Nucleotides, the monomer units of RNA and DNA, consist of a pentose sugar, either ribose in RNA or deoxyribose in DNA, a phosphate group, and a nitrogenous base. As the name implies, a pentose is a 5-membered, puckered ring. Attached to the ring is the phosphate group, which is a phosphorous atom with 4 covalently attached oxygen atoms. Nucleotides also have either a pyrimidine or purine base, attached to the pentose sugar. Pyrimidines are planar, six-membered rings of 5 carbon and 1 nitrogen atom.
Purines are planar, fused N-containing rings.
The purine and pyrimidine bases of RNA and DNA are shown below. DNA and RNA RNA and DNA are polymers of nucleotides. All rights reserved.