Building the Protein: Translation at the Ribosome

The workshop and its adapters

The message is decoded into a protein at the ribosome — the cell's protein workshop. But the ribosome cannot read amino acids directly; it only reads codons on the mRNA. Something has to bridge the two languages. That bridge is transfer RNA, or tRNA — a small adapter molecule that does two jobs at once.

Each tRNA carries one specific amino acid on one end. On the other end it displays a three-letter sequence called an anticodon. The anticodon is the complement of a codon: it pairs with the matching codon on the mRNA by the same A–U, G–C rules you already know. So a tRNA reads 'this codon' and delivers 'that amino acid.' It is, quite literally, the dictionary made physical.

How the chain grows

1. Start. The ribosome locks onto the mRNA and finds the start codon AUG. The first tRNA, carrying methionine, pairs its anticodon there.
2. Match the next codon. A tRNA whose anticodon fits the next codon arrives and base-pairs into place, bringing its amino acid alongside the first.
3. Link. The ribosome forms a bond between the two amino acids, extending the chain by one, then lets the now-empty tRNA leave.
4. Step and repeat. The ribosome slides exactly one codon along, keeping the reading frame, and the cycle repeats — read codon, fetch amino acid, link.
5. Stop. When a stop codon appears, no tRNA matches it. The ribosome releases the finished protein chain and lets go of the mRNA.

The chain of amino acids then folds into its working shape, and the gene has finally become a molecule that does something — an enzyme, a structural fiber, a signal. This whole decoding process is translation, the second arrow of the central dogma.

One message, all the way through

Let's run a single gene end to end, tying together everything in this track. Start with a short stretch of DNA, transcribe it into mRNA, then translate the mRNA at the ribosome. Watch how the original order of DNA bases ends up dictating the exact order of amino acids — the promise made back in Guide 1.

GENE → PROTEIN, the whole road in one example

  STEP 0  DNA
    coding   5'-A T G  T G C  A A C  U..→ T G A-3'
    template 3'-T A C  A C G  T T G  ...  A C T-5'  ◀ read this

  STEP 1  TRANSCRIPTION  (pair off template, T→U)
    mRNA   5'-A U G  U G C  A A C  ...  U G A-3'

  STEP 2  TRANSLATION  (ribosome reads codons; tRNA brings amino acids)
    codon    AUG    UGC    AAC   ...   UGA
    anticodon UAC   ACG    UUG   ...  (none — STOP)
    amino    Met  - Cys  - Asn  - ...   release

  PROTEIN:  Met - Cys - Asn - ...   then folds into shape

  The DNA letter order set the codon order, which set the amino-acid order.
  Change one DNA base and you may change one amino acid — or, in frame, the whole chain.