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Saddleback BIO 3A - MITOSIS – Asexual Reproduction

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1Biology 3A Laboratory MITOSIS – Asexual Reproduction OBJECTIVE • To study the cell cycle and understand how, when and why cells divide. • To study and identify the major stages of cell division. • To relate the mitotic process to living organisms. • To understand the differences between plant and animal cell division. • To graphically illustrate the roles of cell division and cell elongation in root growth. • To learn how chromosomes are stained. INTRODUCTION Mitosis is a form of cell division that eukaryotic cells employ for growth, repair or regeneration of somatic (body) cells. Some eukaryotic cells can even use mitosis as a form of asexual reproduction to increase their numbers. Certain bacteria, cyanobacteria and many single-celled organisms reproduce by another form of mitosis called binary fission. The ability of cells to reproduce more of its own kind is a basic principle of the cell theory. Every cell in our body has a cell cycle (Figure 1) which is divided into two main phases: Interphase (non-dividing phase) and Mitotic phase (dividing phase). Interphase is the longest phase of the cell cycle. It comprises about 90% of the cell cycle and is subdivided into three phases: G1, S and G2. Most cells are arrested in the G1 phase of Interphase. During this G1 phase, cellular processes such as protein synthesis and metabolic activities occur. The S phase of interphase is when DNA replication takes place which results in the copying of the genetic information. The G2 (Gap) phase is another cellular growth phase in which the cell with its copied genetic information is preparing for the mitotic phases. Figure 1: A typical cell cycle. The Mitotic Phase is the division of the duplicated genetic material (karyokinesis) and can be subdivided into four phases: prophase, metaphase, anaphase and telophase. Prophase is longest of the mitotic phases. During this phase, the nucleoli disappears, the nuclear envelope fragments and begins to disappear. The chromosomal material (chromatin) condenses becoming more distinct and visible as chromosomes. Each duplicated chromosome consist of two sister chromatids that are held together at the centromere. During metaphase, the chromosomes move to the metaphase plate and line up single file at the center of the cell. In Anaphase, the centromeres break and the sister chromatids separate and move toward opposite poles of the cell. After separation, each chromatid is considered a chromosome. In Telophase, as the cell continues to elongate, the nuclear envelope and nucleoli reappear. Cytokinesis, the division of the cytoplasm, occurs in conjunction with2telophase to create two separate daughter cells. In animal cells, a cleavage furrow develops to pinch the cell into while in plants, the cell plate begins to form between the new daughter cells. At the end of karyokinesis and cytokinesis, each daughter cell has a single nucleus with the same number of chromosomes as the parent cell. A. PLANT AND ANIMAL MITOSIS Although the process of mitosis is the same for both plants and animals, there are differences in presence or absences of certain cellular structures. Telophase and cytokinesis occur in both plant and animal cells. However, plant cells form a cell plate during telophase, which will eventually separate the two daughter cells. Animal cells form a cleavage furrow when the chromosomes are near the poles. Animal cells also have centrioles (asters) which assist in the shortening of the microtubules during anaphase. When you are trying to locate and identify the various phases, not all of the cells will show the “typical” stages of mitosis. During the tissue preparation procedure, thin sections are taken through the whitefish blastula (early embryonic developmental stage) and the onion root tip in such a way that not all of the nuclear material may be present. If you encounter such a cell that does not contain any nuclear material, skip that cell and move on to one with distinct nuclear material present. Procedure A: 1. Obtain a prepared slide of an onion root tip (Allium sp.). Note: there will be several root tips on this slide. 2. Place a piece of white paper in front of the slide in order to notice the three lightly stained root tips. 3. Place the slide on the microscope stage and focus on the root tip under scanning power (4X). 4. Find the rounded root cap and move to the area just above the root cap. This is the region in the root where mitosis is occurring. 5. Identify and draw the 5 mitotic stages on your worksheet. Do not forget to label the appropriate components. 6. Obtain a prepared slide of a whitefish blastula. Note: there will be several thin sections through the blastula on this slide. 7. Place a piece of white paper in front of the slide in order to notice the three lightly pinkish stained blastula sections. 8. Repeat the process as before with the onion root tip. 9. Make a note of the differences between plant and animal mitosis. B. MITOTIC STAGE FREQUENCY During the cell cycle, a cell will spend only a portion of its time in a particular phase. Interphase is the longest of all the phases, while prophase is the longest of the mitotic phases. To understand the relative time a cell spends in a particular phase, you will determine the frequency for each of the five major phase of mitosis for your group, your cumulative lab section’s data and finally the cumulative data for all lab sections. By increasing the sample size (large population), this should increase the accuracy of the results.3The onion root tip is divided into three or more (or less) distinct regions: the region of cell division, the region of elongation and the region of maturation. However, there is no clear distinction between these regions. Keep in mind that the frequency of cell division will vary at different levels and in different tissues of the root tip. Some cells may even have elongation during interphase between successive divisions. Procedure B: 1. Work in pairs. 2. Obtain a prepared slide of an onion root tip (Allium spp.). Note: there will be several root tips on this slide. 3. Place the slide on the microscope stage and focus on the root tip under scanning power. 4. Find the rounded root cap and move to the area just above the root cap as before. 5. Focus on low power and scan for a section in which the chromosomes appear distinct. 6. Change to high


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