By Kevin Beck, Updated Aug 30, 2022
Nucleotides and nucleosides are both the building blocks of DNA and RNA, but the key distinction is the presence of a phosphate group. A nucleoside consists of a nitrogenous base linked to a five‑carbon sugar (ribose or deoxyribose). When one or more phosphate groups attach to this sugar, the resulting structure is a nucleotide. This seemingly small structural difference influences how these molecules interact, how they form the double‑helix of DNA, and how RNA functions in protein synthesis.
A nucleoside is made up of two parts: a nitrogenous base and a sugar. The sugar can be ribose (in RNA) or deoxyribose (in DNA). The nitrogenous base falls into one of two categories: purines (adenine and guanine) or pyrimidines (cytosine, thymine, and uracil). In DNA, the four bases are adenine, guanine, cytosine, and thymine; RNA replaces thymine with uracil.
The addition of a phosphate group—or a chain of up to three phosphates—to the sugar transforms the nucleoside into a nucleotide. This change is the defining feature that separates nucleotides from nucleosides and determines how they can link together to form polymers.
DNA’s double‑stranded structure relies on complementary base pairing: adenine pairs exclusively with thymine, while cytosine pairs with guanine. In RNA, the single‑stranded molecule can fold back on itself to create transient double‑stranded regions where adenine pairs with uracil and cytosine pairs with guanine. These specific pairings ensure accurate genetic information transfer during transcription and translation.
When a nucleoside acquires a single phosphate, it becomes a nucleotide monophosphate. Nucleotides can further bind additional phosphates to form diphosphates and triphosphates, which play crucial roles in cellular energy transfer and signaling. For example:
Understanding these molecules’ structures and interactions provides insight into the fundamental processes that sustain life—from DNA replication to cellular metabolism.
