Both DNA and RNA polymers are composed of monomers. These monomers are called nucleotides. A nucleotide has four parts; however, the monomers of DNA and RNA differ slightly from each other, thus giving the molecules different structures and functions.
A nucleotide comprises a phosphate group, a five-carbon sugar and a nitrogenous base. The five-carbon sugar in DNA is deoxyribose whereas the sugar in RNA is ribose. Four different nitrogenous bases make up DNA and RNA, but the bases differ slightly. In DNA, the bases are adenine, thymine, cytosine and guanine; in RNA, the bases are the same except that uracil replaces thymine. Adenine and guanine are called purines; thymine, cytosine and uracil are pyrimidines.