MT: Sexual Reproduction Understand that homologous chromosomes separate randomly during meiosis to produce haploid gametes. Copyright Pearson Prentice Hall

What is happening here? Fertilization! Copyright Pearson Prentice Hall

= when a male and female gamete join fertilization = when a male and female gamete join Copyright Pearson Prentice Hall

Chromosome # Most organisms are diploid and thus want an offspring that is also diploid So, each gamete has to be haploid so when they join they create a diploid organism Copyright Pearson Prentice Hall

½ + ½ = 1 haploid + haploid = diploid OR Female gamete + male gamete = offspring ½ + ½ = 1 Copyright Pearson Prentice Hall

Find chromosome # Human __ chromosomes Human 46 chromosomes HAPLOID # Human __ chromosomes Dog __ chromosomes Carrot __ chromosomes Goldfish __ chromosomes Wheat __ chromosomes Tobacco __ chromosomes DIPLOID # Human 46 chromosomes Dog 78 chromosomes Carrot 18 chromosomes Goldfish 94 chromosomes Wheat 42 chromosomes Tobacco 48 chromosomes Copyright Pearson Prentice Hall

What happens during the process of meiosis? making of non-identical haploid gametes Copyright Pearson Prentice Hall

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

Interphase 1 DNA copies itself centrioles nucleus Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes. nucleus Copyright Pearson Prentice Hall

Prophase I Homologous chromosomes pair up to form a tetrad. 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

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

Homologous chromosomes line up in middle of cell Metaphase 1 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

Anaphase 1 The fibers pull the homologous chromosomes 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

The cell separates into two cells. Telophase 1 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

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

Nuclear membrane disappears. 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

Individual chromosomes line up in the middle of cell. 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

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

Meiosis II results in four non-identical haploid cells. 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

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

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

Websites to check out 1. http://www.cellsalive.com/meiosis.htm 1. http://www.cellsalive.com/meiosis.htm 2. http://www.pbs.org/wgbh/nova/baby/divi_flash.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html 4. http://www.youtube.com/watch?v=uh7c8YbYGqo Copyright Pearson Prentice Hall