A nucleoside consists of a nitrogenous base covalently attached to a five-carbon sugar. A nucleotide is a nucleoside with one or more phosphate groups attached.

The common biological bases fall into two structural families:

• purines: adenine (A) and guanine (G), which contain two fused rings;
• pyrimidines: cytosine (C), thymine (T) and uracil (U), which contain one ring.

The sugar is usually ribose in ribonucleotides and 2-deoxyribose in deoxyribonucleotides. Deoxyribose differs from ribose at the $2'$ carbon: it has H where ribose has OH.

A base plus ribose forms a ribonucleoside such as adenosine. Adding phosphate gives nucleotides such as AMP, ADP and ATP.

Sugar carbon numbering

The sugar carbons are conventionally labeled $1'$ through $5'$ so they are not confused with positions in the nitrogenous base.

• the base is attached at the $1'$ carbon;
• the $5'$ carbon commonly carries phosphate;
• the $3'$ hydroxyl is central to forming nucleic-acid chains.

Phosphate groups give nucleotides charge and connectivity

Under typical aqueous biological conditions, phosphate groups carry substantial negative charge. This makes free nucleotides and nucleic-acid backbones strongly hydrophilic and causes them to interact electrostatically with oppositely charged ions and molecular groups.

Nucleotides perform several different roles in biology. They are the monomers of DNA and RNA, and some also act as transferable chemical-group carriers or signaling molecules.

ATP is therefore not a separate kind of chemistry unrelated to nucleic acids. It is a ribonucleotide—adenosine with three phosphate groups—whose phosphate-transfer reactions make it especially important in cellular energy coupling.

The shared nucleotide architecture lets one chemical framework support both information storage and reaction coupling.