1. Why is mitosis alone insufficient for the life cycle of sexually reproducing eukaryotes?

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1 Chapter 13: Meiosis and Sexual Life Cycles 1. Why is mitosis alone insufficient for the life cycle of sexually reproducing eukaryotes? 2. Define: gamete zygote meiosis homologous chromosomes diploid haploid 3. With a partner, do the chromosome dance for meiosis. Make sure that you distinguish between tetrads, chromosomes, and chromatids, and note at each stage the number of sister chromatids per chromosome. 4. Summarize the key events of prophase I. 5. In which phase do sister chromatids segregate? In which phase do homologous chromosomes segregate? 6. Compare and contrast meiosis and mitosis. 7. Draw zygotic meiosis, gametic meiosis, and sporic meiosis (alternation of generations). For each, note the organisms that use it. 8. Describe Miller s Ratchet, DNA repair, and the Red Queen hypotheses for sex. 1 of 5

2 Chapter 13: Meiosis and Sexual Life Cycles I. Different modes of reproduction require different types of cell division A. asexual reproduction creates offspring that are genetically identical to each other and to the parent cell (clones) 1. only mitotic cell division, or something very similar, is required 2. the parent may split, bud, or fragment; sometimes, this involves mitotic cell division with unequal partitioning during cytokinesis (cellular budding) 3. asexual reproduction is typically rapid and efficient compared to sexual reproduction B. sexual reproduction occurs when specialized sex cells called gametes fuse to form a single cell called a zygote 1. usually the gametes that fuse are produced by different individuals, but they may be produced by the same individual 2. in plants and animals the gametes are called the egg and the sperm 3. the offspring are not genetically identical to their parents 4. this genetic recombination may render the offspring better adapted to the environment than either parent, or it may be more poorly adapted than either parent 5. sexual reproduction must contain a mechanism to half the number of chromosomes at some point without such a mechanism, the number of chromosomes would double with each generation halving the chromosome number is accomplished through meiosis II. Diploid cells give rise to haploid cells during meiosis A. the somatic (body) cells of animals and higher plants are diploid cells 1. each chromosome in a diploid cell has a partner chromosome 2. the partners are called homologous chromosomes 3. one member of each pair came from the father (paternal homolog), and one from the mother (maternal homolog) 4. thus, for humans, the 46 chromosomes are in 23 pairs 5. most pairs of homologous chromosomes contain very similar, but not identical, genetic information in each member of the pair (more on this in the next unit, genetics) 6. sex chromosomes aren t strictly homologous (an X chromosome has different genes than a Y chromosome), but they act as if they are homologous during meiosis 7. a set of chromosomes (n) has one member for each homologous pair; a diploid cell has two complete sets (2n), while a haploid cell has one set (n) 8. sometimes, cells have extra sets (3n or more; polyploid cells common in plants, rare and usually fatal in animals) B. meiosis reduces chromosome number, producing up to 4 haploid cells from one diploid cell 2 of 5

3 1. meiosis has two successive cell divisions after only one DNA replication 2. the two cell divisions are called meiosis I and meiosis II homologous chromosomes separate during meiosis I sister chromatids separate during meiosis II meiosis is best understood by following the chromosomes (and their kinetochores) don t confuse homologous chromosomes with sister chromatids III. Meiosis I and meiosis II are each divided into prophase, metaphase, anaphase, and telophase (PMAT) with accompanying cytokinesis A. prophase I chromatin condenses to form chromosomes, and homologous chromosomes pair 1. the process of homologous chromosomes pairing lengthwise is called synapsis the resulting structure, with 4 total chromatids (two sisters from each homologous chromosome), is called a bivalent or a tetrad the tetrad is held together by a synaptonemal complex during early prophase I typically, enzymes cause breaks in the chromatids and genetic material may be exchanged between chromatids (crossing-over or genetic recombination) genetic recombination greatly increases the potential for genetic variation in offspring 2. as in mitosis, sister chromatids are held together at centromeres and have kinetochores, but their kinetochores are side-by-side and attach to spindle fibers from the same pole (thus both sister chromatids are later pulled to the same pole) 3. by the end of prophase I: the spindle has formed the nuclear membrane has vesicularized nucleoli have disintegrated homologous chromosomes are attached by their kinetochores to spindle fibers from opposite poles homologous chromosomes are held together only at chiasmata, the sites where crossing-over occurred B. metaphase I tetrads line up along the midplane of the cell the presence of tetrads (bundles with 4 total chromatids) is the key distinguishing feature of metaphase I of meiosis C. anaphase I homologous chromosomes separate and are moved toward opposite poles 1. each pole gets one set of homologous chromosomes 3 of 5

