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Performance Task:
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Note to the teacher: This task will require several
days for the students to complete. There are required time waits in several steps. You will need to help students arrange their
time to accommodate these. In this task you will grow onion roots and use the squash preparation
method to prepare slides to observe the stages of cellular mitosis under a microscope. You will need the following
materials and equipment for this activity:
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onion bulbs tooth picks 250 ml beaker
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medicine dropper glass slides 2 Petri dishes
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cover slips Aceto-orcein stain 18% Hydrochloric acid
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single edge razor blades compound microscope Carnoy fluid (with chloroform)
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70% isopropyl alcohol solution paper towels or bibulous paper
Follow the procedure outlined in each of the stages below:
Stage 1: Growing the Onion Root
- Obtain onion bulbs or grown mature onions from a grocery or seed store.
If you purchase them from the grocery store, select red onions as white or yellow onions are usually
treated to prevent sprouting.
- Remove the dried outer layer and one fleshy layer. Trim away dried roots
carefully. Do not trim away any part of the bulb.
- Suspend the lower third of the onion in a beaker of water by using
toothpicks that are pushed into the sides of the bulb. Rest the toothpicks on the mount of the beaker.
- Place the onion and beaker in a dark room for several days. The dark
room will assure that the roots will grow straight. When the roots are one to two centimeters long, they are
ready for cutting.
Stage 2: Preparing the Slides
- Cut the roots and transfer them to Carnoy fluid and let set for four
hours. After four hours, transfer the roots to a 70% isopropyl alcohol solution for storage.
- Label one Petri dishes "HCl solution" and the other
"Carnoy solution."
- Place the 18% Hydrochloric solution in the properly labeled Petri dish
and, using forceps, transfer a root tip from the alcohol solution to the HCl Petri dish and allow to stand for
four minutes. Handle the root tip from the cut end to avoid damage to the meristemic region.
- After the four minutes has elapsed, transfer the root tip to the Carnoy
fluid and allow to set for four more minutes. Then set the root aside, but do not allow it to dry out. Keep it
moist by folding it in a wet paper towel.
- Using a single edge razor blade, cut off 1-2 mm of the root tip and
place the tip on a cleaned microscope slide. Chop the root tip into many fine pieces.
- Cover the root tip with a few drops of Aceto-orcein stain and allow to
set for two minutes. Do not allow the root tip to dry. If it appears to be drying out, add another drop of
stain. After two minutes, blot away the stain with a paper towel or bibulous paper. Be careful not to touch
the root tip.
- Cover the root tip with 1-2 drops of water. Gently lower a cover slip
over the root tip. Cover the slide with a paper towel or bibulous paper and firmly press on the cover slip
with your thumb. Do not twist the cover slip as this will destroy the cells and may cause the slide and
cover slip to break.
- Remove the paper towel. The slide is now ready for viewing.
Stage 3: Viewing the Slides
- Using the low power lens, observe the cells in several stages of
mitosis.
- Find a cell in each stage and observe each with the high power lens.
- While observing under high power, illustrate a cell in each stage,
labeling the important features of each cell. Illustrate the surrounding cells as well.
Note to the teacher: If you would like to have an extension to this
activity, have the students, using the knowledge and skills learned from this process, design an experiment to
determine the duration of each mitotic stage of mitosis in onion roots.
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Rubric:
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4 Points = The student competently follows
directions in each of the stages of the task. He/she produces a
well-prepared set of slides, enabling him/her to see the various stages of
mitosis. The illustrations are drawn to scale, are properly labeled, are
easily identifiable, and show that the student has a thorough understanding
of mitosis.
3 Points = The student needs some coaching in order
to carry out the inquiry. The prepared slides are not neatly made. The
student's illustrations are incomplete and indicate some lack of
understanding of mitosis.
2 Points = The student has much difficulty carrying
out the inquiry. He/she does not follow directions accurately nor
carefully, and the prepared slides are poorly made. The student's
illustrations are either not labeled or mislabeled, are not drawn to scale,
and are incomplete. The student shows little understanding of mitosis.
1 Point = The student does not complete the task. He/she has little
understanding of how to complete a scientific inquiry and does not indicate
any understanding of mitosis.
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