2.13—Protein synthesis

Protein synthesis has two linked stages: transcription copies a gene into mRNA, then translation reads the mRNA to assemble a polypeptide. The DNA template (antisense) strand is used for transcription; codons are read on mRNA and anticodons pair on tRNA.

During transcription, RNA polymerase opens the DNA locally and joins complementary RNA nucleotides. The mRNA leaves the nucleus and binds to a ribosome. In translation, tRNA brings amino acids whose anticodons match successive mRNA codons; peptide bonds join the chain from the start codon until a stop codon.

A template triplet TAC produces an mRNA codon AUG. The ribosome recognises AUG as the start signal and the matching tRNA brings methionine; later codons determine the rest of the sequence.

RNA polymerase makes RNA during transcription; DNA polymerase is used in DNA replication. The mRNA sequence matches the coding strand except that RNA uses U instead of T.