You can easily model DNA in a variety of media due to the puzzle-like structure of the bonds. With coloured paper, you can create a sturdy structure which is suitable for classroom use.
Fold one white piece of construction paper in half across the 9-inch length, creating two 4.5-by-12-inch sides. Apply glue liberally on the inside of the fold, and crease together. Apply pressure with ruler's straight edge, sliding from the bottom to top edge of the paper to distribute the glue. Repeat with a blue piece of construction paper, using less glue. Set the blue piece aside.
Place the ruler along the long edge of the folded paper and make horizontal pencil dashes every 2 inches. Move the ruler over to the opposite edge of the paper and repeat. Flip the ruler horizontally and connect each set of dashes to create six sections. Mark the centre point of the 4.5-inch side on both ends and connect with a light, dashed pencil line. Cut the paper along the horizontal lines, creating six bars. Discard the top and bottom bar.
Take one bar and place it flat on the table, on a vertical plane. Erase the straight centre line and replace with a jagged centre line in pencil, on both sides of the glued bar. Repeat for the next bar. Place both bars on horizontal planes against the table. Lightly, in pencil, write "R" on the right half of each bar, and "G" on the left. Flip the bars over and write "R" on the left half and "G" on the right half. Take the red marker and colour each area marked "R," and along half the jagged midline, for both front and back. Take the green marker and repeat the process for halves marked with a "G". With the black marker, outline the jagged centre line darkly. Fold 1/2 inch ends on one bar, in opposite directions, creating a three-dimensional "Z". Repeat for the second bar.
On remaining two bars, replace the straight centre line with squiggly lines on both sides. Lightly, in pencil, write "P" on the right half of each bar, and "Y" on the left. Flip the bars over and write "P" on the left half and "Y" on the right half. Take the purple marker and colour each area marked "P," and along half the squiggly midline, for both sides. Take the yellow marker and repeat for sides with a "G." Outline the squiggly midline darkly with the black marker. Fold 1/2 inch ends on the bar, in opposite directions, creating a three-dimensional "Z". Repeat for the second bar.
Take the glued blue piece of construction paper and fold it in half, creating two bars of paper 2.25 by 12 inches. Cut along the fold. Place these pieces 4 inches apart. Take one R/G bar and apply glue to outward-facing side of the folded tabs. Create the first rang of the ladder by affixing one sticky tab to the centre line of first blue side, 1/2 inch down, and the opposite sticky tab to centre line of second blue side, 1/2 inch down. Repeat with a P/Y bar, placing it 1 inch below the R/G bar. Complete the ladder with remaining R/G and P/Y bars.
Take one pipe cleaner and slip it inside the folded and glued piece of one blue side, displacing glue with the sharp end. Repeat with the second pipe cleaner on the opposite blue side. Twist the ends of the model so the ladder spirals. With the remaining piece of white paper create a key, writing "Model Key" at top. Draw a spot with the red marker, and write "Adenine" next to it. Repeat for the green marker, writing "Thymine," the purple marker, writing "Cytosine," and the yellow marker, writing "Guanine." With the blue marker, draw a small bar and write "Phosphate deoxyribose backbone."