Glossary
nucleotide
A nucleotide is the building block of DNA and RNA: a phosphate, a five-carbon sugar, and one of the nitrogenous bases.
A nucleotide is the repeating unit of DNA and RNA, and it has three parts: a phosphate group, a five-carbon sugar, and a nitrogenous base. The sugar decides which nucleic acid it is — deoxyribose in DNA, ribose in RNA. The base is one of four: adenine and guanine, the larger two-ring purines, and cytosine and thymine, the smaller single-ring pyrimidines; RNA carries uracil in place of thymine. Nucleotides are joined into a strand by bonds between the phosphate of one and the sugar of the next, so the strand has a sugar-phosphate spine with the bases sticking out. The bases pair across two strands — adenine with thymine, guanine with cytosine — and that pairing is what lets one strand be copied into its partner. ATP is a nucleotide too, with three phosphates instead of one, which is a reminder that the same three-part design serves both storage and energy transfer.
Taught in
Related terms
- allele — An allele is one version of a gene; a person carries two for most genes, one on each chromosome of a homologous pair, and the pair is the genotype.
- ATP — ATP is the molecule cells use to carry energy — breaking off its third phosphate releases the energy a reaction needs.
- virus — A virus is genetic material wrapped in a protein coat — it has no cell, no metabolism, and can only reproduce inside a host cell.
- prokaryote — A prokaryote is a cell with no nucleus and no membrane-bound organelles — its DNA sits loose in the cytoplasm, as it does in bacteria.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)