Mitosis and Meiosis. Part I Mitosis

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1 Mitosis and Meiosis Name Date Part I Mitosis It was discovered in 1858, by Rudolf Virchow, that new cells can only arise from previously existing cells. This is done in two ways: mitosis and meiosis. Body cells divide exclusively by mitosis followed by cytokinesis, while germ cells produce gametes by the process of meiosis. Plant cells grow by enlargement, essentially by taking up water. When they reach a certain size, they divide, forming two identical daughter cells. The various parts of the cell are divided in such a way that the new daughter cell is identical to the parent cell. Strictly speaking, mitosis implies only the division of the nucleus, and is therefore distinct from cell division, in which the cytoplasm is divided. In most organisms, cells divide by ingrowth of the cell wall, if present, and the contraction of the cell membrane, a process that cuts through the spindle fibres. In land plants and a few algae, cell division takes place by the formation of a cell plate. Small droplets appear across the equatorial plate of the cell and gradually fuse, forming a disc that grows outward until it reaches the wall of the dividing cell, which completes the separation of the two daughter cells. Mitosis, or nuclear division, ensures the equal division of the nuclear material between the daughter cells in organisms. During mitosis the chromosomes condense, and move to the centre of the cell where they fully contract. They then split longitudinally into two identical halves that appear to be pulled to opposite poles of the cell by a series of microtubules. In these two genetically identical groups, the coiling of the chromosomes relaxes again, and they are reconstituted into the nuclei of the two daughter cells. It is a continuous process that can be divided into five major phases: interphase, prophase, metaphase, anaphase, and telophase Interphase: The chromatin, if visible at all, can only be seen as small grains or threads. Interphase is generally considered to be a resting phase. However, the cell is replicating the genetic material, preparing for mitosis. Prophase: The beginning of mitosis is illustrated by the chromosomes gradually becoming visible. They start out as elongated threads that shorten and thicken. Chromosomes become more condensed and undergo spiral contractions, like a thin wire being turned into a coiled spring. This coiling involves the entire DNA protein complex. Each chromosome is composed of two longitudinal halves, called chromatids, joined in a narrow area known as the centromere, where the chromatids are not coiled. The centromere, located on each chromosome, divides the chromosomes into two arms of varying lengths. As prophase progresses, the nucleoli grow smaller and finally disappear. Shortly after, in most cell types, the nuclear envelope breaks down, putting the contracted chromosomes into direct contact with the cytoplasm; this marks the end of prophase.

2 Metaphase: The chromosomes, still doubled, become arranged so that each centromere is on the equatorial region of the spindle. Each chromosome is attracted to the spindle fibres by its centromere; often the arms of the chromosome point toward one of the two poles. Some of the spindle fibres pass from one pole to the other and have no chromosome attached. Anaphase: The chromatids separate from one another and become individual chromosomes. First, the centromere divides and the two daughter chromosomes move away from the equator toward opposite poles. Their centromeres, which are still attached to the spindle fibre, move first, and the arms drag behind. The two daughter chromosomes pull apart; the tips of the longer arms separate last. The spindle fibres attached to the chromosomes shorten as the chromatids divide and the chromosomes separate. The fibres appear to move, but in fact the microtubules are continuously formed at one end of the spindle fibre and disassembled at the other. In the process, it appears as if the spindle fibres were tugging the chromosomes toward the poles by their centromeres. By the end of anaphase, the two identical sets of chromosomes have separated and moved to opposite poles. Telophase: The separation is made final; the nuclear envelopes are organized around the two identical sets of chromosomes. The spindle apparatus disappears. Nucleoli also reform at this time. The chromosomes become increasingly indistinct, uncoiling to become slender threads again. Cytokinesis: As mitosis ends, cytokinesis begins, resulting in the formation of two daughter cells. The cleaved membrane slowly draws together; forming a narrow bridge, then separates the cell into two daughter cells. The cells now enter interphase. Mitosis ends when the processes are complete and the chromosomes have once more disappeared from view. The two daughter cells enter interphase. The two daughter nuclei produced are identical to one another and to the nucleus that divided to produce them.

3 In order to investigate the process of mitosis, plant and animal tissues where cells are dividing rapidly must be examined. In animals, the most rapidly growing and dividing tissues are found in the embryonic stages of development. Although most animal tissues continue to undergo mitosis throughout the life cycle of the organism, they do so very slowly when compared to their embryos. Some animal cells, like most plant tissues, rarely replicate after the organism reaches maturity. In plants, these tissues are primarily found in the tips of stems and roots. The root tip plants are exceptionally good places to look for cells undergoing mitosis. Plant root tips consist of several different zones where various developmental and functional processes of the root are performed. The primary region for the formation of new cells is the apical meristem. The root cap offers protection for the rest of the root, the region of elongation is the area where the bulk of cell growth occurs, and the region of maturation is where tissue differentiation occurs. Apical meristem Root tip Part II Meiosis Sexual reproduction provides a mechanism to produce genetic variation, as the genes of two different individuals are arranged in various ways. This requires a reduction in the chromosome number of the parent cell, normally diploid, to half that, or haploid. The type of cell division resulting in half the chromosome number of the parent cell is called meiosis. In meiosis, a cell divides into four haploid gametes. When two gametes egg and sperm combine during fertilisation, forming a zygote, the diploid chromosome number is restored. Meiosis consists of one DNA replication and two nuclear divisions, meiosis I and II. This results in the formation of four daughter cells, each with only half the number of chromosomes as the parent.

