Erwin Chargaff studied samples of DNA and discovered that the amount of adenine was almost equal to the amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine.
This is known as Chargaff's Rule. http://mooni.fccj.org/~ethall/dna/dna.htm
P.A. Levene discoverd DNA can be broken down into a sugar, a phosphate group, and four nitrogen-containing bases (adenine, guanine, thymine, and cytosine).
Also a nucleotide contains one base, a sugar, and a phosphate group.