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First teaching 2023

First exams 2025

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Nucleotides (CIE AS Biology)

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Nucleotide Structure

  • Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules
  • Like proteins (polypeptides) and carbohydrates (polysaccharides), these nucleic acids are polymers 
  • 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

  • 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 Diagram

basic structure of a nucleotide diagram

The basic structure of a nucleotide

Nucleotide Structure Table

Properties DNA RNA
Pentose sugar Deoxyribose Ribose
Bases Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Adenine (A)
Uracil (U)
Cytosine (C)
Guanine (G)
Number of strands Double-stranded
(double helix)
Single-stranded

Nucleotides in DNA & RNA Diagram

nucleotides-in-DNA-and-RNA-diagram

There is much overlap of the nucleotides found in DNA and RNA, the only difference is that RNA contains uracil (U) instead of thymine (T)

ATP

  • Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
  • ATP is another type of nucleotide 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)

ATP Structure Diagram

structure-of-ATP

The structure of AMP, ADP and ATP

Examiner 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
    • This means that they have a double-ring structure
  • The bases cytosine, thymine, and uracil are pyrimidines 
    • These bases have a single-ring structure

Purines & Pyrimidines Diagram

purines-and-pyrimidines-(1)purines-and-pyrimidines-(2)

The molecular structures of purines and pyrimidines are slightly different

Examiner Tip

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

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.