6.1.1—Nucleotide structure

A nucleotide is one building block of a nucleic acid. It contains three components: a pentose sugar, a phosphate group and a nitrogenous base. Many nucleotides can join to form a polynucleotide such as DNA or RNA.

• Pentose sugar: DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose. The sugar identity helps distinguish which nucleic-acid context the nucleotide belongs to.
• Phosphate group: The phosphate contributes to the sugar–phosphate backbone when nucleotides join in a nucleic-acid strand. It is one component of a nucleotide, not a whole nucleic acid.
• Nitrogenous base: The base is classified as a purine (adenine or guanine, double-ring) or a pyrimidine (cytosine, thymine or uracil, single-ring). The classification names the base structure; it does not replace the sugar or phosphate components.
• ATP boundary: ATP is a phosphorylated nucleotide: adenine and ribose are attached to three phosphate groups. Its phosphate groups make it suitable for energy transfer in cell processes, but ATP is not a DNA or RNA strand and is not itself a nucleic acid.

The three-part structure supports two different biological roles: repeated sugar–phosphate connections allow nucleotides to build a nucleic-acid chain, while the attached bases provide the base identities used in sequence and pairing rules. ATP uses a nucleotide-like structure with additional phosphate groups for energy transfer, so “nucleotide” is a component-level description rather than a synonym for DNA, RNA or ATP.

A nitrogenous base alone is not a nucleotide, and a nucleotide is not the same as a nucleic-acid polymer. Purine/pyrimidine describes the base; deoxyribose/ribose describes the sugar. ATP is a phosphorylated nucleotide with an energy-transfer role, not a DNA/RNA building strand.