Bios 208 1st Edition Lecture 29 Outline of Last Lecture I. Living OrganismsII. Comparison of Asexual and Sexual ReproductionIII. Fertilization and meiosis alternate in sexual life cyclesIV. Sets of Chromosomes in Human CellsOutline of Current Lecture I. Behavior of Chromosome Sets in the Human Life CycleII. The Variety of Sexual Life CyclesIII. The Stages of MeiosisIV. A Comparison of Mitosis and MeiosisV. Sources of Variation in sexual life cyclesCurrent LectureI. Behavior of Chromosome Sets in the Human Life CycleA. Fertilization is the union of gametes (the sperm and the egg)B. The fertilized egg is called a zygote and has one set of chromosomes from each parentC. The zygote produces somatic cells by mitosis and develops into an adultD. At sexual maturity, the ovaries and testes produce haploid gametesE. Gametes are the only types of human cells produced by meiosis, rather than mitosisF. Meiosis results in one set of chromosomes in each gameteG. Fertilization and meiosis alternate in sexual life cycles to maintain chromosome numberII. The Variety of Sexual Life CyclesA. The alternation of meiosis and fertilization is common to all organisms that reproduce sexuallyB. The three main types of sexual life cycles differ in the timing of meiosis and fertilizationC. Gametes are the only haploid cells in animalsD. They are produced by meiosis and undergo no further cell division before fertilizationE. Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organismF. Plants and some algae exhibit an alternation of generationsG. This life cycle includes both a diploid and haploid multicellular stageThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.H. The diploid organism, called the sporophyte, makes haploid spores by meiosisI. Each spore grows by mitosis into a haploid organism called a gametophyteJ. A gametophyte makes haploid gametes by mitosisK. Fertilization of gametes results in a diploid sporophyteL. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stageM. The zygote produces haploid cells by meiosisN. Each haploid cell grows by mitosis into a haploid multicellular organismO. The haploid adult produces gametes by mitosisP. Depending on the type of life cycle, either haploid or diploid cells can divide by mitosisQ. However, only diploid cells can undergo meiosisR. In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspringS. Meiosis reduces the number of chromosome sets from diploid to haploidT. Like mitosis, meiosis is preceded by the replication of chromosomesU. Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis IIV. The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosisW. Each daughter cell has only half as many chromosomes as the parent cellIII. The Stages of MeiosisA. After chromosomes duplicate, two divisions followB. Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomesC. Meiosis II (equational division) sister chromatids separateD. The result is four haploid daughter cells with un-replicated ChromosomesE. Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatidsF. The sister chromatids are genetically identical and joined at the centromereG. The single centrosome replicates, forming two centrosomesH. Division in meiosis I occurs in four phasesa) Prophase Ib) Metaphase Ic) Anaphase Id) Telophase I and cytokinesisI. Prophase Ia) Prophase I typically occupies more than 90% of the time required for meiosisb) Chromosomes begin to condensec) In synapsis, homologous chromosomes loosely pair up, aligned gene by gened) In crossing over, non-sister chromatids exchange DNA segmentse) Each pair of chromosomes forms a tetrad, a group off our chromatidsf) Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred J. Metaphase Ia) In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each poleb) Microtubules from one pole are attached to the kinetochore of one chromosome of each tetradc) Microtubules from the other pole are attached to the kinetochore of the other chromosomeK. Anaphase Ia) In anaphase I, pairs of homologous chromosomes separateb) One chromosome moves toward each pole, guided by the spindle apparatusc) Sister chromatids remain attached at the centromere and move as one unit toward the poleL. Telophase I and Cytokinesisa) In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatidsb) Cytokinesis usually occurs simultaneously, forming two haploid daughter cellsc) In animal cells, a cleavage furrow forms; in plant cells, a cell plate formsd) No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because thee) Chromosomes are already replicatedM. Division in meiosis II also occurs in four phasesa) Prophase IIb) Metaphase IIc) Anaphase IId) Telophase II and cytokinesisN. Meiosis II is very similar to mitosisO. Prophase IIa) In prophase II, a spindle apparatus formsb) In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plateP. Metaphase IIa) In metaphase II, the sister chromatids are arranged at the metaphase plateb) Because of crossing over in meiosis I, the two sister chromatids of each chromosomeare no longer genetically identicalc) The kinetochores of sister chromatids attach to microtubules extending from opposite polesQ. Anaphase IIa) In anaphase II, the sister chromatids separateb) The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite polesR. Telophase II and Cytokinesisa) In telophase II, the chromosomes arrive at opposite polesb) Nuclei form, and the chromosomes begin decondensingS. Cytokinesis separates the cytoplasmT. At the end of meiosis, there are four daughter cells, each with a haploid set of un-replicated chromosomesU. Each daughter cell is genetically distinct from the others and from the parent cellIV. A Comparison of Mitosis and MeiosisA. Mitosis conserves the number of chromosome sets, producing cells that are genetically