Haploid nucleus

Sperm 2n

Haploid nucleus

©2000 Timothy G. Standish

Fertilization Results In A  Diploid Zygote Egg 1n

Haploid nucleus

Sperm 2n

Haploid nucleus

©2000 Timothy G. Standish

Fertilization Results In A  Diploid Zygote Egg 1n

Haploid nucleus Haploid nucleus

Sperm 2n

©2000 Timothy G. Standish

Fertilization Results In A  Diploid Zygote Egg 1n

Haploid nucleus Haploid nucleus

Sperm 2n

©2000 Timothy G. Standish

From Zygote to Embryo Zygote Zygote

2n 2n

©2000 Timothy G. Standish

From Zygote to Embryo Cleavage

©2000 Timothy G. Standish

From Zygote to Embryo Cleavage

©2000 Timothy G. Standish

Stages Of Meiosis

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Meiosis resembles mitosis except that it is actually two divisions not one These divisions are called Meiosis I and Meiosis II Meiosis I results in haploid cells with chromosomes made up of two chromotids Meiosis II is essentially mitosis on haploid cells Stages of meiosis resemble mitosis with two critical differences: the first in prophase I and the second in

©2000 Timothy G. Standish

Stages Of Meiosis ­ Meiosis I

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Prophase I - The beginning phase -

– DNA which was unraveled and spread all over the nucleus is condensed and packaged – Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids) – Crossing over, in which chromatids within tetrads exchange genetic material, occurs ●

Metaphase I - Middle stage - Tetrads line up along the equator of the cell ©2000 Timothy G. Standish

Stages Of Meiosis ­ Meiosis I ●

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Anaphase I - One copy of each chromosome still composed of two chromatids moves to each pole of the cell Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells

©2000 Timothy G. Standish

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Stages Of Meiosis ­ Meiosis II

Prophase II - Cells do not typically go into interphase between meiosis I and II, thus chromosomes are already condensed Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells Telophase II - Cytokinesis and reformation of the nuclear membrane

©2000 Timothy G. Standish

Mother cell

Interphase

Stages Of Meiosis:  Meiosis I

Prophase I: Condensing  Chromosomes

Prophase I:

Tetrad formation/ crossing over

Meiosis II Telophase I

Metaphase I 

Anaphase I  ©2000 Timothy G. Standish

Stages Of Meiosis:  Meiosis II

Telophase I Prophase II

The products of meiosis are 4  haploid cells each with a  unique set of chromosomes.

Metaphase II  The products of  mitosis are 2 diploid  cells with identical  chromosomes. Anaphase II 

Telophase II 

Crossing Over

Prophase I:

Tetrad formation/ crossing over

Because of crossing over, every  gamete receives a unique set of  genetic information. Telophase II 

Metaphase I  Anaphase I 

Telophase I

©2000 Timothy G. Standish

©2000 Timothy G. Standish

©2000 Timothy G. Standish