End Show Slide 1 of 35 Copyright Pearson Prentice Hall Meiosis, fertilization & chromosomal abnormalities Chapter 11-4 and 14-2 End Show 11-4 Meiosis Slide 2 of 35 Copyright Pearson Prentice Hall Each organism must inherit a single copy of every gene from each of its parents. fertilization Egg Sperm Zygote fertilized Egg End Show 11-4 Meiosis Slide 3 of 35 Copyright Pearson Prentice Hall Gametes are formed by a process of meiosis that separates the two sets of genes so that each gamete ends up with just one set. meiosis End Show 11-4 Meiosis Slide 4 of 35 Copyright Pearson Prentice Hall 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 fathers sperm 4 from its mothers egg These two sets of chromosomes are homologous pairs. End Show 11-4 Meiosis Slide 5 of 35 Copyright Pearson Prentice Hall Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid (2N). The gametes of sexually reproducing organisms are haploid (N), they contain a single set of chromosomes For Drosophila (fruit fly) the diploid number is 8 2N=8. For Drosophila (fruit fly) the haploid number is 8 N=4. End Show 11-4 Meiosis Slide 6 of 35 Copyright Pearson Prentice Hall Phases of Meiosis What happens during the process 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. Homologous pair End Show Slide 7 of Meiosis Copyright Pearson Prentice Hall 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. Phases of Meiosis End Show Slide 8 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Interphase I Meiosis I 1 diploid cell 2 Haploid cells End Show 11-4 Meiosis Slide 9 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I End Show 11-4 Meiosis Slide 10 of 35 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 I Prophase I End Show Slide 11 of Meiosis 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. End Show 11-4 Meiosis Slide 12 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I End Show 11-4 Meiosis Slide 13 of 35 Copyright Pearson Prentice Hall Phases of Meiosis MEIOSIS I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell. End Show 11-4 Meiosis Slide 14 of 35 Copyright Pearson Prentice Hall Independent Assortment of Chromosomes The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes This results in 2 n possible combinations of gametes End Show 11-4 Meiosis Slide 15 of 35 Copyright Pearson Prentice Hall Independent Assortment of Chromosomes Mosquito's 2N =6; N=3 2 n = ? different gametes For humans 2 n = 2 23 = 8 million possible combinations End Show 11-4 Meiosis Slide 16 of 35 Copyright Pearson Prentice Hall Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis 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. End Show Slide 17 of Meiosis 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 cells chromosomes has 2 chromatids. End Show Slide 18 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis II Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I Meiosis II 2 Haploid cells 4 Haploid cells End Show 11-4 Meiosis Slide 19 of 35 Copyright Pearson Prentice Hall Phases of Meiosis Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. MEIOSIS II Prophase II End Show 11-4 Meiosis Slide 20 of 35 Copyright Pearson Prentice Hall Phases of Meiosis The chromosomes line up in the center of cell. MEIOSIS II Metaphase II End Show 11-4 Meiosis Slide 21 of 35 Copyright Pearson Prentice Hall Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II End Show Slide 22 of Meiosis Copyright Pearson Prentice Hall Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis End Show 11-4 Meiosis Slide 23 of 35 Copyright Pearson Prentice Hall Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. End Show 11-4 Meiosis Slide 24 of 35 Copyright Pearson Prentice Hall Gamete Formation End Show 11-4 Meiosis Slide 25 of 35 Copyright Pearson Prentice Hall End Show 11-4 Meiosis Slide 26 of 35 Copyright Pearson Prentice Hall After Meiosis: Random Fertilization 1 haploid sperm randomly fertilizes a randomly released haploid egg diploid zygote. Fruit fly math Human math why are the chances less than 1 out of 64,000,000,000,000 that your parents would have two identical children (NOT twins) End Show 11-4 Meiosis Slide 27 of 35 Copyright Pearson Prentice Hall Review: What are the 3 key sources of genetic variation? Crossing over of homologous pairs during prophase I Independent assortment of homologous pairs during Metaphase I Random fertilization End Show 142 Human Chromosomes Slide 28 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders What problems does nondisjunction cause? The most common error in meiosis occurs when homologous chromosomes fail to separate. This is known as nondisjunction, which means, not coming apart. End Show 142 Human Chromosomes Slide 29 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes if that gamete is used to in sexual reproduction the resulting individual will have an abnormal number of chromosomes. End Show 142 Human Chromosomes Slide 30 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Nondisjunction In Meiosis IIn Meiosis II http://www.sumanasinc.com/webcontent/animations/ content/mistakesmeiosis/mistakesmeiosis.swfhttp://www.sumanasinc.com/webcontent/animations/ content/mistakesmeiosis/mistakesmeiosis.swf End Show 142 Human Chromosomes Slide 31 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Down Syndrome If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome. In most cases abnormal chromosome number results in spontaneous abortion long before birth. Down syndrome involves three copies of chromosome 21. End Show 142 Human Chromosomes Slide 32 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Intellectual disabilityIntellectual disability. Most children with Down syndrome have mild to moderate cognitive disability. Heart defectsHeart defects. About half of the children who have Down syndrome are born with a heart defect. Physical features A flat facial profile An upward slant to the eye A short neck Abnormally shaped ears A single, deep transverse crease on the palm of the hand Smaller than average size End Show 142 Human Chromosomes Slide 33 of 25 Copyright Pearson Prentice Hall End Show 142 Human Chromosomes Slide 34 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Sex Chromosome Disorders Nondisjunction in the sex chromosomes has less of an affect on survival End Show 142 Human Chromosomes Slide 35 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders Sex Chromosome Disorders In females, nondisjunction can lead to Turners syndrome. A female with Turners syndrome usually inherits only one X chromosome (karyotype 45,X). Women with Turners syndrome are sterile. End Show 142 Human Chromosomes Slide 36 of 25 Copyright Pearson Prentice Hall Chromosomal Disorders In males, nondisjunction causes Klinefelters syndrome (karyotype 47,XXY). The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing. End Show 11-4 Meiosis Slide 37 of 35 Copyright Pearson Prentice Hall Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. MitosisBothMeiosis