Here's why:

* DNA Structure: DNA is a double helix, meaning it's shaped like a twisted ladder.

* Rungs: The "rungs" of the ladder are formed by pairs of nucleotides.

* Nucleotides: Nucleotides are the building blocks of DNA. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

* Base Pairing: The four nitrogenous bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C). They always pair up in a specific way: A with T, and G with C. This pairing forms the "rungs" of the DNA ladder.

So, a pair of nucleotides, connected by their nitrogenous bases, forms one step in the DNA ladder.