Ribosooms

A ribosome is a molecular machine that converts genetic information encoded in messenger RNA into a polypeptide chain, the first step in protein synthesis. Ribosomes are composed of ribosomal RNA (rRNA) and ribosomal proteins and consist of two subunits that come together during translation.

In bacteria and archaea, ribosomes are 70S, formed from a 50S large subunit and a 30S small subunit. In eukaryotes, ribosomes are 80S, consisting of a 60S large subunit and a 40S small subunit. Mitochondria and chloroplasts retain ribosomes that resemble bacterial ones (often 70S).

Ribosomes are located freely in the cytosol or bound to the endoplasmic reticulum. Free ribosomes synthesize mainly cytosolic and some organelle-targeted proteins, while bound ribosomes on the rough endoplasmic reticulum produce secreted and membrane proteins. In mitochondria and chloroplasts, ribosomes resemble bacterial ones.

During translation, initiation assembles the small subunit with the mRNA and a start codon, followed by recruitment of the large subunit. Elongation adds amino acids via tRNA delivery and peptide bond formation at the peptidyl transferase center, with translocation moving the ribosome along the mRNA. Termination occurs at stop codons with release factors.

The catalytic activity of the ribosome is largely provided by rRNA. Ribosome biogenesis involves transcription of rRNA, processing, and assembly with ribosomal proteins in the nucleolus of eukaryotes. Ribosomes are a common antibiotic target in bacteria; drugs such as tetracyclines, macrolides and aminoglycosides inhibit bacterial translation.

Dysfunction in ribosome production can lead to ribosomopathies, a class of disorders including Diamond-Blackfan anemia and some cancers, reflecting the essential role of ribosomes in cell growth and development.