The rungs of the DNA ladder consist of four nitrogenous bases: adenine, thymine, cytosine and guanine. Each rung of the ladder is composed of two of the nitrogenous bases held together by hydrogen bonds.
The DNA strand is a double helix formed by the biochemical interactions of its base units. The structure of DNA is always the same. The nitrogenous bases (adenine, thymine, cytosine and guanine) form pairs that create the rungs of the ladder. These bases are commonly referred to only by their first letters ("A," "T," "C," and "G").
Adenine and guanine are the purine bases. They have a double-ring molecular structure. In contrast, thymine and cytosine are the pyrimidine bases. They have a single-ring molecular structure. The difference between single- and double-ring form contributes to the helical structure of the DNA strand. The bases always bond together in the same configuration: adenine with thymine and cytosine with guanine. These nucleotides are bonded by two or three weak hydrogen bonds.
A clever way to remember the nitrogenous base pairs of the DNA strand is to memorize "A loves T" and "C loves G." These molecules always bond in this way to form the rungs of the DNA ladder.