CIE IGCSE Biology

Revision Notes

4.1.3 DNA Structure: Extended

Test Yourself

Structure of a DNA Molecule: Extended

  • 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

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

DNA, chromosomes and the nucleus

  • The individual units of DNA are called nucleotides

A nucleotide, IGCSE & GCSE Biology revision notes

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)

DNA base pairs, IGCSE & GCSE Biology revision notes

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

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

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

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.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Lára

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.