4 The example that will be used in the investigation is Sordaria fimicola, a fungus that is haploid for the bulk of its life cycle, including the individual fungal filaments, called hyphae, which normally exist in a mass called a mycelium representing the body of the fungus; and the ascospores, from which mycelia develop. The only diploid portion of the life cycle of S. fimicola occurs when the nuclei of specialized hyphae come together. These hyphae, which belong to different strains of the species, fuse to form a zygote. This zygote then undergoes meiosis to produce the haploid, yielding four haploid nuclei contained in a sac. After meiosis, the four nuclei undergo mitosis, resulting in eight haploid. Part I Mitosis MATERIALS Materials for Parts A and B - whitefish mitosis slide - onion mitosis slide - compound microscope Materials for Part C - onion root tip - Bunsen burner - hydrochloric acid 1M - forceps - toluidine blue 0.5% - paper towel - compound microscope - eye dropper - coverslip - scalpel - microscope slide PROCEDURE Part A: Observing Mitosis in Plant and Animal Cells 1. Observe the prepared microscope slide of onion root tip mitosis, first at 100X, then 400X. Using the Plant Mitosis Chart as a guide, identify cells that represent each mitotic phase. 2. In the Analysis section, draw each phase of plant cell mitosis that you see. Write a brief description of each phase below each drawing. 3. Observe the prepared microscope slide of whitefish blastula. Using the Animal Mitosis Chart as a guide, identify each phase of animal cell mitosis. 4. In the Analysis section, draw each phase of plant or animal cell mitosis that you see. Write a brief description of each phase below each drawing. Part B: Relative Lengths of Phases of Mitosis 5. Examine at least three fields of view of the apical meristem of the onion root tip at 400X. In each view, count the number of cells in the various stages of mitosis. Record this data in Table Calculate the total number of cells counted and the percentage of total cells counted for each stage of mitosis. Record this data in Table 1. Record the percentages in Table 2, as well. 7. Assuming that it takes an average of 24 hours (1,440 minutes) for onion root tip cells to complete the cell cycle, calculate the amount of time cells spent in each phase of the cycle. Use the formula provided below. Enter your results in Table 1. Percent of Cells in Phase 1,440 minutes = minutes cell spent in phase

5 Part II Meiosis MATERIALS - microscope slide -coverslip - inoculating loop - sordaria demonstration cross plate (shared) PROCEDURE 1. Place a drop of water on a clean slide with an inoculating loop. 2. With an inoculating loop, scrape several perithecia (fruiting body s of the sordaria) from the demonstration cross plate. 3. Scrape the perithecia and place in the drop of water on the slide. Avoid picking up agar along with perithecia; it will interfere with results. perithecia 4. Cover the slide with a coverslip. Using a pencil eraser or other blunt instrument, gently press down on the coverslip to squash and spread out the perithecia. The pressure should be sufficient to squeeze the asci (sexual spore-bearing cell) from the perithecia, but not enough to crush the asci themselves. Note: It may be helpful to slide the coverslip around on top of the sample, with slight pressure, to spread out the asci and make them easier to observe. Keep in mind, however, that applying too much pressure may rupture the asci, releasing the individual spores. 5. View the slide under a microscope at 100X. Locate the asci. You may wish to view the slide at 400X to determine the colour of some spores. The slide preparation should show collapsed perithecia and asci clusters (rosettes), with mature spores in various arrangements. Immature spores will all be light coloured. Since S. fimicola is homothallic (having male and female reproductive structures), the preparation will show both hybrid and self-fertilized perithecia of both parental types asci

6 ANALYSIS

7 Stage Interphase # of cells in Field 1 # of cells in Field 2 Table 1 Cells in each stage of mitosis # of cells in Field 3 Total # of cells % of Total # of cells Time of Each stage (min) Prophase Metaphase Anaphase Telophase Total number of cells counted Interphase Prophase Metaphase Anaphase Telophase Total Stage Table 2 Percentage of cells in each stage of mitosis % of Total # of cells QUESTIONS 1. Referring to the percentage of total cells counted in each phase of mitosis, determine which phase takes the longest for the cell to complete, and explain why. Sketch a pie graph of the percentage of cells in each phase to illustrate. Be sure to title your graph and include a key. 2. What is the relationship between the processes of mitosis and cytokinesis?

8 3. Which of the following is significantly different between plant and animal cell mitosis? a. Metaphase b. Anaphase c. Cytokinesis d. Prophase 4. Define the following terms. somatic cell chromatin centromere diploid haploid zygote 5. Create a Venn diagram showing at least two similarities and two differences between mitosis and meiosis.

9 ANIMAL CELL MITOSIS Interphase Prophase Metaphase Anaphase Telophase Cytokinesis

10 PLANT CELL MITOSIS Interphase Prophase Metaphase Anaphase Telophase Cytokinesis