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LabManual MeiosisLab 87 ! Cell Division Meiosis Materials Needed per class: 1 box of 24 microscope slides of meiosis 1 1 box of 24 microscope slides of meiosis 2 red and yellow popbead chromosome kits Objectives To become familiar with the process of meiosis and to be able to identify the principal phases of meiosis To understand how the process of meiosis is similar to mitosis and how it differs from mitosis Introduction The genetic information of a cell is encoded in DNA that, in eukaryotic organisms, is organized into linear chromosomes. All the cells of any species of organism contain a certain constant number (N) of different chromosomes. The chromosomes differ in that each carries genetic information for different traits. For humans, N = 23; each cell of the human body contains 23 different types of chromosomes. In the cells of most multicellular organisms, however, the chromosomes occur in pairs; human cells contain 46 chromosomes, two of each of the 23 different types. The pairs of chromosomes are called homologs, or homologous chromosomes, because they carry genetic information for the same traits. Cells containing pairs of chromosomes are said to be diploid (2N). During sexual reproduction a reproductive cell (gamete, egg) from the female parent fuses with a reproductive cell (gamete, sperm) from the male parent forming a zygote. From the zygote develops the new individual. Thus, through sexual reproduction an offspring receives genetic information from two different parents. However, in order to preserve the diploid number of the species from generation to generation, it is 88 ! necessary that each gamete contain only ONE set of chromosomes; each gamete must receive only one chromosome of each homologous pair. A special type of nuclear division, called meiosis, is required to produce cells with a single set of chromosomes. Cells produced by meiosis and containing only one set of chromosomes are said to be haploid (N). The haploid cells formed by meiosis in animals are gametes (egg and sperm) and in plants, are called spores. Many of the events of meiosis are similar to those of mitosis, and the same four phases (prophase, metaphase, anaphase, and telophase) are recognized. However, in meiosis there are two rounds of division producing a total of four cells. The two rounds of division are called meiosis I and meiosis II, and the individual phases are identified as prophase I, metaphase I, etc.; and prophase II, metaphase II, etc. Meiosis I is frequently called the Reduction Division, for it is in this division that the number of chromosomes is reduced by one half. The second division is sometimes referred to as the Mitotic Division as the events of this division very closely resemble those of mitosis. In this lab you will observe the events of meiosis as they occur in plants, specifically, as they occur in the production of microspores in lilies. Microspores are male spores which are produced in the male reproductive organ, the anther. Within the anther, special diploid cells called microspore mother cells divide meiotically to form 4 haploid cells, the microspores. Each microspore ultimately develops by mitosis into a pollen grain. INTERPHASE I. Before division of the nucleus is initiated, the nucleus is in interphase I, and the cell carries out all its normal metabolic activities. An intact nuclear membrane is visible, as is the nucleolus. The chromosomes are invisible and present as the chromatin net. During interphase I the strands of DNA duplicate forming sister chromatids held together by a centromere. At the appropriate time the cell is stimulated to undergo meiosis. Events of Meiosis I A. Prophase I The events of prophase I are the most complex and unique of any in meiosis. In prophase I of meiosis, unlike the prophase of mitosis, the homologous strands of chromatin are brought physically together. This process is called synapsis. How synapsis actually occurs is not under stood. However, it has been observed that the chromatin attaches to the nuclear membrane, and it is believed that the membrane may then function in aligning the homologs. After synapsing, the chromatin condenses into visible rods. The centromeric region of each chromosome is visible as a constricted region joining the two sister chromatids. Spindle fibers begin to form. The nucleolus disappears. At the end of prophase I the nuclear membrane disintegrates. B. Metaphase I The spindle matures. Spindle fibers attach to the pairs of chromosomes. The chromosomes become aligned on the equatorial plate in homologous pairs. C. Anaphase I 89 ! The homologous chromosomes separate: one chromosome of each pair is pulled to one pole; the other chromosome is pulled to the opposite