Abstract

Objective

The objective of this plant biology science fair project is to investigate how irradiation affects the germination of radish seeds.

Introduction

Mutations are permanent changes in the DNA sequence of an organism, and can be inherited. If the organism is a single cell (like a bacterium), the "daughter cells" that formed when it divided, might each have the mutation, assuming that the mutation is not lethal. If the organism is multicellular, such as a human being, the mutation can be inherited only if it occurs in the cells that form the eggs and sperm. A mutation that occurs in one of the cells on your arm, for example, cannot be passed on to the next generation.

Mutations can be caused by a number of things. Chemicals found in tobacco smoke, for example, cause mutations. Mutations can also be caused by forms of electromagnetic radiation, including ultraviolet (UV) light, X-rays and gamma rays.

UV light from the Sun causes DNA damage to exposed skin. But UV light does not penetrate your skin very far. It stops after it travels a few cell-thicknesses into the skin. X-rays also cause DNA damage. X-rays are more energetic and more penetrating than ultraviolet rays. That is why they are so useful for getting a picture of your bones or teeth—they pass through the soft tissues and are absorbed by the hard tissue. On an X-ray film, the light regions form where lots of X-rays have struck the film (soft tissue). The dark regions form where fewer X-ray photons have passed through (bones). The X-ray you get at the dentist or when you have a chest X-ray is safe because the dose is so low.

In this plant biology science fair project, you will investigate how radish seeds are affected by gamma irradiation. Gamma rays are even more powerful than X-rays. For the purpose of irradiating food, the gamma rays are produced by a highly radioactive version of the element cobalt, called cobalt 60 (see the Bibliography for more about cobalt 60). It is important to understand that the seeds in this science fair project have been "irradiated," which means they were treated with gamma rays. The seeds are not radioactive.

Because they are so energetic, gamma rays can penetrate deeply into tissue. Gamma rays are a form of ionizing radiation, which means that they can form ions, or charged particles, in irradiated tissue. When gamma rays cause DNA damage, most of the damage is due to the reaction of these ions with the DNA molecule. DNA damage caused by gamma rays can result in breakage of both strands of the DNA molecule. The higher the dose of gamma rays, the more damage there is to the DNA.

You will use gamma-irradiated WARD's Rapid Radish^TM seeds in the experimental procedure. The seeds have already been irradiated with several doses of gamma rays. The doses were not so high that the seeds were all killed, as in food sterilization, but the doses were high enough that the growth of some of the seeds could be affected. The unit used to measure the level of gamma irradiation is the mrad. An mrad is a measure of how much energy has been deposited in a material by the irradiation. A rad is equal to 1,000 mrads. The rad is the original unit developed for expressing absorbed dose, which is the amount of energy from any type of ionizing radiation deposited in any medium (e.g., water, tissue, air). A dose of one rad is equivalent to the absorption of 100 ergs (a small but measurable amount of energy) per gram of absorbing tissue. The rad has been replaced by the gray in the SI system of units (1 gray = 100 rad).

If the gamma rays have caused mutations in the DNA sequence of the plants that grow up from the seeds, then the plants have an altered genotype. The genotype of the plant consists of its DNA sequence. If the DNA damage causes a change in the observable appearance or behavior of the plant, then the plant is said to have an altered phenotype. The phenotype you will observe is seed germination, so you will observe the part of the plant that emerges from the seed first, the embryonic root, termed a radicle, or primary root. Let's get started!

Terms, Concepts, and Questions to Start Background Research

• Mutation
• DNA sequence
• Electromagnetic radiation
• Ultraviolet (UV) light
• X-ray
• Gamma ray
• Dose
• Ionizing radiation
• Ion
• rad
• mrad
• Absorbed dose
• Erg
• Gray
• Genotype
• Phenotype
• Germination
• Radicle
• Primary root
• Photon
• Radio wave
• Visible light
• Electromagnetic spectrum

Questions

• What are the wavelengths of the different forms of electromagnetic radiation?
• Based on your research, do gamma rays damage proteins? What effect will protein damage have on the irradiated seeds. Hint: consider the number of copies of a protein in a cell and the number of copies of a gene.
• What is the definition of a dose-response curve?
• Based on your research, what actually happens at the level of the DNA molecule when the cell is exposed to gamma rays?
• How are gamma rays produced for the purpose of irradiating food, or seeds?
• Are gamma rays found in nature? Hint: think "cosmic."
• What are some other types of ionizing radiation?
• What is a gamma knife?

