If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Open the DNA.cml file, in which you will find a portion of a DNA molecule build using Avogadro. As you can see, it is a double alpha helix. Rotate it on its main axis. What do you notice? Do you recognise the so-called “double pitch” of the helix?
Select each half and separate it. You can do this because the two filaments are bound only by hydrogen bonds, dashed in yellow.
It is thanks to the presence of an intermolecular bond that the filaments can coalesce, but it also allows the molecule to split up to give rise to DNA duplication, without even breaking a covalent bond, that would be a much more energy-intensive operation.
Do you recognise the skeleton of phosphorous atoms? What do you notice in the DNA structure?