pole. This separation is accomplished by shortening the spindle fibers attached to the centromeres. Sister chromatids do NOT separate at this time; they remain connected by the centromere. D. Telophase I During telophase I the chromosomes decondense and become invisible again; the spindle dis- appears; the nucleoli reappear; and two nuclear membranes are formed, one around each set of chromosomes. Meiosis I is complete. Procedure 1. Obtain 3 slides: one of early prophase; one of late prophase; and one labeled either division I or heterotypic division. On each slide under scanning power, you will observe six anthers in a ring around a single central ovary, the female reproductive organ. Notice that each anther is composed of four circular chambers called microsporangia, see Fig. 1. The microspore mother cells develop and undergo meiosis within these microsporangia. a. EARLY PROPHASE. Using low and high power observe that the nucleus is still visible, and that the genetic material is evident as very long, very thin threads. The threads are composed of sister chromatids connected by centromeres, but the individual chromatids are not visible. The homologs have not yet synapsed. b. LATE PROPHASE. Synapsis has occurred. Under low and high power observe that the chromatin strands, although still not completely condensed, appear much thicker than the previous slide. Occasionally the individual sister chromatids may be visible also. Notice that the nucleolus is no longer visible, and that the nuclear membrane is disintegrating. Figure 1 Cross Section of an Anther 90 ! Figure 2 Cells undergoing Meiosis I c. HETEROTYPIC DIVISION (DIVISION I). Observe cells in prophase, metaphase, anaphase and telophase of meiosis I. The cells within a sporangium appear to divide synchronously, so most of the cells in a single microsporangium will be in the same phase of division. Therefore, it will be necessary to examine the microsporangia of all the anthers in order to observe all the phases of meiosis I. 2. In Fig. 2, identify and label cells in each of the four phases of meiosis. CYTOKINESIS. During late anaphase and early telophase cytokinesis occurs. A cell plate forms in the center of the cell and is extended gradually until the cytoplasm is divided and two haploid daughter cells are formed. Procedure 1. Using low and high power locate cells in which the cell plate is beginning to form in the center of the cell. 2. Locate a cell in which cytokinesis has been completed. 3. In Fig. 2, identify and label cells which are undergoing cytokinesis. 91 ! INTERPHASE II. When telophase I is completed, the nuclei enter interphase II. In plants this phase is frequently very brief, with the cells promptly entering the second round of division. The process occurs separately in the two daughter cells, but usually synchronously in the two cells. The chromatin does NOT replicate during Interphase II of meiosis. The chromatin already occurs as sister chromatids connected by centromeres. Events of Meiosis II A. Prophase II In prophase II the chromatin condenses again to form conspicuous rod shaped chromosomes. Spindles reappear, and spindle fibers attach to the centromeres of the chromosomes. The nucleoli disappear. At the end of prophase II the nuclear membranes disappear. B. Metaphase II The chromosomes are jostled by the spindle fibers until the centromeres are lined up on the equatorial plate in SINGLE FILE. C. Anaphase II At the beginning of metaphase II replication of the centromeres is completed, and the two centromeres separate. Once the centromeres separate, the two former sister chromatids are independent chromosomes. They separate and migrate to opposite poles. D. Telophase II In telophase II a nuclear membrane forms around each of the sets of chromosomes; nucleoli reappear; the spindle fibers disappear; and the chromosomes decondense into invisible strands of chromatin. Meiosis II is complete. Procedure 1. Obtain a slide labeled either division II or homeotypic division. Some of these slides are longitudinal sections through a single anther rather than a cross section through six anthers. Under scanning power locate the two long microsporangial chambers. Cells undergoing meiosis II can be observed within these chambers. 2. Using low and high power locate cells in prophase, metaphase, anaphase, and telophase of meiosis II. Observe that the cells occur in pairs, the result of the first meiotic division. Observe also that the chromosomes, although condensed, appear thinner than in the early phases of meiosis I. Explain why this would be so. 3. In Fig. 3, identify and label cells in each of the four phases of meiosis II. 92 ! Figure 3 Cells Undergoing Meiosis II CYTOKINESIS. During late anaphase II and early telophase II cytokinesis occurs in both