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Expected Results: Results may vary based on materials used, setup, procedure, and other factors, however, here is an example on what to expect: • The soap breaks down the cheek cells, releasing the DNA into the salty sports drink. The sodium in the salt binds to the phosphate DNA backbone; when it comes into contact with the alcohol, the latter being less polar than water, causes the sodium-DNA complex to precipitate out of solution — forming the white clumps that are visible at the water-alcohol interface. DNA does not dissolve in alcohol, so it forms a solid where the sports drink and the alcohol meet. The white clumps are thou- sands of your DNA strands. When you swish the sports drink, you'll also pick up bacteria in your mouth. So what you see may be a mixture of your DNA and bacteria! Teaching Notes: The more vigorous you swish the sports drink in your mouth, the more cheek cells (DNA) you will have in your cup. Questions and observations for students: • Why did we use the soap? • Why did we use ice-cold alcohol? • What happens if the alcohol isn't cold? • Could a CSI team use this technique to collect DNA for legal purposes? • What techniques are used for forensic science or medicine? We drink sports drinks when we exercise because it replenishes the electrolytes we lose while sweating. Why would it be important to add detergent to the cells you collected with the sports drink? The detergent is amphipathic, meaning that it has one end that is hydrophobic and one side that is hydrophilic. The membrane surrounding each cell is composed of a lipid bilayer, which consists of a hydrophobic center and a hydrophilic outer portion. Because it is amphipathic, the detergent molecules are able to invade the lipid bilayer and cause it to fragment into many small micelles, surround proteins to break up protein interactions, and ultimately release the DNA from the cell! The salt ions in the sports drink help protein fragments and other cell debris to clump together, which is why you may start to see some chunks floating to the bottom of your tube. Polar molecules, like water, have a slight positive charge on one end and a slight negative charge on the other end. DNA is very polar with a net negative charge when it is in solution. Both water and ethanol are polar, but ethanol is much LESS polar than water, particularly when the ethanol is very cold. This means that the difference between the positive and negative charges in ethanol is not as great across the molecule. The DNA is coated in Na+ ions from the salt in the sports drink, making it even more polar. The highly polar DNA cannot dissolve in the much less polar ethanol, so it precipitates into a white, wispy layer when cold alcohol is added. More questions and observations for students: • Do you get more DNA from using your cheeks, the fruit, the leaves, or the seeds? Why do you think this is so? • What do you have in common with a banana? • What other sources of DNA could you use? Disposal/Clean-up: – Discard any leftover product as directed. + ward ' s science Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.wardsci.com Double (Helix) the Fun with this DNA Extraction Activity (continued)