A second type of cell division called meiosis takes place in multicellular eukaryotes. This is a reduction division in which the daughter cells receive exactly half the number of chromosomes of the mother cells. Meiosis occurs in the production of gametes—the sperm of the males and the eggs of the females. When a sperm fertilizes an egg, a zygote is produced with the appropriate number of chromosomes for the species—in humans (and potatoes) the zygote and the somatic (body) cells produced from it have 46 chromosomes. This is the diploid (2n) number of chromosomes, half of which have come from the sperm nucleus, half from the egg. The sperm and egg are haploid ( n); they carry half the number of chromosomes of the body cells (in humans, 23 in each sperm and egg). Meiosis thus makes it possible to maintain a constant number of chromosomes in a species that reproduces sexually by halving the number of chromosomes in the reproductive cells. Meiosis uses many of the same mechanisms as mitosis and is assumed to have been derived from mitosis after the latter procedures were in place in some early organisms millenia ago. Figure 1 shows the stages of mitosis, and Figure 2 shows the stages of meiosis. Note that the names for the stages are the same as those of mitosis, with the addition of a numeral to designate either the first or the second divisional stage. Both divisions are part of meiosis; not until the final four daughter cells are produced is the process complete. Synapsis in Prophase I is a decisive interval in determining the inheritance of the daughter cells. At this time, genetic recombination can occur; that is, daughter cells may receive combined traits of their two parents rather than simply the trait from one or the other. This is possible because the phenomenon called crossing over often occurs when the chromatids lie together—segments containing similar alleles break apart and rejoin to the corresponding segment of the opposite chromatid, thus mixing the traits from individual parents.

1. Meiosis and Sexual
Reproduction
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2. Formation of Haploid Cells
 Meiosis: a form of cell division that halves the number of chromosomes
creating haploid cells (gametes or spores)
 Involves two divisions of the nucleus
 Meiosis I
 Meiosis II
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3.  reduces the number of
chromosomes by half to form
reproductive cells
 when the reproductive cells
unite in fertilization, the
normal diploid number is
restored
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Meiosis

4. Meiosis I
 four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
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5.  Longest and most complex phase (90%).
 Chromosomes condense.
 Synapsis occurs: homologous chromosomes come together to form a
tetrad.
 Tetrad is two chromosomes or four chromatids (sister and nonsister
chromatids).
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Prophase I

6. Crossing over
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7. Metaphase I
 Pairs of homologous chromosomes moved to the middle
 Spindle fibers are attached
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8. Anaphase I
 homologous chromosomes separate
 sister chromatids remain attached
 move toward opposite poles
 Genetic material has recombined
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9. Telophase I and Cytokinesis
 chromatids gather at poles
 cytoplasm divides
 Now have 2 haploid cells, but still have sister chromatids
 no further replication of genetic material
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10. Meiosis II
 Includes four phases
 Prophase II
Metaphase II
Anaphase II
Telophase II
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11. Prophase II
 A new spindle forms around the chromosomes
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12. Metaphase II
 Chromosomes line up at the equator
 Spindle fibers attached at the centromeres
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13. Anaphase II
 Centromeres divide
 Chromatids move to opposite poles
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14. Telophase II and Cytokinesis
 Nuclear envelope forms around each set of chromatids
(chromosomes)
 Spindle breaks down
 Cell cytoplasm divides
RESULT:
4 HAPLOID GENETICALLY
DIFFERENT CELLS
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15. Meiosis and Genetic Variation
 Meiosis allows for rapid generation of new genetic combinations
 Genetic variation is essential for evolution to occur.
 Three things that contribute to genetic variation:
 Independent assortment
 Crossing-over
 Random fertilization
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16. Independent Assortment
 the random distribution of homologous chromosomes during meiosis
 contributes to genetic variation in sexually reproducing organisms
 Occurs in metaphase I
 2N = number of combinations possible of chromosomes (N = 23 chromosomes from mom or dad)
 223 = 8.4 million possible combinations of gametes
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17. Crossing Over
 Produce individual chromosomes that combine genes inherited
from parents
 frequency of crossing over depends on size of chromosome (larger
chromosomes, more crossing over)
 Occurs during prophase I and is the exchange of corresponding
segments of DNA
 contribute to genetic variation within a species
 Genetic recombination has occurred at the end
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18. Random Fertilization
 zygote formed by random joining of two gametes
 one egg cell – 1 of 8.4 million possibilities
 one sperm cell – 1 of 8.4 million possibilities
 223 x 223 = 70 trillion diploid combinations
 This is not including variation from crossing over!!!
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19. Meiosis and Gamete Formation
 In sexually reproducing eukaryotic organisms, gametes form through the
process of
 spermatogenesis in males.
 oogenesis in females.
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21. Sexual Reproduction
 Two types of reproduction
 Asexual: a single parent passes copies to make identical offspring
 Sexual: two parents with gametes
 Reproduction: process of producing offspring
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22. Asexual Reproduction
 the formation of offspring from one parent.
 The offspring are genetically identical to the parent
 Simplest and most primitive method of reproduction
 All types lead to clones of the parent.
 Types of Asexual Reproduction
 Fission (amoebas), fragmentation (planarea), and budding
(hydra) .
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23. Sexual Reproduction
 the formation of offspring through the union of gametes from two parents.
 The offspring are genetically different from their parents.
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24. Genetic Diversity
• Raw material for evolution
• sexual reproduction increases variation in the population by making possible
genetic recombination
• asexual reproduction leads to a lack of genetic diversity among offspring.
– This lack of diversity is a disadvantage in a changing environment.
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25. Evolution of Sexual Reproduction
• Sexual reproduction may have begun as a mechanism to repair damaged
DNA
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26. Sexual Life Cycles in Eukaryotes
 Life Cycle: the entire span in the life of an organism from one generation to
the next
 Three types of sexual life cycles:
 Haploid Life Cycle
 Diploid Life Cycle
 Alteration of generations
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27. Haploid Life Cycle
 the simplest of all life cycles,
 the haploid cell occupies the major portion of the life cycle
 Zygote is the only diploid cell and undergoes meiosis to create new haploid
cells
 Found in protists and fungi and algae
 Example = moss plants
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28. Diploid Life Cycle
 the adults are diploid
 the diploid individual occupies the major portion of the life cycle
 Gametes are the only haploid cells
 Examples are humans and other mammals
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29. Alternation of Generations
 Some organisms have a life cycle that alternates
between diploid and haploid phases. (plants, algae,
and some protists)
 Reproduces by mitosis and meiosis
 In plants, the diploid phase produces spores =
sporophyte (creates 4 haploid spores)
 In plants, the haploid phase produces gametes =
gametophyte
 Example = roses
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