Translation

Translation is the second stage of gene expression, in which the nucleotide sequence of messenger RNA (mRNA) is decoded to produce a specific sequence of amino acids — a polypeptide. Translation occurs at ribosomes in the cytoplasm (or on the rough endoplasmic reticulum). This lesson covers the components of the translational machinery, the mechanism of translation, and how the polypeptide is processed into a functional protein.

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Components of Translation

Messenger RNA (mRNA)

• mRNA carries the genetic information from the nucleus to the ribosomes in the form of codons — sequences of three bases.
• Each codon specifies one amino acid (or a stop signal).
• mRNA is read in the 5' to 3' direction during translation.

Transfer RNA (tRNA)

Key Definition: Transfer RNA (tRNA) is a small RNA molecule that carries a specific amino acid to the ribosome during translation. Each tRNA has an anticodon that is complementary to a specific mRNA codon.

Key structural features of tRNA:

• A cloverleaf shape when drawn in two dimensions, but an L-shaped three-dimensional structure.
• An anticodon — a sequence of three bases on one loop that is complementary to an mRNA codon. The anticodon binds to the codon by complementary base pairing during translation.
• An amino acid binding site at the 3' end (the CCA sequence) — where the specific amino acid is covalently attached.
• tRNA molecules are relatively small (~75–90 nucleotides) and contain several unusual modified bases.

Amino Acid Activation

Before translation, each amino acid must be attached to its correct tRNA molecule. This process is called amino acid activation and is catalysed by a specific enzyme called aminoacyl-tRNA synthetase.

• There is at least one aminoacyl-tRNA synthetase for each of the 20 amino acids.
• The enzyme recognises both the amino acid and the correct tRNA (ensuring fidelity).
• The reaction requires ATP (one molecule of ATP is hydrolysed per amino acid attached).
• The product is an aminoacyl-tRNA (a "charged" tRNA) — the amino acid is attached via a high-energy bond to the 3' end of the tRNA.

Ribosomes

Ribosomes are the site of translation. They are composed of ribosomal RNA (rRNA) and ribosomal proteins.

• Eukaryotic ribosomes are 80S (composed of a 60S large subunit and a 40S small subunit). The "S" stands for Svedberg units, a measure of sedimentation rate during centrifugation.
• Prokaryotic ribosomes are 70S (50S large subunit + 30S small subunit) — this difference is exploited by antibiotics such as erythromycin and tetracycline.
• Each ribosome has three tRNA binding sites:
  • A site (aminoacyl site) — where the incoming aminoacyl-tRNA binds.
  • P site (peptidyl site) — where the tRNA carrying the growing polypeptide chain is held.
  • E site (exit site) — where the "empty" (deacylated) tRNA exits the ribosome.

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The Mechanism of Translation

Initiation

1. The small ribosomal subunit binds to the 5' end of the mRNA and moves along until it reaches the start codon (AUG).
2. The initiator tRNA (carrying methionine, Met) with the anticodon UAC binds to the start codon at the P site by complementary base pairing.
3. The large ribosomal subunit then associates with the small subunit, forming the complete ribosome with the initiator tRNA in the P site.
4. Initiation requires initiation factors (proteins) and GTP as an energy source.

Elongation