From code
to protein.

1. RNA polymerase binds the DNA at the start of a gene; unwinds a short stretch (~12 bp).
2. One DNA strand serves as the template strand; the other (the sense or coding strand) has the same sequence as the eventual mRNA (with T → U).
3. RNA polymerase moves along the template strand in the 3' → 5' direction, producing mRNA in the 5' → 3' direction.
4. Base pairing: A ↔ U, T ↔ A, G ↔ C.
5. Once finished, mRNA detaches; DNA rewinds into the double helix.

In eukaryotes, transcription happens in the nucleus. The mRNA must then leave through nuclear pores to reach the ribosomes in the cytoplasm.

The players

• mRNA — carries the coded message from the nucleus to the ribosome.
• Ribosome — two subunits; small reads mRNA, large catalyses peptide bond formation. Has three tRNA binding sites: A (acceptor), P (peptidyl), E (exit).
• tRNA — each tRNA carries a specific amino acid and has an anticodon matching the codon for that amino acid.

The sequence

1. Initiation. Small ribosomal subunit binds the mRNA at the start codon (AUG). The first tRNA (carrying methionine) binds at the P site. Large subunit joins.
2. Elongation. A new tRNA enters the A site, anticodon matching the next codon. A peptide bond forms between the amino acid at P and the one at A. The ribosome moves one codon along; the now-empty tRNA exits via the E site.
3. Termination. A stop codon (UAA, UAG, or UGA) is reached. No tRNA matches; release factors trigger polypeptide release. Ribosomal subunits dissociate.

• AUG — start codon (also codes for methionine).
• UAA, UAG, UGA — stop codons. Don't code for any amino acid.
• The other 60 codons — code for the remaining 19 amino acids. Multiple codons per amino acid (degeneracy) — usually the third base varies most.
• The code is universal — the same codon table works in nearly every organism, supporting common ancestry.

The ribosome reads three bases at a time, moving 3 bases per step. Once translation begins at the start codon, the reading frame is locked. Insertions or deletions of bases (other than multiples of three) shift the reading frame — almost always producing a non-functional protein. This is why frameshift mutations are usually catastrophic.