Meiosis Gamete Production Timothy G Standish Ph D

Chromosomes, Chromatids and Centromeres A packaged chromosome Chromatid Identical chromatid Chromosome arm Centromere Two

In The Beginning Two � Humans and many other complex multicelled organisms incorporate genetic

Fertilization Results In A Diploid Zygote Egg 1 n Haploid nucleus Sperm 2 n

From Zygote to Embryo Zygote 2 n 2 n © 2000 Timothy G. Standish

From Zygote to Embryo Cleavage © 2000 Timothy G. Standish

From Zygote to Embryo Morula © 2000 Timothy G. Standish

Gametes Are Haploid � Gametes must have half the genetic material of a normal

Stages Of Meiosis � � � Meiosis resembles mitosis except that it is actually

Stages Of Meiosis - Meiosis I � Prophase I - The beginning phase –

Stages Of Meiosis - Meiosis I � � Anaphase I - One copy of

Stages Of Meiosis - Meiosis II � � Prophase II - Cells do not

Mother cell Stages Of Meiosis: Meiosis I Interphase Prophase I: Condensing Chromosomes Prophase I:

Stages Of Meiosis: Meiosis II Telophase I Prophase II The products of meiosis are

Crossing Over Prophase I: Tetrad formation/ crossing over Because of crossing over, every gamete

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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 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

Gametes 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 (1 n) � Regular cells have two sets of chromosomes, this state is called diploid (2 n) © 2000 Timothy G. Standish

Stages 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 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 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 II � � Prophase II - Cells do not typically go 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

Mother cell Stages Of Meiosis: Meiosis I Interphase 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 mitos are 2 diploid cells with identical chromosome 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