Protein Synthesis – Triple Science (HT only)

DNA

DNA is a polymer of molecules called nucleotides. Each nucleotide has the same sugar, phosphate and one of 4 bases (A,T,C,G). A always pairs with T and C always pairs with G.

• Proteins = polymers of amino acids

  • Humans have 20 different amino acids.

  • The order of amino acids determines the protein’s shape and the shape determines the function.

  • Functions include:

    • Enzymes (e.g., amylase)

    • Structural proteins (e.g., collagen)

    • Hormones (e.g., insulin)

  • So the order of amino acids determines the shape and function of the protein .

  • The order of amino acids is determined by the sequences of bases in the gene for the protein.

• Genes determine proteins

  • A gene = sequence of DNA bases.

  • DNA is read in triplets of bases.

  • Each triplet codes for one amino acid.

  • Therefore, the sequence of bases → sequence of amino acids → protein structure & function.

Stages of Protein Synthesis

1. Transcription (in the nucleus)

   • DNA base sequence of the gene is copied into a complementary mRNA molecule.

   • mRNA = single-stranded.

   • mRNA leaves nucleus → enters cytoplasm.

2. Translation (in the cytoplasm at ribosomes)

   • mRNA attaches to a ribosome.

   • tRNA (transfer RNA) molecules carry amino acids to the ribosome.

   • Ribosome reads triplets on mRNA.

   • Amino acids are joined in the correct order to form a polypeptide chain.

   • Polypeptide folds into its unique 3D shape → functional protein.

✅ Exam Tip: Be able to:

• Describe transcription and translation in sequence.

• Explain how triplets on DNA/mRNA determine amino acid sequence.

• Link back to protein function (enzyme, structural, hormone, etc.).