Before beginning any type of DNA model project with your eighth graders, it is best to help them fully understand the structure of the double helix. Although many students of this age come to the table with prior experience describing or even designing model DNA, don't assume that each child knows what this building block of life looks like. Show off your own handmade model or look at a textbook illustration to demonstrate the structure. Discuss that DNA's double helix is made up of a sugar phosphate back bone that looks like twisted sides of a ladder and acid base pairs that make up the ladder rungs. Include all applicable keywords such as the names of the base: adenine, cytosine, guanine and thymine.
Dried pasta is a simple material that you can easily use to make a model of DNA. Use at least two different types of pasta to create the vertical parts of the DNA ladder structure. Pick ziti or penne as the phosphate and pinwheel or macaroni as the sugar. The students will string the pasta onto fishing line or yarn, therefore, it is important that the pasta has a hole in it. Types that are solid such as spaghetti or spiral will not work. You will need 18 pieces of one pasta for the sugars and 16 for the phosphates. String this in an alternating pattern and tie at the ends. Use cut halves of different colored pipe cleaners for the bases, wrapping them around each other to form pairs and again around the strung pasta line to make rungs.
Use brightly colored beads to construct sugar-phosphate backbones or base pairs in an imaginative double helix model. You will need beads in six different colors for the sugars, phosphates and each base. Make a mini model with small beads or a larger design with over-sized novelty types. Alternate the sugars and phosphates by stringing the beads onto a strong or fishing wire. Create base pairs of adenine-guanine and cytosine-thymine on separate pieces of line, with one piece for each coupling. Use at least five or more small beads to represent each base or two to three larger beads. Using only one bead per base may be confusing or difficult to work with. Tie the base strands to the sugar-phosphate backbone to complete the model. Have your eighth graders tie the models to fishing line and hang from the ceiling.
From gum drops to gummy bears, your students will get a kick out of making a colorful candy model. Link together any squishy candy with tooth picks to create the double helix structure. Choose different candies for the backbone and base parings or simply go with a variety of colors. For example, use two different colors of gum drops to represent the sugars and phosphates and four colors of gummy bears to make up the bases. Other sweets and candies that work well include gummy worms, jelly beans or colored marshmallows.