PART II: Identifying and counting cells
For this section of the lab, you will view a micrograph of an onion root tip that shows cells in the different stages of mitosis. Before beginning the steps below, view the following micrograph of an onion root tip to become familiar with the different stages of mitosis, as seen in a micrograph of a cell. Once the micrograph is opened, use the mouse to scroll over a cell. Once the cursor is placed over a cell that should be identified, click on the cell and then identify the stage of mitosis for the cell. Immediate feedback will be provided. Repeat this procedure for all of the cells that are identified on the micrograph. Completing this practice exercise will allow the student to become proficient in correctly identifying cells with their respective stage of mitosis.
1. Using the Virtual Microscope, view an image of an onion root tip and count the number of cells that are in each stage of the cell cycle. As a reminder, the cell cycle consists of the following stages: interphase, prophase, metaphase, anaphase, and telophase/cytokinesis. It may be helpful to record the data in a table similar to the one below to assist in compiling results.
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2. Repeat Step 1 three more times, each time selecting a different slide from the virtual microscope. Count the cells observed in each stage of the cell cycle for each slide and record the information.
3. After the counts have been completed for all four slides, total the number of cells in each stage and find the average. To find the average, add up the number of cells observed in each stage and divide this number by four. Using the table above as a reference, to calculate the average number of cells in interphase, add the numbers found in locations 1, 7, 13, and 19 and then divide this number by four. Enter that result into location 25 above. Repeat this process for each column (stage in the cell cycle).
4. Finally, calculate the percent of time that cells are actually in each stage of the cell cycle. To determine the percentage of time cells are in each stage, divide the average number of cells in each stage by the total average number of cells in each field of view. Using the table above as a reference, first determine the total average number of cells by adding across the row entitled Average Number of Cells/Slide. Add up locations 25, 26, 27, 28 and 29. Enter the total obtained into location 30 above. To calculate the percentage of time cells were in telophase, divide the number in location 29 by the number in location 30, then multiply this number by 100 to obtain the percentage. The equation for calculating this percent is shown below.