6.1 Structure of Nucleic Acids and Replication of DNA

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the structure of nucleotides, including the phosphorylated nucleotide ATP
  2. State that adenine and guanine are purines (double ring), and cytosine, thymine, and uracil are pyrimidines (single ring)
  3. Describe the structure of DNA as a double helix, including antiparallel strands, complementary base pairing, hydrogen bonding differences, and phosphodiester bonds
  4. Describe semi-conservative DNA replication during the S phase, including the roles of DNA polymerase and DNA ligase, and the difference between leading and lagging strand synthesis
  5. Describe the structure of an RNA molecule, using mRNA as an example

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1. Nucleotides — The Building Blocks

Nucleic acids (DNA and RNA) are large molecules made up of many smaller units joined together in a long chain. These smaller units are called nucleotides. Because nucleic acids are made of many repeated units, they are called polymers — and since the units are nucleotides, DNA and RNA are also called polynucleotides.

Every single nucleotide is made up of exactly three parts:

  • A phosphate group — a chemical group containing phosphorus and oxygen atoms. It is always attached to the 5th carbon of the sugar.
  • A pentose sugar — a sugar with 5 carbon atoms (the word "pent" means five). The carbons in the sugar are numbered 1 to 5. The nitrogenous base is always attached to carbon 1, and the phosphate is always attached to carbon 5.
  • A nitrogenous base — a nitrogen-containing molecule that carries genetic information. There are five different bases in total (more on these below).

Think of a nucleotide like a lollipop: the sugar is the round head, the phosphate is attached on one side, and the base sticks out from the other.

The Two Types of Pentose Sugar

The type of sugar in a nucleotide tells you whether it belongs to DNA or RNA:

  • Deoxyribose — found in DNA nucleotides. It has a hydrogen atom (H) at carbon 2.
  • Ribose — found in RNA nucleotides. It has a hydroxyl group (–OH) at carbon 2.

This one tiny difference — H versus OH at carbon 2 — is what makes DNA and RNA chemically different.

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