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Chapter 7 Meiosis and Sexual Reproduction Section 1: Meiosis Section 2: Sexual Reproduction
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Section 1 Meiosis Objectives: Summarize the events that occur during meiosis. Relate crossing-over, independent assortment, and random fertilization to genetic variation. Compare spermatogenesis and oogenesis.
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Section 1 Meiosis Formation of Haploid Cells Meiosis and Chromosome Number Meiosis reduces the number of chromosomes by half to form reproductive cells. (46 to 23) When the reproductive cells unite in fertilization, the normal diploid number is restored.(23+23=46)
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Stages of Meiosis Meiosis I –4 stages Prophase I MetaphaseI Anaphase I TelophaseI /cytokinesis Meiosis II -4 stages Prophase II Metaphase II Anaphase II Telophase II /cytokinesis 2 Stage process
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Meiosis I
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Meiosis II
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Meiosis I PROPHASE I Chromosomes become visible Nuclear envelope disappears homologous chromosomes separate. Crossing-over during prophase I results in the exchange of genetic material between homologous chromosomes.
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Meiosis I Metaphase I Pairs of homologous chromosomes Move to equator of cell Spindle fibers attach to chromosomes
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Meiosis I Anaphase I –Homologous chromosomes separate –Pulled to poles by spindle fibers –Chromatids do not separate at their centromeres –Each chromosomes has 2 chromatids, however the genetic material was recombined during crossing over.
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Meiosis I Telophase I –Chromosomes gather at the poles –Each cell has 1 chromosome from each pair homologous chromosome –Cytokinesis occurs –Chromosomes do not replicate again between Meiosis I & II
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Meiosis I In conclusion: –1 diploid cell divides into 2 diploid cells –The resulting 2 cells have a recombination of genetic material from the parent cell through the crossing over of chromosomes.
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Meiosis II During Meiosis II, the two chromatids of each chromosome separate. As a result of meiosis II, 4 haploid cells are produced from the original diploid cell.
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Meiosis II Prophase II –New spindle forms around chromosomes
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Meiosis II Metaphase II –Chromosomes line up on equator –Attached to spindle fibers at centromeres
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Meiosis II Ananphase II –Centromeres divide –Chromasomes move to opposite poles
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Meiosis II Telophase II/cytokinesis –Nuclear envelope forms around each set of chromosomes –Spindle breaks down –Cytoplasm divides –Results in 4 haploid cells
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Section 7.1 Independent Assortment the random distribution of homologous chromosomes during meiosis contributes to genetic variation in sexually reproducing organisms. 2 23 ( 8 million) gametes can be produced with different gene combinations from 1 original cell Meiosis and Genetic Variation
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Crossing Over and Random Fertilization – –Both crossing-over and the random fertilization of gametes contribute to genetic variation in sexually reproducing organisms. – –2 23 x 2 23 = 64 trillion combinations
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Crossing over
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Importance of Genetic Variation – –Genetic variation is essential for evolution to occur. –More genetic material available in population, the more the population of individuals can vary
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Section 1 In sexually reproducing eukaryotic organisms Meiosis in Males gametes form through the process of spermatogenesis. Meiosis in Females gametes form through the process of oogenesis Meiosis and Gamete Formation
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Meiosis in Males Spermatogenisis –Occurs in the testes –1 germ cell gives rise to 4 sperm cells
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Meiosis in females Oogenisis –Occurs in the ovaries –1 germ cell gives rise to 1 ovum
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Section 2 Sexual Reproduction Objectives: Differentiate between asexual and sexual reproduction. Identify three types of asexual reproduction. Evaluate the relative genetic and evolutionary advantages and disadvantages of asexual and sexual reproduction. Differentiate between the three major sexual life cycles found in eukaryotes.
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Section 2 Sexual Reproduction Sexual and Asexual Reproduction Asexual Reproduction Asexual reproduction is the formation of offspring from one parent. The offspring are genetically identical to the parent. Sexual Reproduction Sexual reproduction is the formation of offspring through the union of gametes from two parents. The offspring are genetically different from their parents.
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Types of Asexual Reproduction There are many types of asexual reproduction such as All types lead to clones of the parent. – –fission – –fragmentation – –Budding – –Sporolation
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Types of Asexual Reproduction Fission: Splitting into two –Bacteria, paramecium Fragmentation: breaking off of pieces from parent grow into new organism. –Some worms, plants and cyanobacteria
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Budding: parent grows bud that may or may not break off and grow into new offspring. –Hydra, yeast, some plants Sporolation: produce spores during meiosis that are released and grow into new offspring without having to pair with another cell. –Fungi, non- flowering plants
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Section 2 Advantages - Disadvantages Genetic Diversity Whereas 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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Evolution of Sexual Reproduction Sexual reproduction may have begun as a mechanism to repair damaged DNA. By recombining genes, enzymes can use genteic material to repair errors or “ bad genes” in a population.
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