ribosome

Ribosomes are the molecular machines responsible for translation, converting mRNA codon sequences into polypeptide chains using tRNA as adaptors. Structurally, eukaryotic ribosomes are 80S, composed of a 40S small subunit (18S rRNA) and 60S large subunit (28S, 5.8S, 5S rRNA), while prokaryotic ribosomes are 70S (30S subunit with 16S rRNA; 50S subunit with 23S and 5S rRNA). This size/composition difference is clinically important because many antibiotics selectively target bacterial 70S ribosomes without affecting human 80S ribosomes, minimizing host toxicity: aminoglycosides, tetracyclines, and spectinomycin bind the 30S subunit; chloramphenicol, macrolides, clindamycin, and linezolid bind the 50S subunit. Mnemonic: 'buy AT 30, CCEL at 50' for 30S (Aminoglycosides, Tetracyclines) vs 50S (Chloramphenicol, Clindamycin, Erythromycin/macrolides, Linezolid) inhibitors.

Functionally, ribosomes can be free in the cytosol, synthesizing proteins destined for the cytoplasm, nucleus, mitochondria, or peroxisomes, or bound to the rough endoplasmic reticulum (RER), synthesizing proteins for secretion, insertion into the plasma membrane, or delivery to lysosomes. The signal recognition particle (SRP) directs ribosomes with a signal sequence to the RER; deficiency or dysfunction of SRP receptor mechanisms can lead to trafficking disorders.

Clinically, ribosomes are relevant in genetic disorders (e.g., Diamond-Blackfan anemia, a ribosomopathy caused by mutations in ribosomal protein genes leading to pure red cell aplasia), and in pharmacology as targets for antimicrobial therapy. Rough ER with abundant ribosomes is prominent in cells with high secretory activity (e.g., plasma cells, pancreatic acinar cells), correlating with basophilic cytoplasm on histology due to rRNA content (Nissl bodies in neurons).