1. Sexual Reproduction and Meiosis
Chapter 7b
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2. Outline
Reduction Division
Unique Features of Meiosis
Prophase I
Metaphase I
Completing Meiosis
Second Meiotic Division
Sexual Reproduction
Origin and Maintenance
Evolutionary Consequences

3. Reduction Division
In sexual reproduction, gametes fuse (fertilization) to produce a zygote.
Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other body cells.
Adult body cells are diploid.
Gamete cells are hapolid.
Alteration of generations.

5. Unique Features of Meiosis
Synapsis
Homologues pair along their length.
Homologous Recombination
Genetic exchange (crossing over) occurs between homologous chromosomes.
Reduction Division
Chromosomes do not replicate between the two nuclear divisions.

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8. Prophase I
Five Sequential Stages
Leptotene - Chromosomes condense
Zygotene - Synaptonemal complex
Pachytene - Crossing over
Diplotene - Cell growth
Diakinesis - Chromosomes recondense

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10. Presence of chiasmata dictates crossing over has occurred.
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11. Metaphase I
Terminal chiasmata holds homologous pair together.
Spindle microtubules attach to kinetochore proteins only on the outside of each centromere.
Each joined pair of homologues lines up on metaphase plate.
Orientation of each pair is random.

12. Chiasmata and Alignment
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13. Completing Meiosis
Anaphase I
Spindle fibers begin to shorten and pull whole centromeres towards poles.
Each pole receives a member of each homologous pair.
Complete set of haploid chromosomes.
Due to random orientation, meiosis results in independent assortment.

14. Completing Meiosis
Telophase I
Chromosomes segregated into two clusters.
Nuclear membrane re-forms around each daughter cell.
Sister chromatids are no longer identical.

15. Second Meiotic Division
Meiosis II resembles normal mitotic division.
Prophase II - Nuclear envelope breaks down and second meiotic division begins.
Metaphase II - Spindle fibers bind to both sides of centromere.
Anaphase II - Spindle fibers contract and sister chromatids move to opposite poles.
Telophase II - Nuclear envelope re-forms.
Final Result - Four haploid cells.

16. Sex
Asexual Reproduction - Individual inherits all its chromosomes from a single parent.
Parthenogenesis - Development of adult from unfertilized egg.
Sexual Reproduction - Produces genetic variability.
Segregation of chromosomes tends to disrupt advantageous combinations.
Only some progeny maintain advantages.

17. Origin and Maintenance Of Sex
Theories
DNA Repair Hypothesis
Only diploid cells can effectively repair certain kinds of chromosome damage.
Contagion Hypothesis
Secondary consequence of the infection of eukaryotes by mobile genetic elements.

18. Origin and Maintenance Of Sex
Red Queen Hypothesis
Current recessive alleles can be stored in reserve for potential future use.
Miller’s Ratchet
Sexual reproduction may be a method of keeping mutational load low.

19. Evolutionary Consequences of Sex
Evolutionary Process is revolutionary and conservative.
Pace of evolutionary change is accelerated by genetic recombination.
Evolutionary change not always favored by selection.
May act to preserve existing gene combinations.

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