Mitochondrial Ribosome

Mitochondrial ribosomes are the protein synthesis machinery inside mitochondria. They are similar to bacterial ribosomes in structure and origin - in fact, mitochondrial ribosomes in eukaryotic cells are about 55S particles (in mammals, composed of a 28S small subunit and a 39S large subunit) which function like prokaryotic 70S ribosomes. Mitochondrial ribosomes translate a few proteins encoded by mitochondrial DNA, mostly components of the oxidative phosphorylation system.

• Endosymbiotic evidence: The similarity of mitochondrial ribosomes to bacterial 70S ribosomes supports the theory that mitochondria evolved from an engulfed bacterium.
• Antibiotic sensitivity: Some antibiotics that inhibit bacterial ribosomes (e.g., chloramphenicol) can also affect mitochondrial protein synthesis because of the ribosomal similarity, which can explain certain drug side effects on human cells.
• Mitochondrial protein synthesis is limited - human mitochondrial DNA only encodes 13 proteins, so mitoribosomes specialize in making those (mostly subunits of the respiratory complexes). The majority of proteins in mitochondria are made by cytosolic 80S ribosomes and imported.

• Endosymbiosis question - If asked for evidence that mitochondria were once bacteria, an answer is: -Mitochondria have their own circular DNA and 70S-type ribosomes similar to bacteria.-
• Antibiotic trap - A question might ask why a bacteriostatic antibiotic can impact eukaryotic cells: the answer could be that it targets 70S ribosomes and thus can also interfere with mitochondrial ribosomes, affecting mitochondrial protein production.
• Structure ID - Mitochondrial ribosomes are not visible under normal microscopy in typical textbooks, but know the term -55S mitoribosome- refers to the mitochondrial protein-making complex (distinct from the 80S cytoplasmic ribosome).