Follow these steps to make your very own 3-D model of DNA. Remember, DNA is shaped like a double helix or twisted ladder.   First, make the sides of the ladder. To do this, cut two strips of colored paper that are at least two feet long and about one inch wide. (If your paper's not long enough, tape a couple of strips together.) Use a pencil and a ruler to mark each inch along one strip. Next, make the steps or rungs of the ladder. In DNA, these steps are made of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Choose four colors from your markers to represent the four bases. Write the colors you choose for each base here:     Take 10 toothpicks. For each one, color half the toothpick the color you chose for A. Color the other half the color you chose for T. Take ten more toothpicks and do the same thing with the colors you chose for C and G. (It's important that A is always paired with T and C is always paired with G.) Now construct your double helix! Grab both strips of paper, put one on top of the other, and tape them together at each end. Using the pencil marks as a guide, poke a toothpick through the middle of the strips at every inch. You can add toothpicks in any order you want. Keep going until your strips are filled up. Be sure to leave some room at the ends of the strips. Make sure all the toothpicks are pushed halfway through the holes. Gently spread the two strips apart until your model looks like a ladder. This is what DNA would look like if it weren't twisted. (Hint: If any toothpicks fall out, put a drop of glue on the hole, stick the toothpick back in, and let the glue dry. You could also wrap a piece of tape around the toothpick.) Now for the twist: Tape one end of your model to a wall or wherever you want to hang your DNA model. (Make sure the place you choose is okay with your parents.) Then, carefully holding the DNA model, twist it at least once around until it looks like a double spiral. Tape the other end to a flat surface. Congratulations - you've done the DNA twist!     Experiment by using different materials to see what works best, like Popsicle sticks or cotton swabs for the steps and pipe cleaners or flexible wire for the sides.   Make a DNA mobile. Use a flexible material (like pipe cleaners or wire) that will twist without being taped down. Tie some string to the top step and hang it in your room.   DNA is very, very long and curves in all directions. First, see how long you can make your DNA model. Then see if you can make your double helix turn and curve in different directions.