the sugar-phosphate backbone
Picture a ladder. The two long side rails are made of one repeating material running the whole length, and the rungs hang between them. In DNA, those side rails are the sugar-phosphate backbone — the strong, monotonous railing that holds the information-carrying bases in order.
The backbone is built by alternating two of the three nucleotide parts over and over: sugar, phosphate, sugar, phosphate, all the way down the strand. The phosphate of one nucleotide bridges from the 3' carbon of one sugar to the 5' carbon of the next, so the backbone reads 'sugar-phosphate-sugar-phosphate' like beads on a wire. The bases are not part of the backbone at all; they branch off the sugars and point inward. Because every phosphate carries a negative charge, the whole backbone is uniformly negative, which is why DNA migrates toward the positive electrode in gel electrophoresis.
Thinking of the backbone as 'just structure' is exactly right and surprisingly powerful. Its sameness is the point: a uniform, featureless rail means any sequence of bases can be carried without changing the molecule's overall shape, so the four-letter message is free to say anything. The backbone is also what enzymes grip and cut, and its negative charge is what proteins, dyes, and the laboratory tools of molecular biology recognise and hold.
In gel electrophoresis, an electric field pulls DNA through a jelly toward the positive pole; it can do this only because every fragment carries the same uniform negative charge along its backbone, so they separate purely by length.
A uniform negative rail — the same shape no matter what it spells.
The bases are not in the backbone. A frequent muddle is to imagine the sequence is written into the rails; in fact the rails are identical everywhere and the message lives entirely in the bases that branch off them.