Separate the foam balls into three piles: 44 balls for the phosphates, 40 balls for the deoxyribose sugars and 40 for the nucleotides.
Use poster paint to paint the phosphates all one color. Poster paint washes off of your hands easily with soap and water, so you do not have to worry about getting it on your hands. Foam balls do not soak up paint evenly and have areas of lighter or darker color, so you don't have to worry about fingerprints
Lay the painted balls on a piece of wax paper and allow them to dry for at least 4 hours.
Leave the balls representing the deoxyribose sugars white.
Divide the nucleotide foam balls into four piles of 10 balls each and designate each pile to a different nucleotide.
Repeat steps 2 and 3, painting each nucleotide a different color. For example, adenosine (A) is red, guanine (G) is blue, cytosine (C) is yellow and thymine (T) is purple.
Place two toothpicks into a phosphate ball. The toothpicks need to be across from each other.
Place a sugar ball on the end of one of the toothpicks.
Place a toothpick in the sugar ball across from the first.
Attach another phosphate to the new toothpick.
Repeat steps 8 through 10, alternately adding phosphates and sugars until you have created a chain of 12 phosphates and 10 sugars with toothpicks sticking out of the terminal phosphates.
Repeat steps 7 through 11 until you have two separate phosphate-sugar chains. These are the sides of your DNA ladder.
Create nucleotide pairs by connecting A to T and G to C with a toothpick. Continue creating pairs until all nucleotide foam balls have been connected. You should have 20 pairs, 10 of each type.
Place a toothpick on either end of the nucleotide pair, in line with the toothpick that already connects the nucleotide balls.
Lay the two phosphate-sugar chains parallel to each other on the work surface.
Start at the bottom-most sugar and attach a nucleotide pair by pressing the outer toothpicks into the sugar on each chain.
Continue attaching nucleotide pairs to the sugars until all pairs have been used. There is no particular order in which you should attach them.
Lay a foam block at both ends of the DNA ladder.
Press the terminal toothpicks of the phosphate-sugar chains into the center of the foam block.
Hold onto each block and carefully lift the model upright.
Hold the lower block still and gently rotate the upper block to create a central twist, dividing the model so that top base pair has rotated 180 degrees from its original position. This will demonstrate DNA's characteristic double-helical twist.