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12.1 Meiosis produces haploid cells from diploid cells.
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• Meiosis is a special form of nuclear division that produces the gametes of the sexual cycle. It involves two chromosome separations but only one chromosome replication.
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1. What are the cellular products of meiosis called, and are they haploid or diploid? What is the cellular product of fertilization called, and is it haploid or diploid?
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12.2 Meiosis has three unique features.
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• The
three unique features of meiosis are synapsis, homologous recombination,
and reduction division.
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2. What three unique features distinguish meiosis from mitosis?
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12.3 The sequence of events during meiosis involves two nuclear divisions.
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• The crossing over that occurs between homologues during synapsis is an essential element of meiosis.
• Because crossing over binds the homologues together, only one side of each homologue is accessible to the spindle fibers. Hence, the spindle fibers separate the paired homologues rather than the sister chromatids.
• At the end of meiosis I, one homologue of each chromosome type is present at each of the two poles of the dividing nucleus. The homologues still consist of two chromatids, which may differ from each other as a result of crossing over that occurred during synapsis.
• No further DNA replication occurs before the second nuclear division, which is essentially a mitotic division occurring at each of the two poles.
• The sister chromatids of each chromosome are separated, resulting in the formation of four daughter nuclei, each with half the number of chromosomes that were present before meiosis.
• Cytokinesis typically but not always occurs at this
point. When it does, each daughter nucleus has one copy of every chromosome.
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3. What are synaptonemal complexes? How do they participate in crossing over? At what stage during meiosis are they formed?
4. How many chromatids are present for each type of chromosome at the completion of crossing over? What two structures hold the chromatids together at this stage?
5. How is the attachment of spindle microtubules to centromeres in metaphase I of meiosis different from that which occurs in metaphase of mitosis? What effect does this difference have on the movement of chromosomes during anaphase I?
6. What mechanism is responsible for the independent assortment of chromosomes?
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12.4 The evolutionary origin of sex is a puzzle.
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• In asexual reproduction, mitosis produces offspring genetically identical to the parent.
• Meiosis is thought to have evolved initially as a mechanism to repair double-strand breaks in DNA, in which the broken chromosome is paired with its homologue while it is being repaired.
• The evolutionary significance of meiosis is that it
generates large amounts of recombination, rapidly reshuffling gene combinations,
producing variability upon which evolutionary processes can act.
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7. What is one of the current scientific explanations for the evolution of synapsis?
8. By what three mechanisms does sexual reproduction increase genetic variability? How does this increase in genetic variability affect the evolution of species?
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