Revision Notes

4.3 DNA Structure

Extended Only

Describing DNA Structure

  • DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms
  • It consists of two strands of DNA wound around each other in what is called a double helix


DNA, chromosomes and the nucleus, IGCSE & GCSE Biology revision notesDNA, chromosomes and the nucleus


  • The individual units of DNA are called nucleotides


A nucleotide, IGCSE & GCSE Biology revision notesA nucleotide


  • All nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached
  • There are four different bases, Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)
  • The bases on each strand pair up with each other, holding the two strands of DNA in the double helix
  • The bases always pair up in the same way:
    • Adenine always pairs with Thymine (A-T)
    • Cytosine always pairs with Guanine (C-G)


DNA base pairs, IGCSE & GCSE Biology revision notesDNA base pairs


  • The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder


The DNA helix is made from two strands of DNA held together by hydrogen bonds, IGCSE & GCSE Biology revision notesThe DNA helix is made from two strands of DNA held together by hydrogen bonds


  • It is this sequence of bases that holds the code for the formation of proteins

Exam Tip

You do not need to learn the names of the bases, just their letter.

Make sure you know which bonds with which, as this is the most commonly asked question about this topic.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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