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Title: Extraction of Strawberry DNA
Principle(s) Investigated:
- DNA is found in all living organisms.
- DNA is a nucleic acid that can be extracted from a cell's nucleus through extraction techniques.
Grade 9-12 Biology:
1.a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.
5.a. Students know the general structures and functions of DNA, RNA, and protein.
Materials:
Fresh strawberries – 3/group
Sandwich bags (zip-up) – 1/group
30 mL of extraction buffer per group (900mL water, 50mL dish wash soap, 2 tsp. salt)
Clear plastic cups
Cheesecloth
Rubber bands
Bamboo sticks
Cold isopropyl alcohol (90%)
Procedure:
1. Make the extraction buffer beforehand:
In a large container, add 900mL of water, 50mL of dish wash soap, and 2 tsp. of salt, close the container and invert it slowly to mix the contents.
2. Tie a cheesecloth round the opening of a clear plastic cup and secure it with a rubber band.
3. Place 3 strawberries in your sandwich bag, make sure most of the air is out of the bag, and seal it.
4. Use your fingers to grind the strawberries into mush for about 2 minutes.
5. Open the bag and add 30mL of extraction buffer.
6. Release the air in the bag, seal it, and again use your fingers to grind and mix the solution. Do this for another minute.
7. Pour the strawberry mixture into your cheesecloth filtration apparatus and let it drip into the cup for about 3-5 min. You may use the end of the bamboo skewer to gently mix the solution so that it filters faster.
8. Remove the cheesecloth and place it in your sandwich bag to discard later.
9. Tilt your cup with the strawberry extract and SLOWLY pour 60mL of cold isopropyl alcohol onto the side of your cup. Make sure not to mix the two liquids.
10. Two layers should form – the strawberry juice layer on the bottom and the alcohol one on top.
11. Dip the bamboo stick where the strawberry extract and alcohol layers meet, twirl the stick and collect your DNA strands.
Student prior knowledge:
- DNA contains set of instructions for all life forms.
- DNA is located in the nucleus of every cell.
- The cell membrane and nuclear envelope are made up of a phospholipid bilayer.
Explanation:
Strawberries are octoploid, because they have eight copies of each chromosome, and thus big genomes, they are ideal for simple extraction techniques. The cells are exposed by grinding the strawberries in the plastic bag. The extraction buffer contains soap and salt. The detergent helps to dissolve the phospholipid bilayers of the cell membrane and organelles, including the nuclear envelope. This releases the cell’s DNA. The salt is used to break up protein chains that bind around the nucleic acids and causes these and other cellular debris to clump together in the extract layer so that they are not precipitated with the DNA. The solution is then filtered through a cheesecloth to separate the proteins and cellular debris from the liquid containing the strawberry’s DNA. Because DNA is insoluble in isopropyl alcohol, adding cold alcohol allows the DNA to precipitate out of the strawberry liquid. The colder the alcohol, the less soluble the DNA will be in it. Adding the cold alcohol will produce two layers: the cold alcohol will be on top while the strawberry extract remains on the bottom. White, cotton-like fibers of DNA will precipitate into the alcohol layer, which can be coiled out and removed using a wooden rod.
Questions & Answers:
1. Would DNA be the same in any cell in the human body? How is it that muscle cells are different from skin cells?
Yes, in a multicellular organism (such as a human or a strawberry), all cells contain the same DNA. However, multicellular organisms have many different types of cells. For example, humans have liver cells, muscle cells, skin cells, brain cells, and the list goes on. It is not the DNA that makes each cell different but rather the way it is regulated by the cell. The combination of genes that are turned on and off are what determine the shape of the specific cell along with its specific function.
2. What happens to RNA during the extraction?
The extract is actually a mix of DNA and RNA, in reality the procedure extracts nucleic acids, which include RNA. However, enzymes called ribonucleases cut much of the RNA that is released when the cells are lysed.
3. Why does the DNA clump together?
Each cell in the body contains about six feet of DNA. In order for this much DNA to fit inside the nucleus, the molecules must coil tightly and clump together to be packed efficiently. When the DNA is extracted, it still retains its clumped shape though it is not as tight as when it’s inside the cell.
Applications to Everyday Life:
1. Genetic testing – Analysis of human DNA can help to detect heritable disease-related genotypes. Genetic testing is used for diagnosing genetic disease in newborns, children, and adults, as well as for identifying future health risks of certain populations.
2. Body identification – DNA analysis is especially important in identifying human remains that cannot be identified by traditional methods such as fingerprints or dental examination. When human bodies are severely fragmented or degraded, DNA analysis can serve a most crucial role in determining the identity of the person.
3. Analysis of forensic evidence – DNA analysis is also important to the field of forensic analysis for crime scenes. It can help in identifying potential suspects whose DNA may match any evidence left at a crime scene, and conversely, help to exonerate people who have been wrongly accused of crimes.
Photographs:
https://lh5.googleusercontent.com/-h415aJ5chtQ/TstBDLAtWbI/AAAAAAAAQjE/HFmEYz1q-vY/s640/Step%2525206.jpg
https://lh6.googleusercontent.com/-B7PaQZKYX24/TstBSuzYOoI/AAAAAAAAQjU/sxYYRyXPSng/s640/step%2525207.jpg
https://lh3.googleusercontent.com/-8jHEAb-A1XQ/TstBGAOHiOI/AAAAAAAAQjM/CcyKXMKZpZw/s640/dna%252520cloud.jpg
https://lh4.googleusercontent.com/-sI3z3_sgANQ/TstBUSuDFgI/AAAAAAAAQjc/DZ8sW1FN8Ds/s512/step%25252011.jpg
Videos:
References:
Lab procedure:
http://www.smccd.edu/accounts/digennaroc/biology110_labs/8.%20Strawberry_DNA_Extraction59-62.pdf
Background information:
http://www.nature.com/scitable/topicpage/gene-expression-regulates-cell-differentiation-931
http://www.nejm.org/doi/full/10.1056/NEJMoa012113
http://www.massfatality.dna.gov/Chapter4/HowImportant/
http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml
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