Procedure:
a typical cardboard tube has a seam that, when viewed from one end, describes a spiral that moves away from the observer. this spiral is called a helix. outline the spiral seam with a colored marker.
using a different colored marker, draw a second spiral midway between the lines of the first one. these two spirals represent the two strands of double-stranded dna.
measure along the tube and mark a dot on each spiral every five cm. label a dot with the letter s to indicate a sugar unit. make a hole in the spirals at each s mark with the thumbtack. move down each spiral and mark a letter p to indicate a phosphate group halfway between the two s dots.
color each toothpick along half its length with a marker. a toothpick represents a base pair in the dna molecule.
starting at the top of the tube, insert a toothpick in one hole at an s label and guide it so it emerges through the hole in the s on the opposite side of the tube. repeat the process for the other holes.

questions:
are the bases on the interior or the exterior of the double helix? are they randomly arranged or neatly stacked?
are the phosphate groups on the interior or the exterior of the dna molecule?
are the sugar groups on the interior or the exterior of the dna molecule?