The steps, or rungs, of the DNA ladder are made up of two bases joined together with either two or three weak hydrogen bonds. The rails of the ladder are made up of alternating sugar and phosphate molecules. The rungs are comprised of two bases between sugar molecules.
The basic building blocks of DNA are called nucleotides. Nucleotides are made up of one sugar molecule, one phosphate molecule and one of four different bases. The purine bases are adenine and guanine, and the pyrimidine bases are thymine and cytosine. The nucleotides of DNA line up with a phosphate and a sugar molecule, making the handrail of the ladder.