6.2 Protein Synthesis

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. State that a polypeptide is coded for by a gene, and that a gene is a sequence of nucleotides that forms part of a DNA molecule
  2. Describe the principle of the universal genetic code, in which different triplets of DNA bases either code for specific amino acids or correspond to start and stop codons
  3. Describe how DNA information is used during transcription and translation to construct polypeptides, including the roles of RNA polymerase, mRNA, codons, tRNA, anticodons, and ribosomes
  4. State that the strand used in transcription is called the transcribed (template) strand, and the other is called the non-transcribed strand
  5. Explain how, in eukaryotes, the primary transcript is modified by removing introns and joining exons to form mature mRNA
  6. State that a gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
  7. Explain that gene mutations result from substitution, deletion, or insertion of nucleotides, and outline how each type may affect the polypeptide produced

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1. Genes and Polypeptides

Every protein in your body is made of one or more polypeptides — long chains of amino acids joined together. The instructions for building each polypeptide are stored in your DNA, in sections called genes.

  • A gene is a specific sequence of nucleotides (the building blocks of DNA) that forms part of a DNA molecule.
  • Each gene codes for one specific polypeptide.
  • One DNA molecule contains many genes — thousands of them, each with a specific start and stop point so the cell knows exactly where each gene begins and ends.
  • The sequence of nucleotides in a gene determines the sequence of amino acids in the polypeptide it codes for. Change the gene, and you can change the protein.

Key idea: DNA → gene → polypeptide → protein

2. The Universal Genetic Code

DNA is made of four bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The order of these bases carries the instructions for making proteins.

The Triplet Code

  • The genetic code is a triplet code — every three consecutive bases on a DNA molecule form one triplet, and each triplet codes for one specific amino acid.
  • For example: the DNA triplet TAC codes for the amino acid methionine; CAG codes for valine.
  • There are 4 possible bases, and each codon has 3 positions, giving: 4 × 4 × 4 = 64 different possible triplets.
  • Since there are only 20 different amino acids, many amino acids are coded for by more than one triplet. This is called degeneracy (the code has "spares").

Why a triplet and not two bases? If we used only two bases per code, we'd get 4² = 16 combinations — not enough for 20 amino acids. Three bases gives 64 combinations — more than enough.

Start and Stop Codons

Not all triplets code for amino acids. Some are signals:

  • The start codon (AUG on mRNA, which corresponds to TAC on the DNA template strand) signals "begin making the protein here." It also codes for the amino acid methionine, so every polypeptide begins with methionine.
  • Stop codons (UAA, UAG, UGA on mRNA) signal "stop — the protein is complete." They do not code for any amino acid.

The Code is Universal

The genetic code is universal, meaning almost every living organism on Earth — from bacteria to humans — uses the same triplets to code for the same amino acids. The same triplet codes for the same amino acid across species. This suggests all life shares a common ancestor, and it also means that genetic information can be transferred between species (the basis of genetic engineering).

The Code is Non-Overlapping

Each base belongs to only one triplet. The code is read in groups of three, one after another, with no overlap. This ensures the correct amino acid sequence is always produced.

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