Laboratory Observing the Cell Cycle of Onion Root Tip Cells Background: Because of their rapid growth, the cells of the root tips of plants undergo rapid cell division. Ornamental onion root tips cells (aka Allium from the Plant Family: Liliaceae (Alliaceae)) because of their accessibility and large cell size make excellent specimens for the observation of mitosis body cell division and interphase the period of time a cell spends in-between cell divisions. You are going to observe the preparation of onion root tip cells using acetocarmine stain, and then, using prepared slides, identify and photograph Allium cells in the various stages of mitosis. Materials: digital compound green onion root tips allium root tip acetocarmine stain light microscope prepared slides pipette forceps beaker slides cover slips absorbent paper razor blades Bunsen burner towels biology text for reference beaker tongs Procedure: Preparation of Cells Using Acetocarmine Stain Using the razor blade, cut a 1-2 mm section of the root tip from the onion. Mount onion root tips on a slide in a drop of acetocarmine stain. Using the forceps, mash the root tip. Using the beaker tongs to hold the slide, heat the slide gently over the Bunsen burner taking extreme care not to boil or evaporate the stain from the slide. Begin again if you boil off all or most of the stain. Add another drop of stain and heat again using the same technique. Place the cover slip over the specimen as you would for a typical wet mount slide. Place a small piece of paper toweling over the cover slip. Press firmly on the towelcovered slide with the flat end of a pen or the eraser of a pencil taking care not to crack the cover slip. This will spread the root tissue and allow you to view the cell nuclei. Observe the slide under the microscope under various powers and record your observations with drawings of what you see. If your specimen is not stained dark enough, repeat the staining and heating process with the same specimen until the desired results are obtained. Observation and photographing of Allium Root Tip Cells Using Digital Microscopes Because of the superior quality of the cells in a prepared slide, we will use prepared allium root tips slides to capture images of the different stages of the cell cycle.
Most of the cells that you observe will be in interphase. They are distinguishable because they have no visible chromosomes. Cells in the various stages of mitosis are distinguishable by looking at the position and state of their chromosomes (see Miller, pages 246-247). Your job is to photograph at least one cell in each phase of the cell cycle: interphase, prophase, metaphase, anaphase, telophase, and cytokinesis. For your assessment for this lab, simply turn in the Procedure, and Analysis Questions and labeled pictures of each of the phases of the cell cycle. Procedure Questions: 1. Why are onion root tip cells used in this experiment? 2. What is mitosis? 3. Why is acetocarmine stain used in this experiment? 4. What is the purpose of heating the onion root tip cells and the acetocarmine stain? 5. Why should you avoid boiling the stain and onion mixture on your slide? 6. What is the purpose of squashing the onion cells with the cover slip?
Results: Format your pictures to fit the following spaces and provide a description or explanation of the stage. Interphase Prophase Metaphase
Anaphase Telophase Cytokinesis
Analysis Questions: 1. In what stage does a cell spend most of its time? 2. Were you able to observe cells in all of the stages of the cell cycle? State some of the possible sources of error in this experiment. 3. Explain what happens to the DNA of a cell during mitosis. Why is this necessary? 4. Explain the concept of pairing chromosomes. What does it mean when you have a pair of chromosomes? 5. Describe the changes that occur in the nucleus during mitosis. Procedural elements of this lab were adapted from A Sourcebook for the Biological Sciences. Third edition. by Evelyn Morholt and Paul F. Brandwein. Orlando, FL. Harcourt Brace & Company. 1986.