What makes up the rungs of a DNA ladder?

Answer 1

I'll assume you're talking about the "double helix" structure which looks like a twisted ladder, rather than an electrophoresis gel plate film.

DNA is a double stranded sequence of nucleic acids... on one side a "protein code" is made using Adenine, Thymine, Guanine, Cytosine.

On one side, each of these nucleic acids is punctuated by a phosphate group.

Each "rung" on the ladder is formed by hydrogen bonds between one side and the other. Adenine bonds to Thymine and Guanine to Cytosine.

Hydrogen bonds are formed when an exposed hydrogen atom, which is covalently bonded to an electronegative atom, is in close proximity to another exposed electronegative atom. This happens because the hydrogen atom has a relatively positive charge and the juxtaposed electronegative atom is relatively... well, negatively charged.