Genetics & Evolution Box
Link to lessons that are part of the Genetics & Evolution Box.
There are numerous write ups for this experiment available on the internet and elsewhere. I first experienced this lesson through UCSF’s Science and Health Education Partnership. I then tried it with Carolina Biological .
The estimates of the length of DNA in a human cell and the number of cells in the human body were taken from Wikipedia (from the Genome and Cell Biology articles).
- Have students answer summary questions about the extraction.
- Why is it necessary to mash the strawberries?
- What is the purpose of the detergent?
- What is the purpose of the salt?
- Name a liquid that DNA is not soluble in.
- Is the DNA that you extracted pure? What else might be attached to the DNA?
- Why might some people get more DNA than others?
- Can you see a single strand of DNA without a microscope? Explain how you were able to see the DNA in this experiment without magnification.
- Have students write down a few sentences to describe what DNA is and what they think DNA looks like. After this lab or the series of DNA modeling activities, they will come back to this naive description to revise their answers with a more scientific one.
- Draw a diagram on the board showing DNA (as a long tangled thread) within the nucleus of a cell. Label the DNA, nucleus, cell membrane, and cell wall. Remind (or teach) students about basic cell structure.
- Tell students that they will be extracting the DNA from a strawberry and will then be able to look at the DNA. Briefly describe the process explaining the purpose of each of the steps.
- Pass out ziplock bags and strawberries. Tell students there are strawberries to eat after the lab is cleaned up.
- Students should put the strawberry in the bag, squeeze out most of the air and seal the bag. The strawberry can then be crushed into juice and pulp. Try to squish all of the chunks into an even, smooth puree. Warn students not to pound the strawberry on the table or risk the bag bursting and getting strawberry pulp all over themselves and the classroom.
- Next, open the bag and add 10 ml of extraction buffer (approximately 10 eyedroppers full). Seal the bag again and gently mix the strawberry juice with the extraction buffer. Warn students not to mix too vigorously or it will generate a lot of bubbles and can’t be filtered effectively. Use a gentle tilting back and forth motion while lightly squeezing the bag.
- Set up a filtration system. I had students wrap a paper towel around their finger then put their paper-wrapped finger into the mouth of the 15 ml tube or 5 oz cup. When you remove your finger, the paper towel should form a well into which the strawberry juice can be poured.
- Carefully pour the extract into the well in the paper towel. Allow the juice to filter through the towel into the container below. Let it drip for 3-5 minutes. Do not squeeze the towel or you will create lots of bubbles, disrupting the interface needed in the next step.
- The paper towels can be put inside the ziplock bags and thrown away.
- Carefully transfer liquid from the 15 ml tube or cup into the clear test tube until the test tube is about a third full.
- Slowly add 3 ml (3 eye droppers full) of ice cold alcohol to the test tube. The alcohol should be added so that it trickles down the side of the tube before pooling on top of the strawberry extract. You should end up with a red bottom layer and a clear top layer.
- Have the students make observations of anything going on in the clear alcohol layer. You may wish to have students write down observations at this point.
- After 2-3 minutes, a skewer or stirring rod can be inserted into the tube and gently swirled around. This will spool the DNA around the stick. The DNA can be pulled out of the tube and stored in a microcentrifuge tube filled with some alcohol. Students may safely touch the DNA although the DNA should NOT be tasted under any circumstances.
- By trapping a piece of string in the lid of the microcentrifuge tube, students can wear their DNA home as a necklace.
- Have students clean up their areas. Nicely cleaned tables and washed hands may be rewarded with a piece of strawberry to eat.
- Purchase strawberries, enough for each student to have one. (They will eat half and use the remainder to extract DNA.)
- Prepare the extraction buffer.
- Put the alcohol in the freezer or on ice.
- Wash the strawberries and remove the green tops. Cut each strawberry in half. Put half in a clean food bowl for students to eat. Put the remainder in a separate bowl for students to extract DNA from.
- Set out the remainder of the materials.
This activity should be part of the standard repertoire of any teacher who teaches genetics. It is essential for students to prove to themselves that DNA exists and that it can be extracted from any cell. Strawberries are used in this activity because they are octaploid, meaning they have 8 copies of every gene rather than the usual 2; thus providing prodigious quantities of DNA to extract. Naturally, strawberries are also relatively inexpensive and readily available. Other sources of DNA to experiment with include kiwis, bananas, and calf thymus.
Each student needs:
- 1 fresh strawberry (frozen strawberries also work fine although they are not nearly as much fun to eat)
- 1 ziplock bag
- 1 15 ml centrifuge tube or 5 oz paper bathroom cup
- 1 clear glass or plastic test tube
- 1 paper towel
- 10 ml extraction buffer (see recipe below)
- 1 bamboo skewer or glass stirring rod (DNA tends to stick more fiercely to bamboo than wood – however, bamboo is MUCH cheaper)
Strawberry DNA The cloudy substance in the upper layer is strawberry DNA.What is DNA? What does it look like? In this activity, students extract DNA from strawberries using diluted dish ...
This lesson was adapted from a lesson by Katie Ward of Aragon High School in San Mateo. After trying Katie’s version with my students, I found several other similar activities on the web. For instance, see the Making Babies lab written by Kevin Hartzog of Thurgood Marshall Academic High School. Kevin Hartzog's website Stars and Seas is extraordinary! With lots of other great lessons and ideas. Also recommended is this Making Babies lab from Friends Academy .
- There are conclusion questions at the end of the handout.
- For homework, I assigned my students several pages from the book The Cartoon Guide to Genetics, by Lary Gonick and Mark Wheelis. Pages 37-55 deal with Gregor Mendel and inheritance patterns. The questions I asked include:
- Who was Gregor Mendel?
- In one experiment, Mendel crossed a tall pea plant with a short pea plant. What kind of eggs and pollen are produced? What is the genotype of the baby plant? What is the phenotype of the baby plant?
- Next, he took these tall hybrids and bred them together. How many of these grandchild plants were tall? How many of these grandchild plants were short? Explain how it is possible for 2 tall pea plants to have a short baby.
- Why were Gregor Mendel’s experiments important?
- A brown eyed mom and a blue eyed dad have a blue eyed baby. What is the genotype of the baby? What is the genotype of the dad? What are two possible genotypes for the mom? Which genotype must she be to have a blue eyed baby? Explain why she must be this genotype.
- Make sure each student has their “Human Traits Survey” completed and available.
- Pass out the “Making Babies” handout and assign partners for the activity.
- Read through the first page together, answering any questions students may have. Work through a couple examples of going from genotype to phenotype and back again with different traits. “If you cannot roll your tongue, what is your genotype?” “If your genotype is Rr for tongue rolling, what is your phenotype?”
- Help students fill in the table on page 2 with their phenotype and genotype. If the student has the dominant trait, assume that they are heterozygous unless they know for SURE that no relative has the recessive trait.
- Next, make gametes. Between the partners, one should make sperm, the other should make eggs. Each person will make 2 gametes by flipping a coin for each gene. We are assuming that the genes are not linked (even though hair color and eye color genes are, in fact, linked).
- Finally, students can create 2 babies by combining sperm #1 with egg #1 and combining sperm #2 with egg #2. Beside the genotype/phenotype descriptions, students should draw a picture of their children in color as they would appear in middle school.