Cut each of the two white pipe cleaners into six 2-inch sections. These 2-inch sections will form the “rungs” of the DNA ladder connecting the two black pipe cleaners. These “rungs” represent the hydrogen bonds that hold the nucleobases together to form the nucleic acid of DNA.
Thread two red craft beads and two blue craft beads onto one of the 2-inch sections of pipe cleaner. Repeat until you have six 2-inch sections with red and blue beads on them. The red beads represent the adenine which is one of the constituent bases of the four nucleic acids that make up DNA. The blue beads represent thymine, another one of the four. Adenine and thymine always come in pairs on the same rung of the DNA ladder.
Thread another 2-inch section of pipe cleaner with one yellow bead, then a purple bead, then another yellow bead. Repeat until you have six 2-inch sections with yellow and purple beads in this pattern. The yellow beads represent guanine and the purple ones represent cytosine.
Set the two black pipe cleaners so that they are 2 inches apart and parallel to each other on your work table.
Begin with an adenine and thymine rung at the top of this ladder. Twist the ends of the 2-inch sections around both sides of the black pipe cleaners. Pinch tightly on these areas so the pipe cleaners stay attached. Place another 2-inch section with the guanine and cytosine beads 1 inch below the first rung. Twist both ends as you did on the first rung.
Alternate the red and blue and yellow with the purple strands down the DNA ladder until each has been firmly attached.
Hold the top part of the ladder in one hand and the bottom in the other and gently twist into the familiar double helix shape of DNA.