AP Biology Lab #3: Mitosis Name Date Period Introduction and Overview All new cells come from preexisting cells. New cells are formed by the process of cell division which involves both replication of the cell s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis). There are two types of cell division, mitosis and meiosis. Mitosis typically results in new somatic (body) cells which are diploid in chromosome number. 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. 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 an organism is mature. In this lab you will observe onion root tips and whitefish blastula under the microscope to observe the stages of mitosis (3A). You will also calculate the relative duration of the phases of the cell cycle in the meristem tissue of the onion root (3B). Exercise 3A-1: Observing Mitosis in Plant and Animal Cells using Prepared Slides. Roots consist of different regions (figure 1). The root cap functions in protection. The apical meristem is the region that contains the highest percentage of cells undergoing mitosis. The zone of elongation is the area in which growth occurs. The zone of maturation is where the cells differentiate to become xylem, phloem, and other tissues. Whitefish blastula (figure 2) is often used for the study of cell division. As soon as the egg is fertilized it begins to divide, and nuclear division after nuclear division follows. Procedure Examine the prepared slides of both the onion root tip and the whitefish blastula. First locate the meristematic region of the onion or blastula of the whitefish with the scanning objective (4x). Then focus on the image using the low power (10x) objective and then finally the high power (40x) to view individual cells. Find a field of view in which the cells are clearly in focus, many nuclei are visible and the cells seem to be in various stages of the cycle. Identify a cell that is representative of each phase and draw the image in Table 1. You must draw a cell in each stage for both the plant cell and the animal cell. Then list the major events that are characteristic of that stage. 1
Table 1. Microscope Observations of Cells undergoing Cell Division Labeled Drawing Stage of Cell Cycle Plant Cell (onion root tip) Animal Cell (whitefish blastula) Major Events Occuring Interphase Mitosis Prophase Metaphase Anaphase Telophase Cytokinesis 2
Analysis Questions: 1. How does mitosis differ in plant and animal cells? How does plant mitosis accommodate a rigid, inflexible cell wall? 2. What is the role of the centrosome? Is it necessary for mitosis? Defend you answer. 3. What is the restriction point in mammalian cells? 4. What is the G 0 phase of the cell cycle? How and why do cells enter this phase? 5. What are MPFs, Cdks and cyclins? What is the relationship among these molecules in controlling the cell cycle? 6. Can a cell that is normally non-dividing be stimulated to divide? When does this happen and how is it accomplished in an organism? 3
7. Cancer cells do not respect their neighbors. Describe what this means in more technical terminology. How is this different than normal cell functioning? 3A-2: Time for Cell Replication. To estimate the relative length of time that a cell spends in the various stages of cell division, you will examine the meristematic region of a prepared slide of the onion root tip. The length of the cell cycle is approximately 24 hours for cells in actively dividing onion root tips. Procedure: 1. Observe every cell in one high-power field of view and determine which phase of the cell cycle it is in. This is best done in pairs. The partner observing the slide calls out the phase of each cell while the other partner records. Then switch so the recorder becomes the observer and vise versa. Count 3 full fields of view. 2. Record your data in Table 1. 3. Calculate the percentage of cells in each phase, and record in Table 1. Consider that it takes, on average, 24 hours 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 percentage of cells in that stage. Percentage of cells in stage x 1,440 minutes = minutes of cell cycle spent in stage Results: Table 1: Mitosis in Onion Root Tip Cells Mitosis Phase Interphase Prophase Metaphase Anaphase Telophase Number of Cells Field 1 Field 2 Field 3 Field 4 Total Total Cells Counted Percent of Total Cells Counted Draw and label a pie chart of the onion root tip cell cycle using the date in Table 1. Title: Time in Each Phase 4
Analysis 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 your data in table 1, what can you infer about the relative length of time an onion root tip spends in each stage of cell division? Review the data below and answer the questions that follow. Table 2. Time spent in stages of the cell cycle in normal and cancerous chicken stomach cells. Interphase Prophase Metaphase Anaphase Telophase Total time Normal Chicken Stomach Cells 540 mins 60 mins 10 mins 3 mins 12 mins Cancerous Chicken Stomach Cells 75 mins 15 mins 2 mins 1 minute 3 mins 3. Determine the time of the cell cycle for both normal and cancerous cells. How does it differ? 4. What percent of time does a normal cell spend in interphase? (Show your work!) 5. What percent of time does a cancerous cell spend in interphase? (Show your work!). 5