Nukleotide

A nucleotide is the basic unit of nucleic acids and a versatile molecule in biology. It consists of a nitrogenous base, a five-carbon sugar, and one to three phosphate groups. The sugar is ribose in RNA and deoxyribose in DNA. The bases are divided into purines (adenine and guanine) and pyrimidines (cytosine, thymine in DNA, and uracil in RNA). Nucleotides can exist as monophosphates, diphosphates, or triphosphates; the triphosphate form is especially important for polymerization and energy transfer.

In nucleic acids, nucleotides are linked by phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next, creating a directional chain. DNA typically forms a double helix, while RNA is usually single-stranded and can fold into diverse structures to perform its functions.

Beyond genetic material, nucleotides serve as energy carriers, cofactors, and signaling molecules. ATP and GTP act as primary energy currencies in the cell. Other nucleotide-derived cofactors include NAD+, FAD, and coenzyme A, which participate in oxidation-reduction and metabolic reactions. Cyclic nucleotides such as cyclic AMP (cAMP) and cyclic GMP (cGMP) function as second messengers in many signaling pathways.

Synthesis and metabolism: nucleotides are produced by de novo synthesis or salvage pathways that recycle nucleosides. Purine nucleotides are built from small precursors, while pyrimidine nucleotides are assembled first as ribonucleotides before being converted to deoxynucleotides for DNA. Catabolism leads to breakdown products such as uric acid in humans. Disorders of nucleotide metabolism can affect growth, development, or excretion and include conditions like gout or orotic aciduria.