Introduction to Genetics: Meiosis Ch. 11 Ms. Haut

Introduction to Heredity Each organism must inherit a single copy of every gene from each of its “parents.” Offspring acquire genes from parents by inheriting chromosomes Inheritance is possible because: Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg

Actual passing on of genes depends on the behavior of chromosomes Chromosomes -organizational unit of hereditary material in the nucleus of eukaryotic organisms Contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule, or locus

Copyright Pearson Prentice Hall Chromosome Number Chromosome Number All organisms have different numbers of chromosomes. A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent. 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. Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. Chromosomes are the same size and shape and have the same genes in the same locations Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. For Drosophila, the diploid number is 8, which can be written as 2N=8. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Chromosome Number The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes. These cells are haploid. Haploid cells are represented by the symbol N. For Drosophila, the haploid number is 4, which can be written as N=4. Copyright Pearson Prentice Hall

Human Life Cycle Each human somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid) Karyotype: male Sex chromosomes: determine gender (XX; XY) Autosomes: non-sex chromosomes

Human Life Cycle Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y With 23 chromosomes, they are haploid Haploid sperm + haploid ova zygote (2n) fertilization 2n n haploid number: n = 23 diploid number: 2n = 46

Fertilization Sperm compete to fertilize the egg Only one will win Once sperm penetrates the egg, egg creates a barrier to prevent other sperm from entering Only sperm nucleus is released inside egg Union of nuclei form zygote (fertilized egg)

Chromosomal Inheritance http://stevemorse.org/genetealogy/dna_files/image020.jpg

Copyright Pearson Prentice Hall Phases of Meiosis Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis involves two divisions, meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis 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 Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. MEIOSIS I Prophase I MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis 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. Copyright Pearson Prentice Hall

Crossing Over In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids Creates new combinations of genes not seen in either parent

Copyright Pearson Prentice Hall Phases of Meiosis Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes. Copyright Pearson Prentice Hall

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

Copyright Pearson Prentice Hall Phases of Meiosis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. MEIOSIS I Telophase I and Cytokinesis 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 II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis 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. Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Phases of Meiosis MEIOSIS II Prophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. 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 Phases of Meiosis MEIOSIS II Metaphase II The chromosomes line up in the center 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 Phases of Meiosis MEIOSIS II 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 Phases of Meiosis MEIOSIS II Telophase II and Cytokinesis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Gametogenesis Spermatogenesis In male animals, meiosis results in four equal-sized gametes called sperm. Sperm cells made in testes (over 100 million each day in adult male) Takes over 3 weeks to complete 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 Gametogenesis Oogenesis In many female animals, only one egg results from meiosis. The other three cells, called polar bodies Egg cells made in ovaries Meiosis I takes place and process stops before birth (1-2 million cells at this stage; by puberty only 400,000 left) Meiosis II begins each month once egg cell is released from ovary 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

Formation of Polar Body Occurs because of uneven division of cytoplasm during cytokinesis Egg cells must contain enough supplies to synthesize all the products needed by the developing embryo until the embryo implants into the uterine wall.  Implantation does not occur until 7-10 days after fertilization.  http://www.ccs.k12.in.us/chsBS/kons/kons/image145.gif

Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. Copyright Pearson Prentice Hall

Comparing Mitosis and Meiosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis. Copyright Pearson Prentice Hall

Comparing Mitosis and Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes. Copyright Pearson Prentice Hall

Meiosis Quiz Wiz

Copyright Pearson Prentice Hall If the body cells of humans contain 46 chromosomes, a single sperm cell should have 46 chromosomes. 23 chromosomes. 92 chromosomes. between 23 and 46 chromosomes. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall During meiosis, the number of chromosomes per cell is cut in half through the separation of daughter cells. homologous chromosomes. gametes. chromatids. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall The formation of a tetrad occurs during anaphase I. metaphase II. prophase I. prophase II. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Compared to egg cells formed during meiosis, daughter cells formed during mitosis are genetically different, while eggs are genetically identical. genetically different, just as egg cells are. genetically identical, just as egg cells are. genetically identical, while egg cells are genetically different. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall In many female animals, meiosis results in the production of only 1 egg. 1 egg and 3 polar bodies. 4 eggs. 1 egg and 2 polar bodies. Copyright Pearson Prentice Hall