MITOCHONDRIA AND CHLOROPLASTS SYNTHESIZE THEIR OWN PROTEINS
The mitochondria and chloroplasts found in eukaryotes have their own genome and make some of their own proteins. The symbiotic theory of organelle origin argues that these organelles were once free-living bacteria or blue-green algae (cyanobacteria) that formed a symbiotic relationship with a single-celled ancestral eukaryote. The bacteria supplied energy to the early eukaryote. Over time, the bacteria gave up many duplicate functions and came to rely on the host for precursor molecules. Eventually, the symbiotic mitochondria and chloroplasts lost the majority of their genes, yet today they still maintain a small version of their genome. These genomes have many genes associated with protein synthesis (Fig. 2.19). Organelle genes are often more closely related to bacterial genes than to eukaryotic (nuclear) genes. Moreover, the ribosomes in animal mitochondria are 28S and 39S in size, closer to the 30S and 50S subunits of bacteria. The ribosomal RNA of mitochondria and bacteria are also much more similar in sequence than either is to the rRNA encoded by the eukaryotic nucleus.