DNA is made of repeating units called nucleotides. The four nucleotides in DNA are adenine, cytosine, guanine and thymine. Adenine binds with thymine, and guanine binds with cytosine.
These nucleotides are made of a sugar, deoxyribose, that is bound to a phosphate group and a nitrogenous base. Having different nitrogenous bases is what distinguishes between different nucleotides. Purines have two rings in the nitrogenous base while pyrimidines only have one. Adenine and guanine are purines while thymine and cytosine are pyrimidines Chemical differences in the groups bound to the ring(s) distinguish between adenine and guanine and between thymine and cytosine. Nucleotides bind together to form the double helix structure of DNA. The deoxyribose bind forms the backbone of the DNA, and the nitrogenous bases bind together in the middle to hold both strands of DNA together.