Investigation 7 Part 1: CELL DIVISION: MITOSIS

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1 Investigation 7 Part 1: CELL DIVISION: MITOSIS How do eukaryotic cells divide to produce genetically identical cells? BACKGROUND One of the characteristics of living things is the ability to replicate and pass on genetic information to the next generation. Cell division in individual bacteria and archaea usually occurs by binary fission. Mitochondria and chloroplasts also replicate by binary fission, which is evidence of the evolutionary relationship between these organelles and prokaryotes. Cell division in eukaryotes is more complex. It requires the cell to manage a complicated process of duplicating the nucleus, other organelles, and multiple chromosomes. This process, called the cell cycle, is divided into three parts: interphase, mitosis, and cytokinesis (Figure 1). In the first growth phase (G1), the cell grows and prepares to duplicate its DNA. In the synthesis phase (S), the chromosomes are replicated. In the second growth phase (G2), the cell prepares to divide. In mitosis, the duplicated chromosomes are separated into two nuclei. In most cases, mitosis is followed by cytokinesis, when the cytoplasm divides and organelles separate into daughter cells. This type of cell division is asexual and is important for growth, renewal, and repair of multicellular organisms. Cell division is tightly controlled by complexes made of several specific proteins. These complexes contain enzymes called cyclin-dependent kinases (CDKs), which turn on or off the various processes that take place in cell division. CDK partners with a family of proteins called cyclins. One such complex is mitosis-promoting factor (MPF), sometimes called maturation-promoting factor, which contains cyclin A or B and cyclin dependent kinase (CDK). (See Figure 2a.) CDK is activated when it is bound to cyclin, interacting with various other proteins that, in this case, allow the cell to proceed from G2 into mitosis. The levels of cyclin change during the cell cycle (Figure 2b). In most cases, cytokinesis follows mitosis.

2 Figure 5 illustrates how the chromosomes move during mitosis. It is important for your students to model how the duplicated chromosomes align, separate, and move into new cells. Part 1: Modeling Mitosis this will be done in class: Discuss with your group members The mitosis lab begins with a discussion section during which you ask your students to think about how they developed from a single-celled zygote to a 300-trillion-celled organism. How does the genetic information in a cell from your toe compare to the genetic information in a cell from your arm? After the students have had a sufficient time to discuss this question, ask the following questions: 1). What other purposes besides growth would require cell division? 2). How do cells divide? 3). What must happen to ensure successful cell division? 4). Write down the students answers on the whiteboard, but do not elaborate on their answers. The students will be expected to investigate these questions further throughout the lab. Part 2: How much time is in each stage of Mitosis Introduction All new cells come from previously existing cells. New cells are formed by the process of cell division, which involves both division of the cell s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis). There are two types of nuclear division: mitosis and meiosis. Mitosis typically results in new somatic (body) cells. Formation of an adult organism from a fertilized egg, asexual reproduction, regeneration, and maintenance or repair of body parts are accomplished through mitotic cell division. You will study mitosis in Exercise 3A. Where does one find cells undergoing mitosis? Plants and animals differ in this respect. In higher plants the process of forming new cells is restricted to special growth regions called meristems. These regions usually occur at the tips of stems or roots. In animals, cell division occurs anywhere new cells are formed or as new cells replace old ones. However, some tissues in both plants and animals rarely divide once the organism is mature. To study the stages of mitosis, you need to look for tissues where there are many cells in the process of mitosis. This restricts your search to the tips of growing plants, such as the onion root tip, or, in the case of animals, to developing embryos, such as the whitefish blastula.

3 I. Time for Cell Replication Overview To estimate the relative length of time that a cell spends in the various stages of cell replication, you will examine the meristematic region of a prepared slide of the onion root tip. The meristematic regions are the areas in plants undergoing active cell growth. The length of the cell cycle is approximately 24 hours for cells in actively dividing onion root tips. It is hard to imagine that you can estimate how much time a cell spends in each phase of cell replication from a slide of dead cells. Yet this is precisely what you will do in this part of the lab. Since you are working with a prepared slide, you cannot get any information about how long it takes a cell to divide. What you can determine is how many cells are in each phase. From this, you can infer the percent of time each cell spends in each phase. Procedure: 1. Obtain Mitosis Cards. 2. Count and record the number of cells in each stage. Count until you have reached a total of 200 cells. 3. Record your data in Table 1 Table 1 Field Total Percent of total cells Time in each stage Prophase Metaphase Anaphase Telophase Total Calculate the percentage of cells in each phase. Consider that it takes, on average, 24 hours (or 1,440 minutes) for onion root-tip cells to complete the cell cycle. You can calculate the amount of time spent in each phase of the cell cycle from the percent of cells in that stage. Percent of cells in stage X 1,440 minutes = minutes of cell cycle spent in stage

4 Questions 1. If your observations had not been restricted to the area of the root tip that is actively dividing, how would your results have been different? 2. Based on the data in Table 1, what can you infer about the relative length of time an onion root-tip cell spends in each stage of cell division? 3. Explain how mitosis leads to two daughter cells, each of which is diploid and genetically identical to the original cell. What activities are going on in the cell during interphase? 4. How does mitosis differ in plant and animal cells? How does plant mitosis accommodate a rigid, inflexible cell wall? 5. What is the role of the centrosome (the area surrounding the centrioles)? Is it necessary for mitosis? Defend your answer.

5 Lab 7 Part B Cell Division: Mitosis and Meiosis - Chi- Squared (χ 2 ) Analysis of Data Tip Treatment Number of Cells Prophase Metaphase Anaphase/Telophase Total Control Lectin mm caffeine Null Hypothesis - Table 1 Compiled Class Data for Untreated Cells and Cells Treated with Lectin Group Number of Cells Total Control Treated - Lectin Table 2 Percentage of Cells Expect in Each Phase Cells # of Control Cells % of Control Cells Use the % above to calculate the number of treated cells expected in each phase if your null hypothesis is correct. Table 3 Chi- Squared Analysis χ 2 = Σ o e 2 e Cells # Observed (o) # Expected (e) (o- e) (o- e) 2 (o- e) 2 /e Compare your chi- square value to the critical value for the appropriate degrees of freedom. Determine whether your null hypothesis is rejected or not rejected. χ 2 =

6 Null Hypothesis - Table 1 Compiled Class Data for Untreated Cells and Cells Treated with 1 mm caffeine Group Number of Cells Total Control Treated - Caffeine Table 2 Percentage of Cells Expect in Each Phase Cells # of Control Cells % of Control Cells Use the % above to calculate the number of treated cells expected in each phase if your null hypothesis is correct. Table 3 Chi- Squared Analysis χ 2 = Σ o e 2 e Cells # Observed (o) # Expected (e) (o- e) (o- e) 2 (o- e) 2 /e Compare your chi- square value to the critical value for the appropriate degrees of freedom. Determine whether your null hypothesis is rejected or not rejected. χ 2 =