The part of the DNA that forms outside the ladder structure is the sugar-phosphate backbone.

Here's why:

* The ladder: The "rungs" of the DNA ladder are made up of the nitrogenous bases (adenine, thymine, guanine, and cytosine) connected by hydrogen bonds.

* The backbone: The sides of the ladder, or the backbone, are composed of alternating sugar (deoxyribose) and phosphate molecules. These molecules are linked together by phosphodiester bonds.

Here's a simple analogy:

Imagine a ladder. The rungs are the bases, and the sides of the ladder are the sugar-phosphate backbone. The backbone provides structural support and connects the bases.

In summary:

* Inside the ladder: Nitrogenous bases

* Outside the ladder: Sugar-phosphate backbone