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©2000 Timothy G. Standish Meiosis Gamete Production.

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Presentation on theme: "©2000 Timothy G. Standish Meiosis Gamete Production."— Presentation transcript:

1 ©2000 Timothy G. Standish Meiosis Gamete Production

2 ©2000 Timothy G. Standish Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis.

3 ©2000 Timothy G. Standish In The Beginning Two Humans and many other complex multi-celled 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)

4 ©2000 Timothy G. Standish Egg 1n Haploid nucleus Fertilization Results In A Diploid Zygote Sperm 1n Haploid nucleus

5 ©2000 Timothy G. Standish Sperm 1n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

6 ©2000 Timothy G. Standish Sperm 2n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

7 ©2000 Timothy G. Standish Sperm 1n Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Haploid nucleus

8 ©2000 Timothy G. Standish From Zygote to Embryo Zygote 2n Zygote 2n

9 ©2000 Timothy G. Standish Replication Chromosomes, Chromatids and Centromeres Centromere Chromosome arm Identical chromatid Chromatid Anaphase A packaged chromosome Two identical chromosomes

10 ©2000 Timothy G. Standish Cleavage From Zygote to Embryo

11 ©2000 Timothy G. Standish Cleavage From Zygote to Embryo

12 ©2000 Timothy G. Standish Cleavage From Zygote to Embryo

13 ©2000 Timothy G. Standish Cleavage From Zygote to Embryo

14 ©2000 Timothy G. Standish Morula From Zygote to Embryo

15 ©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)

16 ©2000 Timothy G. Standish Meiosis – A Source of Distinction Meiosis does two things - 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells.

17 ©2000 Timothy G. Standish Meiosis – A Source of Distinction The second accomplishment of meiosis: 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species.

18 ©2000 Timothy G. Standish Keys to Understanding Meiosis Chromosomes are paired. Chromosomes carry genes. The gene forms on a pair of chromosomes may be identical.. Brown eyes Tall Brown eyes Blue eyes Tall Short or different.

19 ©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 chromatids 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

20 ©2000 Timothy G. Standish Oogenesis – A Path of Meiosis in Humans Woman are less busy in meiosis than men – meiosis produces only a few hundred mature eggs over a lifetime.

21 ©2000 Timothy G. Standish Spermatogenesis – Another Path of Meiosis in Humans Men are busy – meiosis produces roughly 250,000,000 sperm per day.

22 ©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

23 ©2000 Timothy G. Standish One Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

24 ©2000 Timothy G. Standish Another Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 ≈ 8,000,0000.

25 ©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

26 ©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

27 ©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

28 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

29 ©2000 Timothy G. Standish The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

30 ©2000 Timothy G. Standish Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

31 ©2000 Timothy G. Standish Boy or Girl? The Y Chromosome “Decides”

32 ©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.

33 ©2000 Timothy G. Standish Between Independent Assortment and Crossing-Over, No Two Gametes Are Identical.

34 ©2000 Timothy G. Standish


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