Nucleotide (basic unit of nucleic acids)

Composition and chemical types

Each nucleotide consists of three parts: a nucleobase, a five‑carbon sugar and at least one phosphate group. The sugar is ribose in ribonucleotides (ribose) and 2‑deoxyribose in deoxyribonucleotides (deoxyribose). Bases fall into two families: the larger purines and smaller pyrimidines. In DNA the common purines are adenine and guanine, and the pyrimidines are thymine and cytosine. In RNA, uracil replaces thymine. Nucleotides also differ by phosphorylation state: nucleoside monophosphates, diphosphates and triphosphates are chemically and biologically distinct.

Structure in nucleic acids and molecular interactions

When nucleotides polymerize, adjoining phosphate and sugar groups form a sugar–phosphate backbone through phosphodiester bonds, which gives strands directionality with 5′ and 3′ ends. Complementary bases on different strands pair by hydrogen bonds—adenine pairs with thymine (or uracil in RNA) and guanine pairs with cytosine—contributing to the three-dimensional shape of DNA and to specific folding and pairing in RNA. Enzymes called polymerases catalyze the addition of nucleotides during replication, transcription and repair, using the energy stored in phosphate bonds to drive polymer formation.

Biological roles and examples

Nucleotides have several major functions beyond encoding genetic information. They act as carriers of metabolic energy (for example, adenosine triphosphate), participate in cellular signaling as cyclic nucleotides, and form parts of key enzymatic cofactors. Common biological roles include:

• Energy transfer: triphosphate forms provide readily usable chemical energy for cellular work.
• Signaling: cyclic derivatives function as second messengers in signal transduction.
• Cofactors and carriers: nucleotide moieties are components of cofactors such as coenzyme A, NAD and FAD, and help shuttle chemical groups in metabolism (metabolism).
• Precursors and building blocks: nucleotides supply activated substrates for synthesis of amino acids, proteins and cell membrane components, and are required during cell division and growth.

History, laboratory uses and medical relevance

Recognition of nucleic acids and their subunits developed over many decades: early chemists isolated material rich in phosphorus from cell nuclei and later work revealed the nucleoside and nucleotide composition and the double‑helical structure of DNA. In modern laboratories nucleotides are manipulated in many ways: they can be labeled for tracking experiments, incorporated as modified analogs in biochemical assays and used to probe structure and function of nucleic acids. For example, researchers label nucleotides using radioactive tracers such as radionuclides in experimental biochemistry. Chemically altered nucleotides and nucleoside analogs also form the basis of several antiviral and anticancer drugs because they interfere with polymerases or DNA synthesis.

Because nucleotides are central to both the storage of genetic instructions and the daily fueling of cellular processes, they remain a core focus in fields from molecular genetics and biochemistry to medicine and biotechnology.

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