IGCSEBiologyCambridge (CIE)Revision Notes4. Biological MoleculesBiological MoleculesDNA Structure

DNA Structure (Cambridge (CIE) IGCSE Biology): Revision Note

Exam code: 0610 & 0970

Written by: Lára Marie McIvor

Reviewed by: Jenna Quinn

Updated on 25 August 2026

Structure of a DNA molecule: Extended

Extended Tier Only

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

Diagram showing DNA double helix forming a DNA strand, packed into a chromosome (with further detail in a label that a chromosome is a long strand of DNA wrapped around proteins) inside the nucleus of a cell with other organelles present (mitochondria, cytoplasm and cell membrane) — **DNA, chromosomes and the nucleus**

The individual units of DNA are called nucleotides

Diagram of a DNA nucleotide labelled with a phosphate group and deoxyribose sugar which forms the DNA strand backbone, and an attached nitrogenous base A in this image (with a label that the nitrogenous base could be A, T, C or G) — **A 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)

Diagram of DNA double helix showing coloured base pairs A–T and C–G joined by hydrogen bonds, and a labelled sugar–phosphate backbone on the outer edge of each strand — **DNA 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

Diagram of DNA showing on the left an unwound DNA double helix, illustrating a sequence of base pairs (with A always paired with T and C paired with G) on two antiparallel strands, and the image on the right shows a double helix, there is a note that genes are sequences of bases, not base pairs — **The 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

Examiner Tips and Tricks

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.

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent