ribosoms

Ribosomes are cellular machines that synthesize proteins by translating genetic information encoded in messenger RNA. They are ribonucleoprotein complexes composed of ribosomal RNA (rRNA) and a large number of ribosomal proteins. The ribosome’s core function is to convert the sequence information in mRNA into a polypeptide chain, using transfer RNA to deliver specific amino acids.

Ribosomes come in two general types distinguished by their sedimentation coefficients. In bacteria and archaea, the functional ribosome is 70S, formed from a 30S small subunit and a 50S large subunit. In eukaryotes, the functional ribosome is 80S, composed of a 40S small subunit and a 60S large subunit. Some organelles, such as mitochondria and chloroplasts, retain 70S-like ribosomes. Ribosomes can be free-floating in the cytosol or bound to the cytosolic face of the rough endoplasmic reticulum, producing proteins destined for secretion or membranes.

Ribosome structure is adapted for its translational role. The small subunit decodes the mRNA, while the large subunit catalyzes peptide bond formation. The catalytic activity largely arises from rRNA, making the ribosome a ribozyme. Translation proceeds through initiation, elongation, and termination, requiring a suite of factors and energy from GTP hydrolysis. In prokaryotes, initiation often involves the Shine-Dalgarno sequence, whereas in eukaryotes initiation relies on the 5' cap and scanning for the start codon.

Biogenesis of ribosomes occurs mainly in the nucleolus in eukaryotic cells, where rRNA genes are transcribed and assembled with ribosomal proteins imported from the cytosol. Final subunit assembly and export to the cytoplasm yield functional ribosomes ready to participate in protein synthesis.

Ribosomes are targets for several antibiotics that inhibit bacterial protein synthesis, exploiting differences between bacterial and eukaryotic ribosomes. Examples include tetracyclines, aminoglycosides, macrolides, and oxazolidinones.