CIE A Level Biology (9700) 2019-2021

Revision Notes

6.1.1 Nucleotides

Nucleotide Structure

  • Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules (giant molecules)
  • Like proteins (polypeptides) and carbohydrates (polysaccharides), these nucleic acids are polymers (‘poly’ meaning ‘many’)
  • This means they are made up of many similar, smaller molecules (known as subunits or monomers) joined into a long chain
  • The subunits that make up DNA and RNA are known as nucleotides
  • Therefore DNA and RNA can also be known as polynucleotides


  • Nucleotides are made up of three components:
    • A nitrogen-containing base (also known as a nitrogenous base)
    • A pentose sugar (containing 5 carbon atoms)
    • A phosphate group

Nucleotide structure table

Table 25 Nucleotide structure, downloadable AS & A Level Biology revision notes


  • Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
  • ATP is another type of nucleic acid and hence it is structurally very similar to the nucleotides that make up DNA and RNA
  • It is a phosphorylated nucleotide
  • Adenosine (a nucleoside) can be combined with one, two or three phosphate groups
    • One phosphate group = adenosine monophosphate (AMP)
    • Two phosphate groups = adenosine diphosphate (ADP)
    • Three phosphate groups = adenosine triphosphate (ATP)

Exam Tip

Don’t worry – you are not expected to know the structural formulae for the nucleotides that make up DNA and RNA or AMP, ADP and ATP (as in the diagram above)! You just need to learn the different groups that they are made up of (phosphate groups, pentose sugars and nitrogenous bases).

Remember that adenine is a nitrogenous base whereas adenosine is a nucleoside (a base – adenine, attached to a pentose sugar).

Purines & Pyrimidines

  • The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines
  • The bases adenine and guanine are purines – they have a double ring structure
  • The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure

Exam Tip

You don’t need to know the structural formulae of these bases, just which are purines and which are pyrimidines.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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