©2000 Timothy G. Standish Meiosis Gamete Production Timothy G. Standish, Ph. D.

©2000 Timothy G. Standish Replication Chromosomes, Chromatids and Centromeres Centromere Chromosome arm Identical chromatid Chromatid Anaphase A packaged chromosome Two identical chromosomes

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

©2000 Timothy G. Standish Sperm 2n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

©2000 Timothy G. Standish Sperm 2n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

©2000 Timothy G. Standish Sperm 2n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

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

©2000 Timothy G. Standish Cleavage From Zygote to Embryo

©2000 Timothy G. Standish Cleavage From Zygote to Embryo

©2000 Timothy G. Standish Cleavage From Zygote to Embryo

©2000 Timothy G. Standish Cleavage From Zygote to Embryo

©2000 Timothy G. Standish Morula From Zygote to Embryo

©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 (1n) Regular cells have two sets of chromosomes, this state is called diploid (2n)

©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 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 Interphase Mother cell Stages Of Meiosis: Meiosis I Meiosis II Prophase I: Tetrad formation/ crossing over Prophase I: Tetrad formation/ crossing over Metaphase I Telophase I Prophase I: Condensing Chromosomes Prophase I: Condensing Chromosomes Anaphase I

Telophase I Stages Of Meiosis: Meiosis II Metaphase II Anaphase 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 II

©2000 Timothy G. Standish Prophase I: Tetrad formation/ crossing over Prophase I: Tetrad formation/ crossing over Crossing Over Anaphase I Telophase II Metaphase I Telophase I Because of crossing over, every gamete receives a unique set of genetic information.

©2000 Timothy G. Standish