Extract your own DNA from cheek cells
Grade level(s):Grade 7, Grade 8, Grade 9, Grade 10
All living organisms contain DNA: from bacteria to plants, animals, and humans. DNA is stored in the cell's nucleus and can be extracted using a few simple steps.
DNA, nucleus, extraction, spooling,
What you need:
- test tubes with 5ml of detergent solution - capped tubes with 10ml of rubbing alcohol - bottle with prepared salt solution - dixie cups - plastic coffee stirrer - flat wood sticks - - small Eppendorf tube - pipettes
Students can work in pairs or as individuals. To allow each student to handle materials groups should not be larger than that.
Classroom or laboratory
One class period (40-60 minutes)
Students will extract DNA from their own cheek cells.
Students should know about the basic function of DNA and its location.
Students will observe their own DNA. Students will learn the simple method of DNA extraction and will be able to explain the rationale of each step. Students will be able to explain why DNA extraction is important to scientists.
All living organisms contain DNA. Some fruits are especially suited for DNA extractions due to their multiple sets of chromosomes. For example, strawberries are octoploid, which means they have 8 copies of each chromosome. Because of the large yield, strawberries are often used to extract DNA (see lesson plan for strawberry DNA extraction on this site; http://seplessons.ucsf.edu/node/217/). Human body cells are diploid; they contain two copies of each chromosomes. Extracting DNA from cheek cells yield just a very small amount of DNA. But students still enjoy the activity and are excited about seeing their OWN DNA. Very demystifying!
Individual strands of DNA are too small to be visible to the eye. One million threads of DNA fit onto the period at the end of a sentence using Times New Roman, font 12 in WORD. The reason why we are able to see DNA in this activity is that there are so many of them, clumped together.
DNA extraction is a fairly simple procedure that requires only a few steps: 1. The detergent breaks open the cells by destroying the fatty membranes that enclose the cells as well as the nuclei membranes within the cells. DNA is released into the solution. Detergent and the salt also helps strip away proteins that are associated with the DNA molecules. 2. DNA is NOT soluble in alcohol, whereas other cell parts are. By adding alcohol, DNA precipitates out of the solution and collects at the interface of the alcohol and soap layer. The colder the alcohol, the less soluble the DNA will be in it.
- Prepare the 0.9 percent salt solution (2 teaspoons of table salt dissolved in 1 quart/liter of water)
- Pour 10ml of salt solution in dixie cups; one for each student
- Prepare 25 percent detergent solution (1 volume of detergent mixed with 3 volumes of water)
- Fill large labeled test tubes with 1 teaspoon (5ml) of detergent solution; one for each student
- Fill labeled test tube with 2 teaspoons (10ml) of rubbing alcohol and seal; one for each student. Store in the friedge. The alcohol needs to be cold for the extraction to be successful.
- Prepare a tray with materials for each pair/table or set-up another procedure to hand-out material to students.
Lesson Implementation / Outline
Prepare slides of cheek cells, stained with methylene blue. Have student look at slide under microscope or show on screen. Discuss what they see (methylene blue stains the nucleus) and the size of nucleus, chromosomes, DNA.
Students can either perform the extraction individually/with their partner following their worksheet or teacher can model the extractions while the whole class follows along step by step. Directions for the extraction (also see attached worksheet):
- Have students swirl the 10ml of salt solution in their mouths for 30 seconds. This will remove dead cells lining the mouth.Have
- students spit their solution back into their dixie cup and then pour it into the large test tube containing the detergent solution.Students
- should then cap the test tube and GENTLY rock it on its side for 2-3 minutes. IMPORTANT: Don't shake the test tube or mix it to vigorously. DNA will break into smaller fragments and will be harder to see later on.After
- 3 minutes, have students uncap their tube, slightly tilt it and carefully pour the chilled alcohol down the side of the test tube. The alcohol and the detergent should form two distinct layers with the alcohol sitting on top.Have
- students let the tube stand for one minute. Then, have them use the coffee stirrer to slowly move some of the ethanol into the soap layer. DNA will start to precipitate out of the soap solution. Have students twirl the stirrer to spool the DNA strands around it. If the DNA fragments are too short to wind up, students can use the pipette the suck up the fragments.Students
- Students can transfer the DNA into a small tube (Eppendorff tubes work well), filled with rubbing alcohol, and take it home. The DNA should be stable in that form for a long time.
Ask questions that make students think about the rational of steps of the extraction process: Why are we using the detergent? What does it do to the cell? Ask questions to check if students understand the size of DNA molecules. Make sure they understand that they would NOT be able to see one individual strand of DNA. The white strands that are becoming visible contain many DNA strands clumped together.
Extensions and Reflections
There is a great number of fruits that can be used for DNA extractions. See lesson plan "Strawberry DNA extraction" on this site. It's a good idea to compare the amount that DNA that different extractions yield. Nice discussion starter for differences in the number of chromosome sets in various organisms.
Have students observe human cheek cells and chromosomes under the microscope to reinforce the idea of the scale of DNA molecules.
The alcohol needs to be VERY cold. Store in refrigerator before class.
Weblinks and References
|See your DNA.pdf||28.69 KB|
|How does extraction process work.doc||194.5 KB|