Transcription and Translation

Transcription is the synthesis of messenger RNA (mRNA) using one strand of DNA as a template. In eukaryotes it happens in the nucleus.

The enzyme. RNA polymerase unwinds the DNA double helix and reads one strand.

• The strand that is read is the template strand (also called the antisense strand).
• The other strand — not read — is the coding strand (or sense strand); its sequence matches the mRNA (except T→U).

Building the mRNA.

1. RNA polymerase binds DNA and separates the two strands.
2. Free RNA nucleotides pair with exposed bases on the template strand by complementary base pairing.
3. RNA polymerase joins the nucleotides with covalent (phosphodiester) bonds, building the mRNA in the 5'→3' direction.
4. The completed mRNA detaches and the DNA re-winds.

Base pairing rule — U replaces T. RNA has uracil (U) instead of thymine. So when reading the DNA template:

• DNA A → RNA U
• DNA T → RNA A
• DNA C → RNA G
• DNA G → RNA C

Worked mapping. If the template strand reads 3'‑TAC GGA ATT‑5', the mRNA reads 5'‑AUG CCU UAA‑3'.

The genetic code is the set of rules linking mRNA base sequences to amino acids.

Triplet. Bases are read in groups of three called codons. Each codon specifies one amino acid (or a start/stop signal). With 4 bases there are 4³ = 64 codons.

Degenerate (redundant). There are 64 codons but only 20 amino acids, so most amino acids are coded for by more than one codon (e.g. both GGU and GGC code for glycine). This reduces the impact of some point mutations.

Universal. The same codons specify the same amino acids in (almost) all organisms — bacteria, plants, humans alike. This is evidence of a shared evolutionary origin and underpins genetic engineering (a human gene can be expressed in a bacterium).

Non-overlapping. Each base belongs to only one codon; the ribosome reads consecutive, non-overlapping triplets from a fixed start point.

Start and stop.

• AUG is the start codon — it codes for methionine and sets the reading frame.
• UAA, UAG, UGA are stop codons — they code for no amino acid and end translation.

Translation is the synthesis of a polypeptide from the codon sequence of mRNA. It occurs at ribosomes in the cytoplasm.

The players.

• mRNA carries the codons.
• Ribosome has a small subunit (binds mRNA) and a large subunit (catalyses peptide bond formation). The large subunit has binding sites for tRNA — the A site (accepts the incoming tRNA) and the P site (holds the tRNA carrying the growing polypeptide).
• tRNA (transfer RNA) has an anticodon that pairs with the mRNA codon, and carries the matching amino acid.

The steps.

1. Initiation — the small ribosomal subunit binds the mRNA and moves to the start codon AUG; a tRNA with anticodon UAC (carrying methionine) pairs at the P site; the large subunit joins.
2. Elongation — the next codon is exposed at the A site; a tRNA with the complementary anticodon binds, delivering its amino acid.
3. A peptide bond forms between the amino acids in the P and A sites.
4. The ribosome moves one codon along the mRNA (translocation); the now-empty tRNA leaves, the growing chain shifts to the P site, and the A site opens for the next tRNA.
5. Steps 2–4 repeat, elongating the polypeptide one amino acid at a time.
6. Termination — when a stop codon (UAA, UAG, UGA) reaches the A site, no tRNA matches; the completed polypeptide is released and the ribosome separates.

Complementary base pairing again. Anticodon–codon pairing follows the same rules (A–U, G–C), ensuring the correct amino acid is added for each codon.