Provide each student with 18 beads in one color and 16 in another. The 18 beads will represent the sugar and the 16 will be the phosphate. Make sure the students know which is which. For example, 18 red sugars and 16 blue phosphates.
Cut two pieces of fishing line to approximately 12 inches per student. Adjust the size for the beads. Have the students start with the sugar color bead and string both kinds, alternating colors. Each piece of line should have 17 total, with nine sugar beads and eight phosphates. Do not push the beads all the way together. Leave a centimeter or so of space in between to tie on the bases later. Tie both ends to the top and bottom beads.
Cut the straws into 1 1/2-inch sections. Assign different colors for each of the bases. For example, adenine will be purple, guanine will be yellow, cytosine will be white and thyamine will be white with a red stripe.
Cut pipe cleaners into sections that are approximately 4 inches in length. Pair the bases into adenine/thymine and cytosine/guanine couples. Thread the straw section pairs onto the pipe cleaner sections. Create eight of each base pair.
Twist the edges of the pipe cleaners around the fishing line in between the beads to form the rungs of the double helix ladder.