1. Meiosis and genetic variation
A study in creating sex cells
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2. Genome Genome: Complete complement of an organism’s DNA.
Includes genes (control traits) and non-coding DNA organized in chromosomes.
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3. Genes Eukaryotic DNA is organized in chromosomes.
Genes have specific places on chromosomes.
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4. Heredity
Heredity – way of transferring genetic information to offspring
Chromosome theory of heredity: chromosomes carry genes.
Gene – “unit of heredity”.
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5. Reproduction
Asexual
Many single-celled organisms reproduce by splitting, budding, parthenogenesis.
Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).

6. Reproductions: some vocabulary
Gamete: the name given to sex cells (either a sperm or an egg)
Fertilization: the process when the sperm cell and the egg cell fuse together
Zygote: The cell created when the egg and sperm cell fuse together

7. Sexual reproduction Fusion of two gametes to produce a single zygote.
Gametes have half the number of chromosomes
In humans: 23 chromosomes (1n)
The zygote is formed when the gametes fuse
In humans, this forms 46 chromosomes (2n)
Introduces greater genetic variation, allows genetic recombination.
With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

8. Importance of Sex Cells
You need to reduce the number of chromosomes in sex cells to half (1n)
Otherwise when they fused, you would keep doubling the number of chromosomes
Example:
In humans, generation 1 would have 46
Generation2 would have 92
Generation3 would have 184, etc.
These organisms would not survive

9. Importance of Sex Cells
Sexual repro starts with sex cell & ends with fertilization
Zygote is formed--in human now is diploid or 2n with 46 chromosomes

10. Why do we need to create sex cells?
During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by shuffling of the DNA. This means that the genes have more possibili
No daughter cells formed during meiosis are genetically identical to either mother or father
During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

11. In humans If you have 1 chromosome, you have 2 possibilities
With 23 chromosomes in in each gamete
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations

12. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
64 trillion combinations for a diploid zygote!!!
That’s more than all humans that have ever lived.
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13. SEX IS COSTLY
Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required
Intimate contact provides route for infection by parasites and disease (AIDS, syphillis, etc.)
Genetic costs: in sex, we pass on only half of genes to offspring.
Males are an expensive luxury - in most species they contribute little to rearing offspring.
QUESTION: Why is genetic diversity so important?

14. But …
More genetic diversity = more potential for survival of species when environmental conditions change.
Shuffling of genes
Fertilization: combines genes from 2 separate individuals
DNA back-up and repair.
Asexual organisms don't have back-up copies of genes
Sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged.
Sexual mechanisms, especially recombination, are used to repair damaged DNA
The undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

15. Meiosis KM

16. In humans …
23 chromosomes donated by each parent (total = 46 or 23 pairs).
Gametes (sperm/ova):
Contain 22 autosomes and 1 sex chromosome.
Are haploid (haploid number “n” = 23 in humans).
Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid.
Diploid cell; 2n = 46. (n=23 in humans)
Most cells in the body produced by mitosis.
Only gametes are produced by meiosis.
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17. MEIOSIS I Unlike mitosis, meiosis occurs in two parts
Meiosis I is similar to mitosis where you are creating a 2n number of chromosomes
Meiosis II is the process where you create the 1n number of chromosomes in the sex cells

18. Meiosis KM

19. Meiosis I --Sex Cell Formation
In meiosis, there are 2 divisions of the nucleus: meiosis I & meiosis II
Prophase I: double stranded chromosomes and spindle fibers appear; nuclear membrane and nucleolus fade (CROSSING OVER occurs here)
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20. Meiosis I --Sex Cell Formation
Metaphase I: chromosome pairs (chromatids) line up
spindle fibers attach to centromeres and centrioles
Anaphase I: chromotids separate from matching pair (independent assortment occurs here)
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21. Meiosis I --Sex Cell Formation
Telophase I: cytoplasm divides and 2 cells form
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22. Meiosis 1 First division of meiosis
Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage.
Metaphase 1: Homologous chromosomes align at the equatorial plate.
Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

23. Summary of Meiosis I
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24. Meiosis II
At the end of meiosis I, you have created 2 cells with a 2n number of chromosomes
The purpose of meiosis II is to take those 2 cells and create a total of 4 cells, each with n number of chromosomes
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25. Meiosis II --Sex Cell Formation
Prophase II: chromatids and spindle fibers reappear
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26. Meiosis II --Sex Cell Formation
Metaphase II: chromatids line up in the center of the cell
spindle fibers attach to centromere & centriole
Anaphase II: centromere divides
chromosomes split and move to opposite poles
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27. Meiosis II --Sex Cell Formation
Telophase II: spindle fibers disappear
nuclear membrane forms around chromosomes at each end of cell
each nucleus has half the # of chromosomes as the original (haploid)
now there are 4 sex cells (daughter cells)
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28. Meiosis II Second division of meiosis: Gamete formation
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes align at the equatorial plate.
Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.
Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

29. Summary of Meiosis II

30. Meiosis I and Meiosis II
OVERVIEW OF MEIOSIS
Meiosis I and Meiosis II

31. Animation
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32. Meiosis creates genetic variation
There are a couple of methods to increase genetic variability in sex cells
Crossing-over
Occurs mainly during prophase I
When adjacent chromatids break of a portion of their structure and switch places
Independent assortment
Occurs mainly during anaphase I
When homologous chromosomes separate randomly and move to opposite poles of a dividing cell

33. Crossing over
Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids.
Crossing over produces recombinant chromosomes.
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34. Independent assortment
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35. Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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36. REMINDER: KARYOTYPES Karyotype:
ordered display of an individual’s chromosomes.
Collection of chromosomes from mitotic cells.
Staining can reveal visible band patterns, gross anomalies.

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39. Down's Syndrome
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40. Homologues Chromosomes exist in homologous pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes, they have homologues.
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41. DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
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42. Meiosis – key differences from mitosis
Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids).
In Metaphase I, homologous pairs line up at metaphase plate.
In Anaphase I, independent assortment of chromosomes.
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43. Mitosis vs. meiosis
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44. Meiosis KM