Chapter 10 Outline and Terms 10.1. Halving the Chromosome Number (p. 160) A. Sexual reproduction 1. Requires gamete formation and then fusion of gametes to form a zygote. 2. If gametes contained same number of chromosomes as body cells, doubling would soon fill cells. 3. Belgian cytologist found Ascaris egg and sperm contain two chromosomes; body cells contain four. B. Life Cycles Vary 1. Life cycle refers to all reproductive events between one generation and next. 2. A zygote always has a full or diploid (2n) number of chromosomes. 3. Mitosis is nuclear division that maintains a constant chromosome number (see Ch. 9). 4. Meiosis is nuclear division reducing chromosome number from diploid (2n) to haploid (n) number. 5. Haploid (n) number is half the diploid number of chromosomes. 6. Meiosis occurs at different points during life cycle of various organisms. (Fig. 10.A) a. In animals, it occurs during production of gametes; adult is diploid and gametes are haploid. b. In plants, meiosis produces spores that divide mitotically to become haploid generation; haploid generation produces gametes. c. In fungi and some algae, meiosis occurs after zygote formation; organism is haploid and produces haploid gametes. C. Chromosomes Come in Homologous Pairs
1. In a diploid cell, chromosomes occur as pairs. a. Each set of chromosomes is a homologous pair; each member is a homologous chromosome or homologue. b. Homologues look alike; they have same length and centromere position; have similar banding pattern. c. A locus (location) on one homologue contains the same types of gene which occur at the same locus on the other homologue. 2. Chromosomes duplicate just before nuclear division. a. Duplication produces two identical parts called sister chromatids, held together at centromere. b. Nonsister chromatids do not share the same centromere. 3. One member of each homologous pair is inherited from either male or female parent; one member of each homologous pair is placed in each sperm or egg. 10.2. How Meiosis Occurs (p. 162) A. Purpose of Meiosis 1. Meiosis keeps chromosome number constant across chromosome number. 2. Gametes have only one member of each homologous pair. B. Meiosis Has Two Divisions 1. Meiosis involves two nuclear divisions and produces four haploid daughter cells; each, therefore, has half the total number of chromosomes as the diploid parent nucleus. (Fig. 10.1) [transp. 61] 2. Meiosis I is the nuclear division at the first meiotic division. a. Prior to meiosis I, DNA replication occurs and each chromosome has two sister chromatids. b. During meiosis I, homologous chromosomes come together and line up (cause unknown) in synapsis.
3. Meiosis II c. During synapsis, the two sets of paired chromosomes lay alongside each other as bivalents or a tetrad. d. Crossing-over is an exchange of homologous segments between nonsister chromatids of bivalent during meiosis I; results in genetic recombinations. (Fig. 10.2) [micro. slide 28] e. After crossing-over occurs, sister chromatids of a chromosome are no longer identical. (Fig. 10.2) a. No replication of DNA needed between meiosis I and II because chromosomes were already doubled. b. During meiosis II, centromeres divide; daughter chromosomes derived as sister chromatids separate. c. Chromosomes in the four daughter cells have only one chromatid. d. Counting number of centromeres verifies that parent cells were diploid, each daughter cell is haploid. C. Crossing-Over Introduces Variation 1. Crossing-over results in exchange of genetic material between nonsister chromatids. 2. At synapsis, homologous chromosomes are held in position by a nucleoprotein lattice (the synaptonemal complex). (Fig. 10.2) 3. As the lattice of the synaptonemal complex breaks down at beginning of anaphase I, homologues are temporarily held together by chiasmata, regions where the nonsister chromatids are attached due to crossing-over. (Fig. 10.3) [micro. slide 29] 4. The homologues separate and are distributed to separate cells. 5. No DNA replication follows the first meiotic division. 6. Centromeres divide during meiosis II. 7. Due to crossing-over, daughter chromosomes derived from sister chromatids have different mix of genes.
