What is The ribosome? — Biology

Definition 19.8 University Biology — Year 1 · Chapter 19 — Gene Expression: Transcription and Translation

The ribosome is a particle of two subunits, each of ribosomal RNA and proteins (in bacteria: a small 30S subunit and a large 50S subunit, together 70S, $2.5\,\mathrm{MDa}$ ; eukaryotic ribosomes are larger, 80S). The small subunit binds the messenger and decodes it; the large subunit holds the tRNAs and forms the peptide bond — the catalyst is the ribosomal RNA itself. Three tRNA sites span both subunits: A (aminoacyl, where the next charged tRNA enters), P (peptidyl, holding the growing chain), E (exit).

The ribosome: two subunits clamped on the messenger, two transfer RNAs in the cleft, the growing chain leaving through a tunnel in the large subunit.

One round of elongation. A charged tRNA is admitted to the A site when its anticodon matches; the ribosomal RNA joins the chain to its amino acid; the ribosome moves on by one codon, and the empty tRNA leaves. Two GTP per residue, plus the two high-energy bonds spent in charging the tRNA.

Examples

Example 19.12 (The numbers of a cell)

A growing E. coli holds some $20\,000$ ribosomes, each adding twenty residues a second: $4 \times 10^{5}$ residues a second, a million proteins of $300\,$ residues in an hour — its own content, which is what doubling every hour requires. Half its energy goes to protein synthesis. A liver cell holds ten million ribosomes and makes proteins for weeks of use rather than for division; a plasma cell, secreting antibody, devotes nearly all of them to one protein and pours out two thousand molecules a second.

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Examples

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