Nucleic acids are macromolecules that exist as polymers called polynucleotides. As indicated by the name, each polynucleotide consists of monomers called nucleotides. A nucleotide, in general, is composed of three parts: a five-carbon sugar (a pentose), a nitrogen-containing (nitrogenous) base, and one to three phosphate groups. The beginning monomer used to build a polynucleotide has three phosphate groups, but two are lost during the polymerization process. The portion of a nucleotide without any phosphate groups is called a nucleoside.
To understand the structure of a single nucleotide, let’s first consider the nitrogenous bases. Each nitrogenous base has one or two rings that include nitrogen atoms. (They are called nitrogenous bases because the nitrogen atoms tend to take up H+ from solution, thus acting as bases.) There are two families of nitrogenous bases: pyrimidines and purines. A pyrimidine has one six-membered ring of carbon and nitrogen atoms. The members of the pyrimidine family are cytosine (C), thymine (T), and uracil (U). Purines are larger, with a six-membered ring fused to a five-membered ring. The purines are adenine (A) and guanine (G). The specific pyrimidines and purines differ in the chemical groups attached to the rings. Adenine, guanine, and cytosine are found in both DNA and RNA; thymine is found only in DNA and uracil only in RNA.
Now let’s add the sugar to which the nitrogenous base is attached. In DNA the sugar is deoxyribose; in RNA it is ribose. The only difference between these two sugars is that deoxyribose lacks an oxygen atom on the second carbon in the ring, hence the name deoxyribose.
So far, we have built a nucleoside (base plus sugar). To complete the construction of a nucleotide, we attach one to three phosphate groups to the 5′ carbon of the sugar (the carbon numbers in the sugar include ′, the prime symbol. With one phosphate, this is a nucleoside monophosphate, more often called a nucleotide.
Urry, Lisa A.. Campbell Biology (p. 84). Pearson Education. Kindle Edition.