Problem: Is the DNA in strawberries like that in humans?
Materials (per student group)
- Ziploc sandwich bag
- 1 strawberry
- DNA extraction liquid (soapy, salty water)
- Filtering apparatus: cheesecloth, funnel, small beaker
- Ice cold ethanol
- Clear glass vial
- Microscope slide
- If the green leaves on the strawberry have not yet been removed, do so by pulling them off.
- Put the strawberry into the Ziploc bag and smush for about two minutes. You need to completely crush the strawberry. You do not want this mixture to be really bubbly. The less bubbles the better.
- When you’re finished smushing, put 10 ml of the DNA extraction liquid into the bag.
- Smush for another minute. Be careful not to make too many soap bubbles.
- Use the scissors to cut a corner of the Ziploc to allow mixture to be poured into the filter.
- Filter the liquid through the cheese cloth into the beaker. If it runs slowly, you may squeeze the cheese cloth to get the liquid through.
- Next, pour the filtered strawberry liquid into your glass vial until it is half-filled.
- Then, carefully pour the ethanol into the glass vial.
- Invert the tube several times to mix the liquids. (do not shake)
- Watch for the development of several large air bubbles that have a white cloudy substance attached to them. The cloudy substance is DNA!
- If you would like, take a toothpick and spin and stir the DNA like you are making cotton candy. If you tilt the vile, you’ll get more DNA
- Pull out the DNA. It will look like mucus or egg white. As it dries, it will look like a spider web. The fibers are millions of DNA strands!
- To look at the DNA under a microscope, put the glob on a clean slide and gently stretch it apart using two toothpicks. The fibers will be easier to see in the stretched apart area.
- Pay attention to the characteristics of the DNA as it precipitates.
Draw what your DNA looks like under the microscope below:
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- In order to study our genes, scientists must extract the DNA from human tissue. Would you expect the method of DNA extraction we used for the strawberry to be the same for human DNA? Why or why not?
- Is DNA the same in any cell in the human body? Explain your answer.
- If you wanted to examine a suspect’s DNA, which cells would you use and why?
- Please list two reasons why an investigator might want to study the DNA of a suspect.