1. Outline 11-4: Meiosis & Sexual Reproduction
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2. I. Meiosis A. Special type of cell division
1. Produces haploid gametes 2. Involves two divisions:
a. Meiosis I
b. Meiosis II
3. DNA is duplicated before it begins, just as in mitosis
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3. II. Steps of Meiosis: A. Prophase I 1. Nuclear envelope disappears
2. Homologous chromosomes find each other & pair up (called synapsis)
a. Form tetrads (4 chromatids together)
B. Metaphase I
1. Homologous pairs move to the equator of cell
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4. Prophase I Metaphase I

5. 1. Homologous chromosomes separate to opposite sides of cell
C. Anaphase I
1. Homologous chromosomes separate to opposite sides of cell
a. Chromatids DO NOT separate
D. Telophase I
1. Individual chromosomes gather back into nuclei.
2. Cytokinesis usually happens
3. The two new cells are haploid
a. The chromosomes are still composed of TWO chromatids
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6. Anaphase I Telophase I

7. Meiosis I
Can you name the phases?

8. 1. Period of time in between Meiosis I and Meiosis II
E. Interkinesis
1. Period of time in between Meiosis I and Meiosis II
2. The chromosomes DO NOT replicate
F. Prophase II (Happens to 2 cells)
1. Chromosomes & new spindle appear
G. Metaphase II
1. Individual chromosomes line up at equator of cell
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9. Prophase II Metaphase II

10. 2. Chromatids move to opposite poles I. Telophase II
H. Anaphase II
1. Centromeres divide
2. Chromatids move to opposite poles
I. Telophase II
1. Nuclear envelope forms around each set of chromosomes
2. Cell undergoes cytokinesis
3. Result = 4 new cells, each of which is haploid with one set of single-stranded chromosomes
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11. Anaphase II Telophase II

12. Meiosis II
Can you name the phases?

13. III. Meiosis creates variation
A. 3 Mechanisms create variation:
1. Independent assortment
a. How the pairs line up at metaphase I is completely random
b. Each pair separates independently
c. In humans, there are combinations (= ~ 8 million)
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14. b. Which sperm fertilizes which egg is completely random
2. Random Fertilization
a. Two gametes join
b. Which sperm fertilizes which egg is completely random
c.  total possible combinations = ~ 223 X 223 (64 trillion)
3. Crossing Over (during Prophase I)
a. Homologous chromosomes break and exchange pieces
b. This DNA exchange adds almost unlimited genetic recombination
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15. Crossing Over

16. Differences between male & female
A.Spermatogenesis
1. Formation of sperm in males
2. Occurs in testes
3. Cytoplasm divides equally to produce four sperm
B. Oogenesis
1. Formation of eggs in females
2. Occurs in ovaries
3. Cytoplasm divides unequally
4. Produces 1 large ovum & polar bodies which die
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17. Spermatogenesis

18. Oogenesis

19. V. Changes in Chromosome #
A. People who are missing an autosome usually aren’t born
B. People that have extras will not develop properly
1. Trisomy is having 3 copies of a chromosome
2. Occurs in Down’s Syndrome = trisomy of chromosome #21
C. Detectable by a karyotype, a photo of the chromosomes
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20. 2. This is called nondisjunction  Sperm or egg has either:
D. How could this happen?
1. If chromosomes don’t separate properly from each other when eggs & sperm are forming
2. This is called nondisjunction
 Sperm or egg has either:
• 1extra chromosome, or
• 1 missing chromosome
3. When such a sperm fertilizes a normal egg, result is either extra or 1 missing chromosome
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21. E. Changes in Chromosome Structure
1. Change in the structure of a chromosome is called a mutation
2. Four types:
a. Deletion - piece breaks off
b. Duplication - piece copied
c. Inversion - breaks & reverses
d. Translocation - breaks & reattaches to a nonhomologous chromosome
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22. VI. Two Types of Reproduction
A. Asexual
1. Single parent
2. Passes copies of genes to all offspring
3. Offspring produced are clones, a. Genetically identical organisms
4. Types of asexual reproduction:
a. binary fission
b. fragmentation
c. budding
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23. 2. Each form haploid gametes 3. Offspring get genes from both parents:
B. Sexual Reproduction
1. Two parents needed
2. Each form haploid gametes
3. Offspring get genes from both parents:
a. Have traits of both, but are not exactly like either
4. Therefore, sexual reproduction produces genetic recombination in a species
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24. VII. Advantages/Disadvantages
A. Asexual:
1. Simple & quick
2. In predictable environments, this allows organisms to produce many offspring in short amount of time
3. Don’t waste energy making gametes & looking for mates
4. Could be a disadvantage in a changing environment
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25. 1. Provides genetic variation 2. This is raw material for evolution
B. Sexual Reproduction
1. Provides genetic variation
2. This is raw material for evolution
3. Different genetic combinations can allow organisms to survive in a changing environment
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