Lab 7: Mitosis and the Cell Cycle in Onion Root-Tip Cells

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Lab 7: Mitosis and the Cell Cycle in Onion Root-Tip Cells Powered By Docstoc
					                                                             NAME: __________________
Mitosis and the Cell Cycle in Alium Root-Tip Cells


In this lab, you will observe the various stages of mitosis in onion root tip cells. Cell
division is especially rapid in the growing root tip, therefore, it is easier to observe each
stage of mitosis than in slowly growing tissues. You will view the cells using prepared
slides under a microscope. You will then estimate the proportion of time that cells in
actively dividing tissues, such as the root tip, actually spend in mitosis and cytokinesis.

Materials and Equipment:

       Prepared slides of Alium root apical meristems (root tips)
       Light microscope


           1. Examine your slide under the microscope at low power (4x) to find the
              area of active cell division at the tip of the root.

           2. Once you have found this area, switch to 10x power to observe cells more
              closely, looking for evidence of mitosis (many cells should be in

           3. On high power (40x), spend some time identifying the different stages of
              the cell cycle visible in your root section. If you still cannot find a
              particular stage after a careful search, ask for assistance; it may also help
              to try another slide. Illustrate examples of each mitotic stage (prophase,
              metaphase, anaphase, and telophase) as they appear on the slides – not
              how they are depicted in diagrams.

Prophase                Metaphase               Anaphase                 Telophase


              4. Return to the area of active cell division. Observe every cell in one high
                 power field of view and determine which phase of the cell cycle it is in.
                 By counting the number of cells in each phase, we will be able to estimate
                 the amount of time spent in each phase. This is best done in pairs; the lab
                 partner observing the slide calls out the phase of each cell while the other
                 partner records. [*Be careful not to move the slide!] Then switch so the
                 recorder becomes the observer and vice versa.

                 **A helpful hint would be to scan methodically down each row of cells,
                 from one side to the other, and tally the number in each phase separately.
                 Count at least two full fields of view. Add your data to the table below.

              5. Calculate the percent of the total cells in each phase by dividing the
                 number of cells in a given phase (e.g. prophase) by the total number of
                 cells considered (sum of all cells, in all stages, from both fields).

              6. Estimate the percentage of time the onion root tip cells spend in each
                 different stage of the cell cycle. [*note that the length of the cell cycle is
                 approximately 24 hours for onion root tip cells.] You can calculate the
                 amount of time spent in each phase of the cell cycle from the percent of
                 cells in that stage: (% of cells in phase) × 1440 min = minutes of cell cycle
                 spent in phase

Stage of cell cycle                 Number of        Number of        Total     % of     Est. time
                                    cells:           cells:           # of      total    in each
                                    (Field 1)        (Field 2)        cells:    cells    phase
M-Phase        Prophase


Address the following questions:

(1)    What is a distinguishing characteristic of each stage of mitosis that is visible on
       your slide?





(2)    How did the stages differ in appearance from the diagrams shown earlier in class?

(3)    Were all stages well represented on your slide? Which ones were more difficult to

(4)    Were the stages occurring in a synchronised fashion, or did they appear to be
       random in their distribution throughout the root tip?

(5)    What can you conclude about the proportion of time cells spend in mitosis?

(6)    How did the results for interphase compare to the results from the mitotic stages?
       What could account for the difference?

(7)    Based on your results, which stage was the longest (i.e. took the most time)?
       Which was the shortest?

(8)    Do you think the proportion of time spent in mitosis would be greater or smaller
       in more mature regions of the root?


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