Protein synthesis step by step: transcription, splicing and translation

Transcription

• DNA helicase breaks hydrogen bonds between the two strands.
• One strand — the template strand — is used; free RNA nucleotides align by complementary base pairing (uracil replaces thymine).
• RNA polymerase joins the nucleotides, forming pre-mRNA.

Splicing (eukaryotes only)

Pre-mRNA contains introns — non-coding sequences. These are removed and the exons are joined to form mature mRNA, which leaves the nucleus through a nuclear pore. Prokaryotes have no introns, so no splicing step: examiners often use this as the discriminating point in a compare question.

Translation

• mRNA binds to a ribosome; two codons are exposed at a time.
• A tRNA with the complementary anticodon brings a specific amino acid; hydrogen bonds form between codon and anticodon.
• A peptide bond forms between adjacent amino acids using ATP and an enzyme.
• The ribosome moves along one codon at a time until a stop codon is reached, releasing the polypeptide.

Marks students lose

Writing 'the gene makes the protein' with no named molecules scores nothing. Every sentence should contain a named molecule or enzyme and what it does. Also watch the direction: mRNA is complementary to the template strand and therefore identical (with U for T) to the coding strand.