Nucleotides

Nucleotides are the basic building blocks of nucleic acids, consisting of a nitrogenous base, a five‑carbon sugar, and a phosphate group. When a phosphate is attached to the sugar, the molecule is a nucleotide; without the phosphate it is a nucleoside.

Bases in nucleotides are categorized as purines (adenine and guanine) and pyrimidines (cytosine, thymine in DNA, and uracil in RNA). In DNA, thymine pairs with adenine, while in RNA thymine is replaced by uracil, which pairs with adenine.

Nucleotides form polymers through phosphodiester bonds between the 5' phosphate and the 3' hydroxyl of the next sugar, creating a sugar‑phosphate backbone. DNA is typically double‑stranded and forms an antiparallel double helix with base pairing (A with T, G with C). RNA is usually single‑stranded, and base pairing can occur within the molecule, forming various structures.

Nucleotides are classified by the sugar component: deoxyribonucleotides (for DNA synthesis) and ribonucleotides (for RNA synthesis). They also serve as energy carriers (ATP, GTP), cofactors (NAD+, NADP+, FAD, FMN, CoA), and signaling molecules (cyclic AMP, cyclic GMP).

Biological roles extend beyond information storage and transfer: nucleotides participate in metabolism, serve as substrates for replication and transcription, and function in signaling and enzymatic processes. Nucleotide pools are maintained by salvage pathways and de novo synthesis, and imbalances can affect genome stability and cellular function.