sugar-phosphate backbone

If you picture DNA as a twisted ladder, the sugar-phosphate backbone is the two long rails that the rungs hang from. It is the structural side of DNA: strong, repetitive, and uniform, holding the information-carrying bases in order along the strand.

The backbone is an alternating chain of two parts — a deoxyribose sugar, then a phosphate group, then another sugar, and so on. Each phosphate links the sugar of one nucleotide to the sugar of the next, forming a continuous strand. The nitrogenous bases branch off from the sugars and point inward toward the partner strand.

Because the same sugar-and-phosphate pattern repeats no matter which base is attached, the backbone carries no genetic information itself; it is the scaffold, while the bases are the message. The phosphates also give DNA a negative electrical charge, which is why DNA can be pulled through a gel by an electric field in the lab.

The backbone has a direction. One end of each strand is called the 5' end and the other the 3' end, named after carbon atoms in the sugar. This built-in directionality determines how the strand is read and copied, and it is why the two strands of the helix run antiparallel.