DNA is called the “ladder of life,” and rightly so. Every living organism on planet Earth has DNA as its hereditary molecule. What’s more, the code that DNA contains, in which every three bases is the code for a specific amino acid, is the same for all living things, whether you’re looking at bacteria or buffalo, amoebas or anteaters.
DNA is simple to extract from living cells, but to do anything interesting with that DNA requires specialized equipment and a variety of technical procedures that are well beyond the scope of this article. If you’re trying to put together a good DNA project for a science class, science fair, or competition, enlist the help of a biology teacher or a university professor. They may have some or all of the equipment you need.
EXTRACT DNA FROM LIVING TISSUE
Crude DNA extraction is fairly simple. It involves mechanically breaking down cell walls (if you’re working with plant tissue), dissolving cell membranes and nuclear membranes using detergents (since cell membranes are composed of fats), separating the DNA from cellular proteins using an enzyme, dissolving the DNA in water, then precipitating the DNA using alcohol, since DNA is not soluble in alcohol.
Many tissues work well for DNA extraction, including strawberries, fresh liver, wheat germ, and onion. Strawberries happen to be pleasant to work with, so here’s a recipe for extracting DNA from strawberries:
Put 3 medium strawberries in a blender with 20 ml of water. Whir in the blender until the strawberries are liquefied. Pour into a beaker. The mechanical action of the blades will break apart cell walls. Find a clear shampoo that contains lauryl or laureth sulfate, detergents that are effective at dissolving cellular membranes. Measure 10 ml of clear shampoo in a graduated cylinder and pour into the beaker. Add two pinches of salt. The exact amount isn’t critical, but the positively-charged sodium ions from salt are needed to stick like a shield to DNA and help the DNA molecules cling together.
Place a coffee filter in a glass funnel and place the funnel into a second beaker. Pour the strawberry mixture through the filter. This will remove extra cell debris, but the DNA, which is water soluble, will go through the filter. Place the beaker in a warm water bath at 65 degrees Celsius. The heat will denature cellular proteins, but it’s not hot enough to denature the DNA. Gently stir for about 10 minutes.
Remove the beaker and add a pinch of meat tenderizer, which contains the enzyme papain. This will break down cellular proteins and separate them from the DNA. Place the beaker in an ice water bath until it cools to about room temperature.
Pour some of the liquid into a test tube. Gently pour ice cold ethyl alcohol or rubbing alcohol down the inside of the tube to create a floating layer of alcohol on top. Do not stir or shake. After a few minutes, you’ll see the interface between the alcohol and the strawberry solution turn cloudy as strands of DNA precipitate from the mixture. If you’re skillful, you can twirl some of the DNA around a stirring rod. Or use a pipette to collect the DNA.
You can try extracting DNA from other tissues using the same procedures. Try using samples of the same weight, then use a pipette to suction up the DNA extracted and drop it onto a pre-weighed coffee filter. Let the filter dry and weigh it again. Subtract the two weights to find out how much DNA you extracted. Do this with several different tissues and find out which has the most DNA. You can even extract small amounts of DNA from cells scraped from the inside of your cheek. You’ll need much smaller amounts of the extraction chemicals, of course.
WHAT CAN YOU DO WITH DNA?
After scientists extract DNA, they may cut it with enzymes to extract pieces that they are interested in studying. They may use a PCR machine (polymerase chain reaction) to increase the quantity of DNA in the sample. They may use a technique called gel electrophoresis to separate different pieces of DNA in a slab of agar by running an electrical current through the agar.
To learn how gel electrophoresis works, the Learn Genetics site at the University of Utah has an interactive lesson. This page also describes microarrays, another tool for working with DNA:
Gel electrophoresis isn’t difficult to do, but it does require specialized equipment. Electrophoresis boxes and kits can be purchased from science supply companies such as Carolina Biological Supply (http://www.carolina.com/) or Flinn Scientific (http://www.flinnsci.com/). Ask your science teacher or a university professor if there is equipment that you can use, because these supplies aren’t cheap.
If you have good woodworking skills, you can build a gel electrophoresis chamber from Plexiglas. The complete instructions are on the Learn Genetics site from the University of Utah (registration required for download):
If you’re working with a science teacher, you might offer to build electrophoresis chambers if the teacher will help you get supplies for using them.
What other projects could you do that involve DNA? Here are two from the Learn Genetics site, with complete instructions and materials lists that you might consider:
Does Sunscreen Protect My DNA? (http://learn.genetics.utah.edu/units/basics/print-and-go/sunscreen.cfm)
Mystery Yeast Mutation (http://learn.genetics.utah.edu/units/basics/print-and-go/mutation.cfm)