©2000 Timothy G. Standish

MeiosisMeiosisGamete ProductionGamete Production

Timothy G. Standish, Ph. D.

©2000 Timothy G. Standish

Replication

Chromosomes, Chromatids Chromosomes, Chromatids and Centromeresand Centromeres

Centromere

Chromosome arm

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes

©2000 Timothy G. Standish

In The Beginning TwoIn The Beginning Two Humans and many other complex multicelled

organisms incorporate genetic recombination in their reproduction

Reproduction in which there is a re-mixing 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

Egg1n

Haploidnucleus

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Sperm2nHaploid

nucleus

©2000 Timothy G. Standish

Sperm2n

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Egg1n

Haploidnucleus

Haploidnucleus

©2000 Timothy G. Standish

Sperm2n

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Egg1n

Haploidnucleus

Haploidnucleus

©2000 Timothy G. Standish

Sperm2n

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Egg1n

Haploidnucleus

Haploidnucleus

©2000 Timothy G. Standish

From Zygote to EmbryoFrom Zygote to Embryo

Zygote2n

Zygote

2n

©2000 Timothy G. Standish

Cleavage

From Zygote to EmbryoFrom Zygote to Embryo

©2000 Timothy G. Standish

Cleavage

From Zygote to EmbryoFrom Zygote to Embryo

©2000 Timothy G. Standish

Cleavage

From Zygote to EmbryoFrom Zygote to Embryo

©2000 Timothy G. Standish

Cleavage

From Zygote to EmbryoFrom Zygote to Embryo

©2000 Timothy G. Standish

Morula

From Zygote to EmbryoFrom Zygote to Embryo

©2000 Timothy G. Standish

Gametes Are HaploidGametes Are Haploid Gametes must have half the genetic material of a normal cell If the genetic material in the gametes was not halved, when they

combined the zygote would have more genetic material than the parents

Meiosis is specialized cell division resulting in cells with half the genetic material of the parents

Gametes have exactly one set of chromosomes, this state is called haploid (1n)

Regular cells have two sets of chromosomes, this state is called diploid (2n)

©2000 Timothy G. Standish

Stages Of MeiosisStages Of Meiosis 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 Metaphase I

©2000 Timothy G. Standish

Stages Of Meiosis - Meiosis IStages Of Meiosis - Meiosis I 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 IStages Of Meiosis - Meiosis I 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

Stages Of Meiosis - Meiosis IIStages 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 in haploid cells each with one set of chromosomes made of one chromatid

©2000 Timothy G. Standish

InterphaseInterphase

Mother cell Stages Of Meiosis: Stages Of Meiosis: Meiosis IMeiosis I

Meiosis IIMeiosis II

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Stages Of Meiosis: Stages Of Meiosis: Meiosis IIMeiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

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

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

©2000 Timothy G. Standish

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing over

Crossing OverCrossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

©2000 Timothy G. Standish