Sexual Reproduction: MeiosisPlants too…Reproduce SexuallyMeiosisAs a comparison: In Mitosis..In Meiosis:Haploid vs. DiploidSlide 7Mitosis vs. MeiosisThe Process of Meiosis: 2 Separate StepsMeiosis I: 4 StagesProphase I in Meiosis has a unique event -- the pairing of homologous chromosomes. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure.Late Prophase IMeiosis I: Metaphase IMeiosis I: Anaphase IMeiosis I: Telophase IMeiosis II:Meiosis II: Prophase IIMeiosis II: Metaphase IIMeiosis II: Anaphase IIMeiosis II: Telophase IIMeiotic ProductsMeiosis OverviewWhy Sex??GametogenesisGenes are located on ChromosomesFISH TechniqueSexual Reproduction:MeiosisChapt. 9Plants too…Reproduce SexuallyMeiosis•Cell division process in which the number of chromosomes is cut in half…•Results in the formation of gametes …such as- eggs and sperm •Gametes have ½ chromosomes of adult • Fusion of an egg and sperm results in a zygote • Zygote now has the same number of chromosomes as adultAs a comparison: In Mitosis..• All chromosomes are passed to each new cell…as chromatids• Each chromosome splits at its centromere region• In this example: 46 chromatids go to each new cellThis happens for each chromosome aboveIn Meiosis:During the first stage of Meiosis-Only one of each pair will go to each new cellThis is called the separation ofHomologous chromosomesEach new cell will then have 23 chromosomesHaploid vs. Diploid•Typically, each cell of your body has 46 chromosomes..23 from each parent•So, you have what we call a Diploid value of 46•Or, referred to as 2N = 46•Your gametes, however, have 1N values•1N = 23….This is a Haploid condition•All your normal body cells are diploid, only your gametes are haploidDiploid numbers of some organisms Homo sapiens (human) 46 Mus musculus (house mouse) 40Zea mays(corn or maize) 20Drosophila melanogaster (fruit fly) 8Xenopus laevis (South African clawed frog) 36Caenorhabditis elegans (microscopic roundworm) 12Saccharomyces cerevisiae (budding yeast) 32Canis familiaris (domestic dog) 78Arabidopsis thaliana (plant in the mustard family) 10Muntiacus reevesi (the Chinese muntjac, a deer) 23Muntiacus muntjac (its Indian cousin) 6Myrmecia pilosula (an ant) 2Parascaris equorum var. univalens (parasitic roundworm)2Cambarus clarkii (a crayfish) 200Equisetum arvense (field horsetail, a plant) 216The complete set of chromosomes in the cells of an organism is its karyotype. Text: pg 167Mitosis vs. Meiosis•Mitosis: each division gives 2 identical products2N Cell 2(2N) Cells 4(2N) Cells•Meiosis: 2 division steps which reduce the number of chromosomes in half2N Cell 4(1N) CellsThe Process of Meiosis: 2 Separate Steps•Meiosis I: Homologous chromosomes line up and then separate. In Meiosis 1, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in Meiosis that generates genetic diversity. •Meiosis II: Similar process to mitosisMeiosis I: 4 Stages•Prophase I: Replicated chromosomes condense and homologs join together in a foursome–Text pg. 171•Homologs may exchange entire regions of genes…Crossing over •Human female eggs remain in Meiosis I until puberty… 12-13 years Two homologous chromosomes come together…and may cross and exchange genesProphase I in Meiosis has a unique event -- the pairing of homologous chromosomes. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. Crossing-over may occur at this point. During crossing-over chromatids break and may be reattached to a different homologous chromosome.Prophase I in MeiosisLate Prophase I•The nuclear membrane disappears. •One kinetochore forms per chromosome rather than one per chromatid, •The chromosomes attached to spindle fibers begin to move.Meiosis I: Metaphase I•Each homologous set of chromosomes line up along cell center •Text pg. 168•Orientation on the metaphase plate is random…with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.Meiosis I: Anaphase I•Homologs are pulled apart to opposite poles•Chromosomes, each with two chromatids, move to separate poles.• Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. •Text pg. 168Meiosis I: Telophase I•Homologous chromosomes have fully separated•Results in a Haploid (1N) set of chromosomes at each pole•Nuclear envelopes may reform, or the cell may quickly start meiosis 2.•Text pg. 169http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.htmlMeiosis II: •Essentially a mitotic division of the products of Meiosis I that now separates the chromatids •Meiosis 2 is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. •Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.htmlMeiosis II: Prophase II•During Prophase II, nuclear envelopes (if re- formed during Telophase I) dissolve, and spindle fibers reform. •All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis. •Text pg. 168Meiosis II: Metaphase II•Metaphase II is similar to mitosis metaphase, with spindles moving chromosomes into the equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region.•Chromosomes align along center•Text pg. 168Meiosis II: Anaphase II•During Anaphase II, the centromeres split and the former chromatids are segregated into opposite sides of the cell.•Text pg. 168Meiosis II: Telophase II•Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.•End up with 4 haploid (1N) products•Text pg. 169Meiotic Products•Final products of meiotic division are:•4 cells containing a haploid set (1N) of chromosomes•These 1N cells become gametes in animals•But, in plants, they may grow into new 1N individuals. •Text pg