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Mader: Biology 8th Ed.
Meiosis and Sexual Reproduction
Chapter 10
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Mader: Biology 8th Ed.
Outline
• Reduction in Chromosome Number• Homologous Pairs• Meiosis Overview• Genetic Recombination
– Crossing-Over– Independent Assortment– Fertilization
• Meiosis I• Meiosis II• Human Life Cycle
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Halving the Chromosome Number
• In sexually reproducing organisms, meiosis reduces the chromosome number from diploid to haploid.
– Gametes are haploid, and the fusion of two gametes forms a diploid organism.
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Homologous Pairs
• Homologous chromosomes (homologues) are paired chromosomes that code for the same traits (one inherited from each parent).
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Overview of Meiosis
• Meiosis requires two nuclear divisions and produces four haploid daughter cells.
– Daughter cells receive one of each kind of parental chromosome, but in different combinations.
– Synapsis results in a bivalent.
!Homologues align and separate.
"Meiosis I
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Overview of Meiosis
• Daughter chromosomes, derived from sister chromatids, move to opposite poles.
– Chromosomes in the four daughter cells have only one chromatid.
!Meiosis II
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Meiosis Overview
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Meiosis Overview
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Genetic Recombination
• Meiosis helps ensure genetic recombination.
• In a changing environment, asexual reproduction might saddle an offspring with a parent’s disadvantageous gene combination.
– Sexual reproduction might give offspring better chance of survival.
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Genetic Recombination
- Crossing-over is an exchange of genetic material between nonsister chromatids of a bivalent during meiosis I.
!Homologues are held together by chiasmata, regions where the nonsister chromatids are attached due to cross-over.
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Crossing-Over
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Fig. 10.3
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Genetic Recombination
– Independent assortment allows homologous chromosomes to separate independently.
!When homologues align at the equator, maternal or paternal homologue may be oriented toward either pole.
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Independent Assortment
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Genetic Recombination
– Fertilization
!Gametes produced by one person are expected to be genetically different from the gametes produced by another person.
"Fusion of gametes causes combination of parental genes.
# (223)2 combinations
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Phases of Meiosis
• Prophase I
– Homologous chromosomes pair during synapsis.
• Metaphase I
– Homologous pairs align at the metaphase plate.
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Meiosis I
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Phases of Meiosis
• Anaphase I
– Homologous chromosomes separate, and move towards poles.
• Telophase I
– Daughter cells have one chromosome from each homologous pair.
• Interkinesis
– Chromosomes still contain two chromatids.
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Meiosis I
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Fig. 10.6
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Meiosis II
• Duplicated, haploid chromosomes align at metaphase plate.
– Sister chromatids separate at the centromere, giving rise to daughter chromosomes.
!At end of telophase II and cytokinesis, there are four haploid cells.
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Meiosis II
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Meiosis II
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Fig. 10.7
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Meiosis vs. Mitosis
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Meiosis vs. Mitosis
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Fig. 10.8
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Table 10.1
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Table 10.2
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Human Life Cycle
• In animals, meiosis occurs only during gamete production.
– In males, meiosis is a part of spermatogenesis.
– In females, meiosis is a part of oogenesis.
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Spermatogenesis
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Oogenesis
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Review
• Reduction in Chromosome Number• Homologous Pairs• Meiosis Overview• Genetic Recombination
– Crossing-Over– Independent Assortment– Fertilization
• Meiosis I• Meiosis II• Human Life Cycle
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