Sexual Reproduction and Meiosis Chapter 7b Copyright © McGraw-Hill Companies Permission required for reproduction or display

Outline Reduction Division Unique Features of Meiosis Prophase I Metaphase I Completing Meiosis Second Meiotic Division Sexual Reproduction Origin and Maintenance Evolutionary Consequences

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.

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

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Presence of chiasmata dictates crossing over has occurred. Copyright © McGraw-Hill Companies Permission required for reproduction or display Copyright © McGraw-Hill Companies Permission required for reproduction or display

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.

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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.

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

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.

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.

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.

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.

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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