DNA is a complex macromolecule and so it is no surprise that students often find it difficult to get their heads around the double-helix.
The structure of a polynucleotide can be effectively modeled using sweets and string or cocktail sticks. Marshmallows actually work best for the sugar phosphate backbone but here I’ve used fruit chews instead. I also used four different coloured jelly beans to represent the bases which make up the ‘rungs of the ladder.’ The sweets won’t last long so keep them until the next lesson on DNA replication and then illustrate the action of helicase by asking the students to cut them down the middle (through the ‘hydrogen bond’ cocktail sticks) and throw them away.
The sweet model illustrates the polynucleotide structure really well but it is a bit fragile to twist into a double-helix.
Instead, use this excellent resource on mygenome.org to build your own origami DNA. There is a step-by-step instructional video which the students accessed from their iPads, allowing them to pause and rewind as and when they needed to.
And here is the finished result!