Define a nucleotide: which component set is minimally required? Select the composition that best describes a nucleotide building block of DNA or RNA.

Introduction / Context:Nucleotides are the monomeric units of nucleic acids and the currency of many metabolic reactions (for example, ATP). Distinguishing nucleotides from nucleosides clarifies structure and function.

Given Data / Assumptions:

• Nucleoside = base + sugar.
• Nucleotide = nucleoside + phosphate(s).
• Number of phosphates may vary (mono-, di-, triphosphate).

Concept / Approach:The minimal definition of a nucleotide requires a pentose sugar ((deoxy)ribose), a nitrogenous base, and at least one phosphate group. While triphosphates are common as substrates for polymerases, the term “nucleotide” does not require three phosphates.

Step-by-Step Solution:Identify components: base + sugar + ≥1 phosphate → nucleotide.Reject definitions lacking a base (that would be a sugar phosphate) or insisting on three phosphates (too restrictive).

Verification / Alternative check:Biochemistry texts define AMP, ADP, and ATP as nucleotides differing only in phosphate count; all share base, sugar, and phosphate.

Why Other Options Are Wrong:“Sugar + phosphate” omits the base (not a nucleotide). “Paired bases only” lacks sugar and phosphate. “Three phosphates always” is incorrect because monophosphates are also nucleotides.

Common Pitfalls:Confusing nucleoside (no phosphate) with nucleotide (has phosphate).

Final Answer:A sugar, a base, and a phosphate.