Bibliography

• Health Physics Society. (2008, July 2). Radiation Basics. Retrieved February 27, 2009, from http://www.hps.org/publicinformation/ate/faqs/radiation.html
• National Aeronautics and Space Administration. (2007, March 27). The Electromagnetic Spectrum. Retrieved March 3, 2009, from http://science.hq.nasa.gov/kids/imagers/ems/gamma.html
• Dolan DNA Learning Center, Cold Spring Harbor Laboratory. (n.d.). DNA from the Beginning: Mutations are changes in genetic information. Retrieved March 3, 2009, from http://www.dnaftb.org/27/concept/index.html
• Wikipedia Contributors. (2009, March 2). Seed germination. Wikipedia: The Free Encyclopedia. Retrieved March 3, 2009, from http://en.wikipedia.org/w/index.php?title=Germination&oldid=274316504
• Wikipedia Contributors. (2009). Cobalt-60. Wikipedia: The Free Encyclopedia.. Retrieved March 13, 2009 from http://en.wikipedia.org/w/index.php?title=Cobalt-60&oldid=276181821

Materials and Equipment

• Irradiated "Rapid Radish" seeds. These can be ordered online from Science Kit & Boreal Lab, product #: WW45656M03. There are about 20–30 seeds for each dose in this product. Order more seeds if you would like to work with a greater number.
• CD "jewel" cases, clear (12)
• Paper towels
• Permanent marker
• Scotch® tape
• Lab notebook
• Modeling clay, enough to hold the CD cases upright
• Tray to hold the CD cases
• Eye dropper
• Optional: Camera
• Graph paper

Experimental Procedure

• The experimental procedure described below focuses on the germination of the seeds. This has the advantage of providing results in a fairly short time frame. As an option, you can grow the plants to maturity to investigate how the irradiation affects the growth and appearance of the plants. The science supply stores that sell the Rapid Radish seeds also sell kits to help you grow the seeds to maturity if you decide to do that.
• Keep in mind that you will need to perform three trials of this science fair project. To save time, the trials can be run concurrently.

1. Label the compact disc cases with your permanent marker, as follows:
   1. Case 1: Control (no irradiation)
   2. Case 2: 50 mrad
   3. Case 3: 150 mrad
   4. Case 4: 500 mrad
2. Fold a paper towel so that it will fit into one of the CD cases.
3. Use the eye dropper to moisten the towel with water. It should not have any dry spots, but should not be dripping wet either.
4. Place the paper towel in the CD case.
5. Repeat steps 2–4 for each of the CD cases.
6. Place six seeds, with the appropriate irradiation dose, in each CD case.
   1. Place the seeds on the paper towel.
   2. The seeds should be between the plastic cover of the jewel case and the paper towel.
   3. Place them along the top so that the rootlets have room to grow. The CD cases will be placed vertically upright, so the roots can grow downward.
   4. Use the same pattern for all of the cases.
7. Close the cases.
8. Add a small piece of Scotch tape around each edge to make sure the CDs stay closed.
9. Place the CDs upright on the tray. Use the modeling clay to hold them in a vertical position. Separate them so that they are several inches apart, and you can easily view the seeds.
10. Place the tray with the seeds in a well-lit room.
    1. Keep out of direct sunlight.
    2. The temperature should be between 16°C and 22°C (60–70°F).
11. Record the date and time that the experiment was started in your lab notebook.
12. Observe the seeds over the course of the next week.
    1. Keep the paper towels moist.
    2. Use the eye dropper to carefully add water to the paper towels, as often as necessary. Reseal the cases with tape each time, to keep the cases closed.
    3. Try to keep the conditions (light, temperature, amount of water added) the same for all of the seeds. Record all procedures and data in your lab notebook.
13. Throughout the week, record how many seeds germinate, and record your observations about the appearance of the growing rootlets.
    1. Note the appearance of the seeds at least twice per day.
    2. Make sketches or take pictures of the seeds twice per day.
    3. Record the length of the rootlets.
14. Use your judgment about when to stop observing.
15. Graph the germination rate (in percent) vs. the irradiation dose.
    1. For example, if 2 out of 6 seeds germinate, the germination rate is 33 percent.
16. Graph the average length of the rootlets vs. irradiation dose.
17. Repeat steps 1–16 two more times, with fresh materials, to get a set of three trials. Note: The trials can be run concurrently.

Variations

• Plant the seeds in moist soil and watch them grow. Follow the directions that accompany the seeds for growing the plants. Compare the heights and appearances of the plants treated with the various doses of gamma irradiation. For more information about how to measure the growth of the plants, visit the Science Buddies page, Measuring Plant Growth.
• Try breeding the plants after they have grown, using the directions that came with the kit. Do you see new phenotypes in the next generation?

• For more science project ideas in this area of science, see Plant Biology Project Ideas.

Credits

David Whyte, PhD, Science Buddies

• Scotch® is a registered trademark of 3M.
• WARD's Rapid Radish^TM is a registered trademark of WARD'S Natural Science Establishment, Inc.

Last edit date: 2009-04-03 11:45:00