Knowledge Organiser: Protein Synthesis (OCR A Higher only)
Key Terms
- Transcription: DNA → mRNA, in the nucleus, by RNA polymerase
- Translation: mRNA → polypeptide, at the ribosome
- Codon: 3-base sequence on mRNA coding for one amino acid
- Anticodon: 3-base sequence on tRNA, complementary to codon
- Peptide bond: covalent bond linking amino acids
- Frameshift: shift in reading frame caused by deletion/insertion
Key Processes
- Transcription: RNA polymerase uses one strand of DNA as a template to build a complementary mRNA strand; base pairing rules: A→U, T→A, G→C, C→G
- mRNA carries the genetic code out of the nucleus through nuclear pores to the ribosome in the cytoplasm
- Translation: ribosome reads mRNA codons; tRNA molecules bring matching amino acids (anticodon pairs with codon)
- Amino acids are joined by peptide bonds to form a polypeptide chain
- AUG is the start codon (codes for methionine); stop codons (UAA, UAG, UGA) signal the end of translation
- One amino acid is added per codon (triplet of bases) — three bases specify one amino acid
Sickle Cell Facts
- DNA: GAG → GTG (A→T substitution)
- mRNA: GAG → GUG
- Amino acid: Glutamic acid → Valine
- Valine is non-polar → Hb molecules stick together
- Red blood cells become sickle-shaped
- Cannot carry O₂ efficiently; block capillaries
Must-Know Facts
- There are 64 possible codons (4³) but only 20 amino acids — the genetic code is degenerate (multiple codons can code for the same amino acid)
- The genetic code is universal — the same codons specify the same amino acids in virtually all organisms on Earth
- Substitution mutation: only one codon is changed; may be silent if the new codon codes for the same amino acid
- Deletion or insertion mutation: causes a frameshift — every codon from the mutation point onwards is altered, usually producing a non-functional protein
- A substitution is generally less damaging than a deletion/insertion because only one amino acid may change rather than the entire protein sequence
- OCR A Higher tier only — this topic is not assessed in AQA, Edexcel, WJEC, or OCR B specifications
Common Mistakes
- Confusing transcription and translation: Transcription copies DNA into mRNA in the nucleus; translation reads mRNA to build a protein at the ribosome. Students often reverse the locations or describe them as a single process.
- Saying DNA leaves the nucleus during protein synthesis: DNA stays in the nucleus. It is mRNA that carries the genetic code out through the nuclear pores to the ribosome in the cytoplasm.
- Mixing up codons and anticodons: A codon is a triplet of bases on mRNA; an anticodon is the complementary triplet on tRNA. They are complementary and base-pair together during translation.
- Stating all mutations change the protein: The genetic code is degenerate — multiple codons can code for the same amino acid. A substitution mutation may be silent if the new codon still codes for the same amino acid.
Memory Aid
- DNA → template for transcription
- MRNA → leaves nucleus through nuclear pores
- Polypeptide → assembled at ribosome by tRNA
- The mRNA codon is the lock (fixed in place on the ribosome)
- The tRNA anticodon is the key (the tRNA moves in, matches, and delivers its amino acid)
- The key is complementary to the lock — if codon is AUG, anticodon is UAC
- Substitution = small effect (one codon at most)
- Deletion/Insertion = Destroys reading frame (frameshift — all subsequent codons wrong)