Copyright Pearson Prentice Hall Meiosis biology Sexual Reproduction Copyright Pearson Prentice Hall

Sperm and ova (egg) are created! Sperm and ova are haploid Phases of Meiosis What happens during the process of meiosis? Sperm and ova (egg) are created! Sperm and ova are haploid Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Chromosome Number Chromosome Number All organisms have different numbers of chromosomes. Human = 46 Carrot = 18 Dog = 78 Goldfish = 94 These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Chromosome Number *These two sets of chromosomes are homologous. One chromosome from mom and one chromosome from dad. Copyright Pearson Prentice Hall

Homologous Chromosomes *Def: Pair of chromosomes that contain the same alleles for traits and phenotypes EX: chromosome with eye color allele from mom is homologous to chromosome with eye color allele from dad Dad (2,4) Mom (1,3)

Chromosome Number QUIZ How many chromosomes are in the picture? 46 How many homologous chromosomes are in the picture? 23

Telophase I and Cytokinesis Phases of Meiosis I Meiosis I Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Interphase I centrioles DNA copies itself Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes. nucleus Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Prophase I Nuclear membrane disappears. Homologous chromosomes pair up to form a tetrad. Tetrad = 4 sister chromatids Crossing over occurs MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Copyright Pearson Prentice Hall

Which pair up to make homologous chromosomes? Pink with pink; yellow with yellow; blue with blue How many pairs of homologous chromosomes are shown? 3 How many chromosomes are shown? 6

CROSSING OVER *Homologous chromosomes pair up One sister chromatid breaks off and exchanges itself with a sister chromatid from the other homologous chromosome. Results in variation! Nature likes this because some will be more fit than others. Occurs randomly at different spots on sister chromatid each time of meiosis. That is why you don’t look exactly like your siblings.

Does crossing over always occur at the same allele? When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles. Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged. Does crossing over always occur at the same allele? NO! It is random Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Metaphase I Homologous chromosomes line up in middle of cell Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Anaphase I The fibers pull the homologous chromosomes apart toward opposite ends of the cell. MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Telophase I Nuclear membranes form. The cell separates into two cells. MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis Meiosis involves two divisions *meiosis I *meiosis II Copyright Pearson Prentice Hall

Telophase II and Cytokinesis Phases of Meiosis II Meiosis II During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Meiosis II Telophase I and Cytokinesis I Metaphase II Anaphase II Telophase II and Cytokinesis Prophase II Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Prophase II Nuclear membrane disappears. MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Metaphase II Individual chromosomes line up in the middle of cell. MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Anaphase II The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Telophase II Meiosis II results in four non-identical haploid cells. MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Meiosis Why is meiosis important? Creates gametes that are HAPLOID. Due to crossing over, meiosis also creates variation. Copyright Pearson Prentice Hall