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Cell Biology
LAB EXPERIMENT 4: Mitosis in Onion Root Tip Cells
Objective After completing this exercise, you should be able to:
- Better understand the process and stages of mitosis.
- Prepare your own specimens of onion root in which you can visualize all of the stages of mitosis.
- Apply an analytical technique by which the relative length of each stage of mitosis can be estimated
1. Introduction
The genetic information of plants, animals and other eukaryotic organisms resides in several (or many) individual DNA molecules, or chromosomes. For example, each human cell possesses 46 chromosomes, while each cell of an onion possesses 8 chromosomes. All cells must replicate their DNA when dividing. During DNA replication , the two strands of the DNA double helix separate, and for each original strand a new complementary strand is produced, yielding two identical DNA molecules. DNA replication yields an identical pair of DNA molecules (called sister chromatids ) attached at a region called the centromere.
DNA replication in eukaryotes is followed by the process called mitosis which assures that each daughter cell receives one copy of each of the replicated chromosomes. During the process of mitosis, the chromosomes pass through several stages known as prophase, metaphase, anaphase and telophase. The actual division of the cytoplasm is called cytokinesis and occurs during telophase. During each of the preceding stages, particular events occur that contribute to the orderly distribution of the replicated chromosomes prior to cytokinesis.
The stages of mitosis
Prophase. During prophase, the chromosomes supercoil and the fibers of the spindle apparatus begin to form between centrosomes located at the pole of the cells. The nuclear membrane also disintegrates at this time, freeing the chromosomes into the surrounding cytoplasm.
Prometaphase. During prometaphase, some of the fibers attach to the centromere of each pair of sister chromatids and they begin to move toward the center of the cell.
Metaphase. At metaphase the chromosomes have come to rest along the center plane of the cell.
Anaphase. During anaphase, the centromeres split and the sister chromatids begin to migrate toward the opposite poles of the cell.
Telophase. During telophase, the chromosomes at either end of the cell cluster begin to cluster together, which facilitates the formation of a new nuclear membrane. This also is when cytokinesis occurs, leading to two separate cells. One way to identify that telophase has begun is by looking for the formation of the cell plate , the new cell wall forming between the two cells.
Why use onion roots for viewing mitosis?
- The roots are easy to grow in large numbers.
- The cells at the tip of the roots are actively dividing, and thus many cells will be in stages of mitosis.
- The tips can be prepared in a way that allows them to be flattened on microscopes slide (“squashed”) so that the chromosomes of individual cells can be observed.
- The chromosomes can be stained to make them more easily observable.
2. Procedure for preparing root tip squashes
While actively growing onions are present in the lab for you to observe, you will be provided with roots that have been previously harvested and treated with a fixative to stabilize the cells.
You will work in groups of two for this lab exercise.
The first step will be to ‘soften’ the roots so that they later can be spread on a microscope slide.
- Using scissors, cut 2 roots tips about 1 cm long, and transfer them into a plastic micro-tube. (One of the rots will be an extra one.)
- Fill the tube about 2/3 full with 1N HCl from a dropper bottle.
*** **Caution: Work with the HCl carefully, it is a strong acid. *****
- Place the tube in a 60 °C water bath, and allow the roots to incubate for 12 minutes.
After the 12 minute incubation period, remove the tube from the water bath.
Rinse the roots in H 2 O
- Using forceps, carefully transfer the root tips to a small petri plate.
- Using a plastic ‘squeeze’ pipet, carefully remove the HCl from the micro-tube and transfer it to the “discard flask”.
- Rinse the root tips 3 times with water from the dropper bottle, disposing of the rinses in the discard flask.
Staining the chromosomes.
- After removing the water from the third rinse, cover the root with the Feulgen stain.
Caution: Although the Feulgen stain does not appear colored, it will strongly stain skin and clothing!!!!
- Incubate the roots in the stain for 12 minutes. During this time the very tip of the root will begin to turn red as the DNA stains the numerous small actively dividing cells at the tip. Remove the stain and again rinse the roots.
- Using a plastic ‘squeeze’ pipet, carefully remove the Feulgen stain and discard it in the discard flask.
- Again, rinse the root tips 3 times with water.
Preparing the root tip squash.
- Transfer a root to the center of a clean microscope slide and add a drop of water.
- Using a razor blade cut off most of the unstained part of the root, and discard it.
Cover the root tip with a cover slip, and then carefully push down on the cover slide with the
wooden end of a dissecting probe. Push hard, but do not twist or push the cover slide sideways.
The root tip should spread out to a diameter about 0.5 – 1 cm.
Observations of onion root tip squash. Scan the microscope under the 10x objective. Look for the region that has large nuclei relative to the size of the cell; among these cells will be found cells displaying stages of mitosis. Examples are shown in the figure to the right. Switch to the 40X objective to make closer observations. Since prophase and prometaphase are difficult to distinguish, classify all these cells as prophase