10.3. Meiosis Has Phases (p. 164) A. Number of Phases 1. Both meiosis I and meiosis II have four phases: prophase, metaphase, anaphase and telophase. B. Prophase I (Fig. 10.4) [transp. 62] 1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments; centrosomes migrate away from each other; and spindle fibers assemble. 2. Homologous chromosomes undergo synapsis forming bivalents; crossing-over may occur at this time in which case sister chromatids are no longer identical. (Fig. 10.4) [transp. 62] 3. Chromatin condenses and chromosomes become microscopically visible. C. Metaphase I (Fig. 10.4) [transp. 62] 1. During prometaphase I, bivalents held together by chiasmata have moved toward the metaphase plate. 2. In metaphase I, there is a fully formed spindle and alignment of the bivalents at the metaphase plate. 3. Kinetochores are regions just outside centromeres; they attach to spindle fibers called kinetochore spindle fibers. 4. Bivalents independently align themselves at the metaphase plate of the spindle. 5. Maternal and paternal homologues of each bivalent may be oriented toward either pole. D. Anaphase I (Fig. 10.4) [transp. 62] 1. The homologues of each bivalent separate and move toward opposite poles. 2. Each chromosome still has two chromatids. E. Telophase I (Fig. 10.4) [transp. 62]
1. Only occurs in some species. 2. When it occurs, the nuclear envelope reforms and nucleoli reappear. F. Interkinesis 1. This period between meiosis I and meiosis II is similar to interphase of mitosis. 2. However, no DNA replication occurs. G. Meiosis II (Fig. 10.5) [transp. 63] 1. During metaphase II, the haploid number of chromosomes align at metaphase plate. 2. During anaphase II, centromeres divide and daughter chromosomes move toward the poles. 3. At the end of telophase II and cytokinesis, there are four haploid cells. 4. Due to crossing-over of chromatids, each gamete can contain chromosomes with different types of genes. 5. In animals, the haploid cells develop into gametes. 6. In plants, the daughter cells divide to produce a haploid adult generation. 7. In some fungi and algae, a zygote results from gamete fusion and immediately undergoes meiosis; therefore, the adult is always haploid. 10.4. Viewing the Human Life Cycle (p. 166) A. Life Cycle has Both Meiosis and Mitosis. (Fig. 10.6) 1. In males, meiosis is part of spermatogenesis, the production of sperm, and occurs in the testes. 2. In females, meiosis is part of oogenesis, the production of egg cells, and occurs in the ovaries. 3. After birth, mitotic cell division is involved in growth and tissue regeneration. B. Comparison of Meiosis with Mitosis (Fig. 10.7 [transp. 64]
1. Mitosis occurs more often because it allows growth and repair of body tissues in multicellular organisms; meiosis only occurs at certain times in the life cycle of sexually reproducing organisms. 2. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only one nuclear division; in meiosis there are two nuclear divisions. 3. There is no crossing-over in mitotic prophase; there is crossing-over in prophase I of meiosis. 4. Duplicated chromosomes align on metaphase plate in mitosis; bivalents align on the metaphase I plate. 5. Sister chromatids separate to form daughter chromosomes in anaphase of mitosis; homologous chromosomes separate in anaphase I of meiosis. 6. Meiosis II is like mitosis except the meiosis nuclei are haploid. 7. Mitosis produces two daughter cells; meiosis produces four daughter cells. 8. In mitosis, two daughter cells have same chromosome number as parent cell; in meiosis, four daughter cells are haploid. 9. In mitosis, the daughter cells are genetically identical to each other and to the parent cell; in meiosis, the daughter cells are not genetically identical to each other or to the parent cell. C. Spermatogenesis and oogenesis Produce the Gametes. 1. Spermatogenesis (Fig. 10.8) [transp. 65] a. In the testes of males, primary spermatocytes with 46 chromosomes divide meiotically to form two secondary spermatocytes, each with 23 duplicated chromosomes. b. Secondary spermatocytes divide to produce four spermatids, also with 23 daughter chromosomes. c. Spermatids then differentiate into sperm (spermatozoa). d. Meiotic cell division in males always results in four cells that become sperm. 2. Oogenesis (Fig. 10.8) [transp. 65]
10.5. Significance of Meiosis (p. 169) a. In the ovaries of human females, primary oocytes with 46 chromosomes divide meiotically to form two cells, each with 23 duplicated chromosomes. b. One of the cells, a secondary oocyte, receives most cytoplasm; the other cell, a polar body, disintegrates or divides again. c. A secondary oocyte begins meiosis II and then stops at metaphase II. d. At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where it may meet a sperm. e. If a sperm enters secondary oocyte, oocyte is activated to continue meiosis II to completion; result is a mature egg and another polar body, each with 23 daughter chromosomes. f. Polar bodies serve to discard unnecessary chromosomes and retain most of the cytoplasm in the egg. g. The cytoplasm serves as a source of nutrients for the developing embryo. A. Meiosis produces genetic variation. 1. Without meiosis, chromosome numbers would continually increase. 2. Meiosis ensures daughter cells receive one of each kind of gene; precisely halves the chromosome number. 3. Independent assortment provides 2 n possible combinations of chromosomes in daughter cells. 4. In humans with 23 haploid chromosomes, 2 n = 2 23 = 8,388,608 possible combinations. 5. Variation is added by crossing-over; if only one crossover occurs within each bivalent, 4 23 or 70,368,744,000,000 combinations are possible. 6. Fertilization also contributes to genetic variation; (2 23 ) 2 = 70,368,744,000,000 possible combinations without crossing-over.
7. With fertilization and crossing-over, (4 23 ) 2 = 4,951,760,200,000,000,000,000,000,000 combinations are possible. B. Advantages of Meiosis 1. Tremendous storehouse of genetic variation provides for adaptations to changing environment. 2. Asexual organisms depend primarily on mutations to generate variation.