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Learn7402Paper 1 · Paper 1: Topics 1–4Biological moleculesDNA structure and replication
Biological molecules · Subtopic 3 of 4

DNA Structure and Replication

A nucleotide = phosphate + pentose sugar (deoxyribose in DNA, ribose in RNA) + nitrogenous base. Purines (A, G) have two rings; pyrimidines (C, T, U) one. Nucleotides join by phosphodiester bonds (condensation) into a sugar-phosphate backbone. DNA is a double helix of two antiparallel strands joined by hydrogen bonds: A–T (2), C–G (3). Semi-conservative replication: helicase breaks hydrogen bonds; free nucleotides pair with each exposed strand; DNA polymerase joins them 5′→3′ (continuous leading strand; fragments on the lagging strand joined by ligase). Each new molecule has one old and one new strand, shown by the Meselson–Stahl ¹⁵N/¹⁴N experiment. RNA is single-stranded with ribose and uracil.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe nucleotide and DNA structure.
  • Describe semi-conservative replication and the enzymes involved.
  • Interpret Meselson–Stahl results.
  • Compare DNA and RNA.
Pause and think

Why is it useful that DNA's two strands are held together by hydrogen bonds rather than covalent bonds?

Section 2 of 14

Core idea

Start hereCopying 3 billion letters

Each time a human cell divides it copies about 3 billion base pairs with very few errors.

The big idea

Complementary base pairing lets each DNA strand act as a template, so each new molecule keeps one original strand.

Semi-conservative replication
  1. helicase breaks H-bonds\text{helicase breaks H-bonds}
    Strands separate.
  2. free nucleotides pair: A–T, C–G\text{free nucleotides pair: A–T, C–G}
    Complementary pairing.
  3. DNA polymerase forms phosphodiester bonds\text{DNA polymerase forms phosphodiester bonds}
    Builds new strand 5′→3′.
  4. 2 molecules, each 1 old + 1 new strand\text{2 molecules, each 1 old + 1 new strand}
    Semi-conservative.
Worked exampleChargaff's rule

DNA is 30% adenine. Percentage of cytosine?

Goal
%C.
Strategy
A = T, C = G.
  1. A+T=60%A+T=60\%
    T matches A.
  2. C=G=402=20%C=G=\frac{40}{2}=20\%
    Remainder shared.
Answer20%.
✓ CheckTotal 100% ✓.

Summary

  • Antiparallel double helix.
  • Helicase, polymerase, ligase.
  • Meselson–Stahl.
Key words
Phosphodiester bond
Bond linking nucleotides in a strand.
Semi-conservative
Each new DNA molecule has one original strand.
History1953. Watson and Crick built the double helix model using Rosalind Franklin's X-ray data and Chargaff's base ratios.
Fun factTwo metres of DNA. The DNA in one human cell is about 2 m long, yet fits into a nucleus about 6 µm across.
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