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I. Sexual reproduction. A. Comparison between sexual and asexual reproduction. B. An overview of human reproduction. 1. The human karyotype. a. 44 sex chromosomes per somatic cell. b. 2 sex chromosomes per somatic cell. 2. Germ cells give rise to gametes via meiosis. 3. Gametes are haploid (egg & sperm). 4. 2 haploid gametes fuse to make a diploid zygote. C. Other sexual cycles. 1. Fungi & some algae have a diploid zygote and a haploid multicellular form. 2. Alternation of generation in plants & some algae, multicellular haploid gametophyte & multicellular diploid sporophyte. II. Meiosis. A. Overview. 1. S phase occurs before meiosis same as for mitosis. 2. Meiosis the products will be haploid and there is enough DNA for 4 haploid cells. 3. It takes 2 divisions to divide the DNA among four cells. 4. The two division are called meiosis I and meiosis II. 5. Meiosis frequently takes days to complete. B. Stages of meiosis. 1. Prophase I a. Homologues line up next to each other in a process called synapsis. b. Each set of homologues comprises 4 sister chromatids and is referred to as a tetrad. c. During synapsis portions of the sister chromatids may be exchanged in what is called crossing over. d. This is the longest stage of meiosis. 2. Metaphase I. a. Tetrads line up at the spindle equator. b. One homologue is nearer one pole and the other to the other pole. 3. Anaphase I. a. Centromeres remain intact. b. Homologues of each tetrad migrate to opposite poles. 4. Telophase I a. Each group of chromosomes is organized into a new nucleus. b. Chromosome number of each new cell is now haploid and the DNA has already been replicated. 5. Meiosis II. a. Is essentially the mitotic division of each of the haploid cells. b. The final product is four haploid nuclei. III. Differences between Mitosis and meiosis. Mitosis Meiosis Occurs in N and 2N cells. Occurs in diploid cells. Nucleus divides once. Nucleus divides twice. No synapsis. Synapsis, tetrads, cross over. Sets of sister chromatids line Sets of sister chromatids line up singly at metaphase. up as tetrads. Sister chromatids separate. Homologues separate at first. Produces 2 daughter nuclei of Produces 4 daughter cells with same ploidy as parent cell. haploid nuclei. IV. Genetic recombination. A. Independent assortment of chromosomes (Explain using 3 chromosomes). B. Crossing over. 1. Occurs while chromosomes are in synapsis at prophase I. 2. Sister chromatids cross each other; sections break off and rejoin to the opposite strand. 3. Occurs anywhere along the chromosome and is amazingly precise. 4. Locations where crossing over is occurring are called chiasmata. 5. Mediation of crossing over seems to be by recombination nodules (composed of protein). 6. Each pair of chromatids form 1 to several chiasmata. 7. Does not occur during prophase II and only rarely during mitosis. C. Additional recombination comes from random fusion of gametes. V. Abnormal meiosis. A. Translocations are when all or a part of one chromosome attaches to another. B. Nondisjunction is when chromosomes fail to separate properly. 1. Results in a cell with too many or too few chromosomes. 2. Often a fatal condition, but embryos derived from such cells may survive. 3. A common nondisjunction is trisomy 21, where an individual ends up with 3 chromosome 21s. a. Results in Down's syndrome. b. Usually means mental retardation. c. Older women and/or older men produce children with trisomy 21 most often. 4. Sex chromosomes may also exhibit nondisjunction e.g. XXY or XXX.
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