DNA, Starch & Proteins

Specification Point 9.25C:
  • Recall that:
    a) DNA is a polymer made from four different monomers called nucleotides (names of nucleotides not required)
    b) starch is a polymer based on sugars
    c) proteins are polymers based on amino acids


  • DNA consists of four different monomers called nucleotides abbreviated to A, T, C, and G which are bound together by polymerisation.
  • The nucleotides form two strands that intertwine, giving the famous double helix shape of DNA.
  • It is a complex molecule that contains genetic information.

DNA, Edexcel GCSE Chemistry

Diagram showing the complex double helix structure of DNA


  • Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula Cx(H2O)y.
  • There are simple carbohydrates and complex carbohydrates.
  • Simple carbohydrates are called monosaccharides and are sugars such as fructose and glucose.
  • Complex carbohydrates are called polysaccharides such as starch and cellulose.
  • Complex carbohydrates are condensation polymers formed from simple sugar monomers and, unlike proteins, are usually made up of the same monomers.
  • A H2O molecule is eliminated when simple sugars polymerise. The linkage formed is an -O- linkage called a glycosidic linkage.

Amylose, Edexcel GCSE Chemistry

Diagram of the starch Amylose showing glycosidic linkages (-O-) which bind the monomers together


  • Proteins are condensation polymers which are formed from amino acid monomers joined together by peptide bonds.
  • Amino acids are small molecules containing amine (-NH2) and carboxylic acid (-COOH) functional groups.
  • A peptide link is formed when a carboxylic acid and an amine react:

R-COOH + R-NH2 → R-C=ONH-R + H2O

(R represents a hydrocarbon side chain)

  • Most proteins contain at least 20 different amino acids.
  • These are the monomers which polymerise to form the protein.

Making a Protein, Edexcel GCSE Chemistry

Diagram showing condensation polymerisation in which a protein is formed

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.