DNA consists of two complementary strands of nucleotides twisted together to form a double helix. In this space-filling model, each atom is represented by a small sphere. The yellow and red phosphate groups make the two sugar-phosphate backbones easy to spot. They're on the outside of the double helix. We will highlight the backbones in blue. Now let's look at the crossbars connecting the backbones. Each crossbar is a hydrogen-bonded pair of bases, one from each DNA strand. An adenine is always paired with a thymine, and a guanine with a cytosine. The double helix can also be represented by a ball-and-stick model. For simplicity, a ribbon model is often used, with ribbons for the sugar-phosphate backbones.