1. Meiosis Gamete Production

2. Meiosis – A Source of Distinction
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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.
G. Podgorski, Biology 1010

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

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

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

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

8. From Zygote to Embryo
Zygote 2n
Zygote 2n

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

10. From Zygote to Embryo
Cleavage

11. From Zygote to Embryo
Cleavage

12. From Zygote to Embryo
Cleavage

13. From Zygote to Embryo
Cleavage

14. From Zygote to Embryo
Morula

15. 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. Meiosis – A Source of Distinction
4/14/2017
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.
G. Podgorski, Biology 1010

17. Meiosis – A Source of Distinction
4/14/2017
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.
G. Podgorski, Biology 1010

18. Keys to Understanding Meiosis
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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.
G. Podgorski, Biology 1010

19. 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. Oogenesis – A Path of Meiosis in Humans
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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.
G. Podgorski, Biology 1010

21. Spermatogenesis – Another Path of Meiosis in Humans
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Spermatogenesis – Another Path of Meiosis in Humans
Men are busy – meiosis produces roughly 250,000,000 sperm per day.
G. Podgorski, Biology 1010

22. 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. One Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
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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.
G. Podgorski, Biology 1010

24. Meiosis
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Another Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 ≈ 8,000,0000.
That’s a lot of diversity by this mechanism alone.
G. Podgorski, Biology 1010

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

27. 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
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

29. Meiosis
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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
G. Podgorski, Biology 1010

30. Boy or Girl? The Y Chromosome “Decides”
Meiosis
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Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome
G. Podgorski, Biology 1010

31. Meiosis
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Boy or Girl? The Y Chromosome “Decides”
G. Podgorski, Biology 1010

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

33. Meiosis
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Between Independent Assortment and Crossing-Over, No Two Gametes Are Identical.
G. Podgorski, Biology 1010

34. The
End