4 2. the initial maternal or paternal chromosome sets are mixed and distributed randomly (crossing-over largely blurs such identity anyway) D. telophase I after anaphase is completed, generally: 1. the spindle fibers disintegrate 2. the chromosomes partially decondense 3. nuclear membranes may form around the genetic material 4. cytokinesis occurs E. interkinesis, the period between meiosis I and meiosis II, varies in length and distinctiveness 1. interkinesis differs from interphase because there is no S phase (no DNA replication) 2. typically, interkinesis is brief (some cells skip it altogether) F. prophase II of meiosis is similar to prophase of mitosis, but is usually very short because the chromatin did not completely decondense after meiosis I G. metaphase II similar to metaphase of mitosis 1. chromosomes line up along the midplane of the cell 2. sister chromatids are connected by their kinetochores (now on opposite sides) to spindle fibers from opposite poles H. anaphase II like mitotic anaphase, sister chromatids segregate toward opposite poles I. telophase II much like mitotic telophase: 1. the spindle is disintegrated 2. the chromosomes decondense 3. nuclear membranes reform around the genetic material to form nuclei 4. nucleoli reappear, and interphase cellular functions resume 5. cytokinesis usually begins during telophase II and ends shortly thereafter IV. Summary: key differences between mitosis and meiosis A. mitosis has one DNA replication and one division, resulting in two genetically identical daughter cells; homologous chromosomes do not pair, do not cross-over, and are not segregated B. meiosis has one DNA replication but two divisions (reductive division), resulting in up to four genetically distinct daughter cells; in the process, homologous chromosomes pair (synapsis), cross-over (homologous recombination), and segregate during meiosis V. The products of meiosis can vary between sexes and between species 4 of 5

5 A. in many eukaryotes, the zygote undergoes meiosis, and most of the life cycle is spent as haploid cells; gametes are then produced by mitosis and fuse to make the zygote B. in animals, the somatic (body) cells are typically diploid, and special germ line cells undergo meiosis to form haploid gametes (gametogenesis) 1. male gametogenesis (spermatogenesis) typically produces 4 haploid sperm for each germ cell that undergoes meiosis 2. female gametogenesis (oogenesis) typically produces 1 haploid egg cell (ovum) for each germ cell that undergoes meiosis; the rest of the genetic material goes to polar bodies, cells that get little of the original cytoplasm and eventually die 3. a sperm and an egg fuse to make a diploid zygote, which gives rise to the multicellular animal C. plants and some algae have distinct alternation of generations 1. spores produced after meiosis divide (mitotically) into a multicellular haploid gametophyte, from which gametes are eventually derived 2. the zygote gives rise (through mitotic divisions) to a multicellular diploid sporophyte, which produces specialized cells that undergo meiosis to produce spores VI. The origin and maintenance of sex is an evolutionary puzzle A. Sexual reproduction dilutes the genes from the best adapted individuals, and thus must offer a significant advantage or asexual reproduction will win out B. Even when sex clearly benefits a population or species, it must directly benefit individuals or it will lose out via evolution C. There may be no single hypothesis that explains Why sex? for all sexually reproducing organisms D. Leading hypotheses for why sex occurs include Miller s Ratchet, DNA repair, and the Red Queen 1. Miller s Ratchet asexual populations tend to accumulate harmful mutations by chance over time, with no good way to get rid of them like turning a ratchet, you can t go back; recombination can overcome this 2. DNA Repair many species reproduce sexually only during times of stress; some types of DNA repair (such as fixing double-strand breaks) can only take place with a diploid cell, and that type of repair is most likely needed in times of stress (also, this allows these species to overcome Miller s Ratchet by simple recombination) 3. Red Queen sex allows for populations to store genetic diversity so that it is available for each generation; this only provides an advantage if the environment provides ever-changing physical and/or biological constraints; an evolutionary arms race between parasites and their hosts may be a key factor in producing such treadmill evolution 5 of 5

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