Each 'step' of the DNA ladder is made up of a purine and a pyrimidine pair, i.e. of a double ring and a single ring compound. The two most common pyrimidines of DNA are cytosine (C) and thymine (T). The two most common purines of DNA are adenine (A) and guanine (G). Two purines would occupy too little. Because of the purine-pyrimidines pairing the total number of purines in a double-stranded DNA molecule is equal to the total number of pyrimidines. Thus A/T=1 and G/C=1 or A+G+C=T. The ratio A+T/G+C, however, rarely equals 1, and varies with different species from 0.4 to 1.9. The purine and pyrimindine bases pair only in certain combinations. Adenine pairs with thymine (A:T) and guanine with cytosine (G:C).
The chemical bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and gunaine are purines, while the other two bases are pyrimidines. When they join up, they become base pairs, with adenine pairing up with thymine, and guanine with cytosine. Add to base pairs sugar and phosphate, then what you get are nucleotides.
Nucleotides are arranged in what is known as a double helix. Double helixes resemble a twisted ladder, with the base pairs being the wrungs of the ladder, with the sugar and phosphate being the sides.