How Does Protein Synthesis Work?
Learn how protein synthesis works in AQA A Level Biology: follow transcription from DNA to mRNA, then translation from codons to a polypeptide at the ribosome.
Protein synthesis makes a polypeptide from the instructions in a gene. It has two stages: transcription copies the gene’s base sequence into messenger RNA (mRNA), and translation uses that mRNA sequence to join amino acids in the correct order. The polypeptide then folds to form a functional protein.
Transcription: DNA to mRNA
In eukaryotic cells, transcription takes place in the nucleus. The DNA strands separate, exposing the bases of a gene. One strand acts as a template. Free RNA nucleotides pair with its exposed bases: adenine pairs with uracil in RNA, while cytosine pairs with guanine. RNA polymerase joins the RNA nucleotides together to make a complementary strand called pre-mRNA.
The pre-mRNA contains introns (sections removed before translation) and exons (sections retained). Splicing removes the introns and joins the exons, producing mRNA. The mRNA leaves the nucleus and attaches to a ribosome. In prokaryotes, transcription produces mRNA directly, without this splicing step.
Translation: mRNA to polypeptide
A ribosome reads the mRNA one codon at a time. A codon is a sequence of three bases that specifies an amino acid or a stop signal. Each transfer RNA (tRNA) carries a particular amino acid and has an anticodon complementary to an mRNA codon. As matching tRNA molecules bind at the ribosome, their amino acids are joined by peptide bonds. This repeats until a stop codon is reached, releasing the polypeptide. Translation requires ATP.
For example, if the mRNA codon is UUU, a tRNA with the anticodon AAA can bind and deliver its amino acid. You do not need to memorise which amino acid that codon specifies.
Exam mistake to avoid: transcription makes RNA from DNA; translation makes a polypeptide from the mRNA instructions. tRNA brings amino acids—it does not carry the gene’s copied message.
