describe the structure of nucleotides, including the phosphorylated nucleotide ATP (structural formulae are not expected)

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology – Structure of Nucleic Acids and DNA Replication

Structure of Nucleic Acids and DNA Replication

Learning Objective

Describe the structure of nucleotides, including the phosphorylated nucleotide ATP (structural formulae are not required).

What is a Nucleotide?

A nucleotide is the basic building block of nucleic acids (DNA and RNA). Each nucleotide consists of three distinct parts:

ComponentDescription
Nitrogenous baseA heterocyclic aromatic ring that can be a purine (adenine, guanine) or a pyrimidine (cytosine, thymine, uracil).
Five‑carbon sugarDeoxyribose in DNA and ribose in RNA. The sugar provides the backbone to which the base and phosphate are attached.
Phosphate group(s)One or more phosphate residues attached to the 5′‑carbon of the sugar. Phosphate groups link successive nucleotides together via phosphodiester bonds.

General Nucleotide Formula

In symbolic form a nucleotide can be written as:

\$\text{Base}–\text{Sugar}–\text{(PO}4\text{)}n\$

where n = 1 for a monophosphate nucleotide, 2 for a diphosphate, and 3 for a triphosphate.

Phosphorylated Nucleotide – ATP

ATP (adenosine triphosphate) is a key energy‑carrier molecule in cells. It is a nucleotide that contains:

  • Adenine – a purine base.

  • Ribose – a five‑carbon sugar (ribose, not deoxyribose).

  • Three phosphate groups – linked in a chain (α, β, γ phosphates).

When ATP is hydrolysed, the terminal (γ) phosphate is removed, releasing energy and forming ADP (adenosine diphosphate) plus inorganic phosphate (\$\text{P}_i\$):

\$\text{ATP} \;\xrightarrow{\text{hydrolysis}}\; \text{ADP} + \text{P}_i + \text{energy}\$

Role of Nucleotides in DNA

  1. DNA is composed of two antiparallel strands forming a double helix.

  2. Each strand is a polymer of deoxyribonucleotides linked by phosphodiester bonds between the 3′‑hydroxyl of one sugar and the 5′‑phosphate of the next.

  3. The sequence of nitrogenous bases encodes genetic information.

Key Points to Remember

  • All nucleotides share the same three‑part structure.

  • The type of nitrogenous base determines the specific nucleotide (e.g., adenine → adenine nucleotide).

  • ATP is a ribonucleotide with three phosphates; it is the primary energy currency of the cell.

  • In DNA, the sugar is deoxyribose and the bases are A, T, C, G; in RNA, the sugar is ribose and the bases are A, U, C, G.

Suggested diagram: A schematic of a nucleotide showing the base, sugar, and phosphate groups, plus a separate illustration of ATP with its three phosphate residues.