CorpusHuman Anatomy
Cells & TissuesOrganelle

Ribosome

Ribosoma

Also known as: 80S ribosome, polysome, polyribosome, free ribosomes, bound ribosomes

A non-membranous ribonucleoprotein machine (≈25–30 nm) that translates mRNA into protein; the eukaryotic 80S ribosome is built from a 60S and a 40S subunit.

The Eukaryotic Cell

Overview

Eukaryotic ribosome80S = 60S + 40S
Mitochondrial / bacterial55S / 70S (50S + 30S)
Diameter≈25–30 nm
Number per mammalian cellMillions (up to ≈10⁷ in rapidly growing cells)

Each ribosome is made of ribosomal RNA (rRNA, about two-thirds of its mass) and ≈80 proteins. The small 40S subunit contains 18S rRNA and decodes the mRNA; the large 60S subunit contains 28S, 5.8S and 5S rRNA and catalyses peptide-bond formation. The catalytic centre (peptidyl transferase) is rRNA, making the ribosome a ribozyme. Svedberg values are not additive: 40S + 60S = 80S. The 18S, 5.8S and 28S rRNAs are transcribed as one 45S precursor by RNA polymerase I in the nucleolus; 5S rRNA is made by RNA polymerase III elsewhere in the nucleus. Subunits are assembled in the nucleolus, exported separately through nuclear pores and join only on an mRNA.

Free ribosomes in the cytosol make proteins that remain in the cytosol or are imported into the nucleus, mitochondria or peroxisomes. Membrane-bound ribosomes are the same particles attached to the rough ER by the signal recognition particle pathway; they make secretory, lysosomal and membrane proteins. Several ribosomes translating one mRNA form a polysome (polyribosome), seen in EM as spirals or rosettes.

Mitochondria contain their own 55S ribosomes (39S + 28S), and bacteria have 70S ribosomes (50S + 30S). These differences are exploited by antibiotics that inhibit bacterial ribosomes, while explaining some of their toxicity to mitochondria (e.g. aminoglycoside ototoxicity, linezolid-associated lactic acidosis).

Location

Free in the cytosol, attached to the cytosolic face of the rough ER and outer nuclear membrane, and in the mitochondrial matrix.

Parts & subdivisions

  1. Small (40S) subunit
    18S rRNA + ≈33 proteins; binds mRNA and scans for the start codon; monitors codon–anticodon pairing.
  2. Large (60S) subunit
    28S, 5.8S and 5S rRNA + ≈47 proteins; peptidyl-transferase centre and polypeptide exit tunnel.
  3. A, P and E sites
    Aminoacyl-tRNA enters the A site, the growing chain is held on tRNA in the P site, and empty tRNA leaves from the E site.

Anatomy

Histology

Ribosome-rich cytoplasm is strongly basophilic (e.g. plasma cells, pancreatic acinar cell bases, Nissl substance of neurons). Individual ribosomes are visible only by EM as 15–25 nm electron-dense granules.

Function

  • Translation of mRNA into polypeptides.
  • Bound ribosomes deliver nascent secretory and membrane proteins into the ER.

Clinical significance

Antibiotics targeting bacterial ribosomes

30S inhibitors: aminoglycosides, tetracyclines. 50S inhibitors: chloramphenicol, clindamycin, macrolides (erythromycin), linezolid, streptogramins.

Ribosomopathies

Diamond–Blackfan anaemia (often RPS19 mutation) — congenital red-cell aplasia with thumb and craniofacial anomalies; Treacher Collins syndrome — defective ribosome biogenesis in neural crest.

Mnemonics

Bacterial ribosome inhibitors

"Buy AT 30, CCEL (sell) at 50" — Aminoglycosides, Tetracyclines (30S); Chloramphenicol, Clindamycin, Erythromycin, Linezolid (50S).

Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.