Protein Synthesis: Transcription and Translation

From gene to protein

A gene is a section of DNA that codes for a polypeptide. Protein synthesis converts the DNA code into a protein in two stages: transcription (in the nucleus) and translation (at the ribosome). The base sequence determines the amino acid sequence, and so the protein's structure and function.

Genes and the genetic code

• A triplet/codon of three DNA bases codes for one amino acid.
• The code is degenerate (multiple codons per amino acid), non-overlapping and universal.
• In eukaryotes, genes contain coding sections (exons) and non-coding sections (introns), plus non-coding DNA between genes.

Transcription (DNA → mRNA)

1. DNA helicase unwinds and unzips the DNA at the gene, breaking hydrogen bonds.

2. One strand acts as a template. Free RNA nucleotides pair with the exposed bases (A–U, as RNA uses uracil; C–G).

3. RNA polymerase joins them into a strand of pre-mRNA.

4. In eukaryotes, splicing removes the introns, joining the exons to form mature mRNA.

5. The mRNA leaves the nucleus through a nuclear pore.

Translation (mRNA → polypeptide)

1. The mRNA attaches to a ribosome.

2. tRNA molecules, each carrying a specific amino acid and an anticodon, bind to the complementary codon on the mRNA.

3. The ribosome moves along the mRNA; adjacent amino acids are joined by peptide bonds (using ATP).

4. This continues until a stop codon, releasing the completed polypeptide, which then folds into its functional shape.

Mutations

Changes to the base sequence (substitution, deletion, insertion) can change the amino acid sequence. Because the code is degenerate, some substitutions have no effect (silent). Deletions/insertions cause a frame shift, altering every codon after them — usually with a large effect.

Worked example

A section of template DNA reads TAC. What is the mRNA codon, and (given AUG = start) what does it code for?

• Template TAC → mRNA AUG (A–U, C–G, C–G... i.e. T→A, A→U, C→G) → this is the start codon, coding for methionine. ✓

Common mistakes

• Confusing transcription (making mRNA in the nucleus) with translation (making protein at the ribosome).
• Forgetting RNA uses uracil (A pairs with U, not T).
• Mixing up codon (on mRNA) and anticodon (on tRNA).

Exam tips

• Learn both stages step by step, naming RNA polymerase, splicing, ribosome, tRNA, anticodon, peptide bonds.
• Know exons (coding) vs introns (removed by splicing).
• Explain how different mutations (substitution vs frame shift) affect the protein.

Key facts to remember

• Transcription (nucleus): DNA template → pre-mRNA (RNA polymerase) → splicing removes introns → mRNA.
• Translation (ribosome): tRNA anticodons pair with mRNA codons; amino acids join by peptide bonds until a stop codon.
• A triplet codes one amino acid (degenerate, non-overlapping, universal); mutations (substitution/frame shift) can change the protein.