Transcription & translation

From a gene to a working protein, in two steps.

The idea

In transcription, RNA polymerase binds a promoter, unzips the DNA, and builds a single strand of messenger RNA complementary to the template strand, working 5′→3′ and using uracil in place of thymine. In eukaryotes the pre-mRNA receives a 5′ cap and a poly-A tail and has its introns spliced out before it leaves the nucleus. In translation, a ribosome reads the mRNA three bases at a time; each codon is matched by a transfer RNA whose anticodon is complementary and which carries one specific amino acid, and the ribosome joins those amino acids with peptide bonds. Translation begins at the start codon AUG (methionine) and ends at a stop codon — UAA, UAG, or UGA — which has no matching tRNA and releases the finished polypeptide.

Work through an example

Template strand 3′-TAC GGA CTC ATT-5′ is transcribed into mRNA 5′-AUG CCU GAG UAA-3′ (T→A, A→U, C→G, G→C). Reading the codons: AUG = methionine, CCU = proline, GAG = glutamic acid, UAA = stop. Twelve nucleotides therefore encode a three-amino-acid peptide, Met–Pro–Glu. The coding (sense) strand, 5′-ATG CCT GAG TAA-3′, matches the mRNA letter for letter except for T in place of U.

What to watch for

mRNA is built from the template (antisense) strand, so it is complementary to that strand and identical to the coding strand — students who transcribe the coding strand get the wrong message. Also, the code is degenerate, not ambiguous: 61 sense codons specify 20 amino acids, so leucine has six codons while methionine and tryptophan have one each, but no codon ever means two different things.