Ribósóma

Ribósóma, commonly known in English as the ribosome, is the cellular machine that translates messenger RNA into polypeptides, enabling protein synthesis. It consists of two subunits, a small subunit that decodes the genetic message and a large subunit that catalyzes peptide bond formation. The ribósóma is composed of ribosomal RNA (rRNA) and ribosomal proteins, and it contains binding sites for messenger RNA and transfer RNA.

In prokaryotes, ribosomes are 70S, made of a 50S large subunit and a 30S small subunit; in eukaryotes, they are 80S, made of 60S and 40S subunits. Mitochondrial and chloroplast ribosomes are bacterial-type and typically 70S. In eukaryotic cells, ribosomes can be free in the cytosol or bound to the endoplasmic reticulum, forming rough ER.

During translation, the small subunit binds the mRNA and, with initiation factors, recruits an initiator tRNA and the large subunit to form the complete ribosome. In elongation, aminoacyl-tRNAs enter the A site, peptide bonds form, and the ribosome translocates along the mRNA. Termination occurs at a stop codon with release factors, releasing the completed polypeptide.

Biogenesis is the process by which ribosomal components are synthesized in the nucleus and cytoplasm, assembled in the nucleolus with rRNA, and then exported to the cytoplasm. rRNA genes are transcribed by RNA polymerase I (except 5S rRNA by polymerase III).

Clinical relevance includes the fact that bacterial ribosomes are targets of many antibiotics that inhibit protein synthesis. Defects in ribosome assembly can contribute to human ribosomopathies, such as Diamond-Blackfan anemia.

Discovery notes that the ribosome was visualized by electron microscopy in the 1950s, with foundational work by George Palade and colleagues.