Select a root of an onion that is 2 to 3 cm long. Place it on a clean slide: Cut off 1 to 2 cm of the root tip. Discard any remaining upper portion.
Place three drops of 1 N hydrochloric acid on the root tip.
Wait about 1 minute. While you are waiting, heat the slide by passing it back and forth over a flame (CAUTION: Hold the slide with forceps so you won’t burn your fingers. Do not allow the liquid on the slide to boil.) Heat the slide in this way two or three times. Carefully blot off the acid with a strip of paper towel.
Cover the root tip with two or three drops of toluidine blue O stain. Warm the slide for about one minute as you did before. Carefully blot off the excess stain.
Add a drop of fresh stain. Apply a cover glass. Place the slide in a folded paper towel on a hard surface. Press the cover glass with your thumb, using a steady, firm pressure. Be careful not to break the cover glass.
Clean the stain from the slide and cover glass. Examine under low power of your microscope. When you have located the cells, switch to high power. The cells may be stained so darkly that you cannot see the individual parts. If this happens, dilute the stain by placing 1 or 2 drops of water at one edge of the cover glass. Place a piece of paper towel at the opposite edge to pull the water through. Repeat this until you can see the stained portions of the cell.
Find these stages of mitosis:
- The nuclear material forms long, slender threads that are stained. At this stage, the separate chromosomes cannot be identified. They form a loose ball of tangled and twisted threads.
- The individual chromosomes are much shorter and thicker. At this stage, each chromosome has two strands, or chromatids. The strands are held together at the centromere.
- The chromatids have separated and are now two separate groups of chromosomes.
- The two nuclei are present in the cell with strands of chromosomes still visible.
Make simple sketches of what you find. Describe the structures you see.
What problems did you have in making this study?
Study a prepared slide of dividing cells in the onion root tip. (The stain used for the slide is different from the one you used.)
Scan the entire length of the section of root tip, using low power. Where do you observe cells dividing? Where are the cells not dividing?
Locate some cells in division. Switch to high power and look for the following stages:
- Early appearance of chromosomes in the nucleus. Find round structures that stain at this early stage. These are the nucleoli.
- The chromosomes are easy to see and grouped in the middle of the cell.
- Chromosomes have separated and moved toward the poles.
- A new cell wall is formed between the new cells.
What has happened to the nucleoli?
What is the condition of the nuclear membrane?
Find the spindle. The end of the spindle fibers that are near the center of the cell are attached to the centromeres of the chromosomes. How many chromosomes can you see? What part do you think the spindle fibers play in moving the chromosomes?
Look for the beginning of the new cell wall. Where does it appear?
Cells divide rapidly in embryos (young organisms, at very early stages of development). These cells are good material in which to study mitosis. Carefully examine a prepared slide of either Ascaris (a worm) or whitefish embryos. Look for the following stages:
- The chromosomes are long and threadlike.
- The double chromosomes are attached to spindle fibers at the center of the cell. Look at the poles of the spindle and compare them with those of the plant cells you studied.
- The chromosomes are separating and the cell is pinching in two.
How many individual chromosomes are in one cell? How do the poles of the spindle of an animal cell differ from those of a plant cell?
Compare the separating of chromosomes and dividing of these cells with what you saw in plant cells.
What structures do you see in the dividing animal cell that were present in the dividing plant cell?