DNA to Protein

Transcription happens in the nucleus. RNA polymerase opens a short bubble in the DNA itself (no separate helicase), reads the template strand and adds each new RNA letter to the RNA's 3' end. The ribosome holds tRNAs in three sites: A (arriving), P (holding the chain) and E (exit). No tRNA matches a stop codon. A release factor enters the A site and frees the chain. Pairing at a codon's third base is loose (wobble), so one tRNA can read several codons.