life11e_ch14

Mitochondria and chloroplasts make some of their own proteins and import others

As you saw in Chapter 5, mitochondria and chloroplasts are semiautonomous organelles, each having a small amount of DNA. Both organelles contain the entire protein synthesis machinery, including a specific RNA polymerase, and their own specialized ribosomes that differ from the ones in the cytoplasm. The genetic coding capacity of the organelle DNA is small, however:

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Mitochondrial DNA encodes mitochondrial rRNAs, a few tRNAs, and 12 to 20 proteins, most of them involved in the electron transport chain.
Chloroplast DNA encodes chloroplast rRNAs, some tRNAs, and about 40 proteins, many of which are involved in photosynthesis.

These organelles contain dozens more proteins, all of which are imported from the nuclear–cytoplasm protein synthesis system. In some cases (e.g., rubisco in the chloroplast) an organelle protein has several subunits, some of which are made locally inside the organelle and others of which are imported from the nucleus–cytoplasm system. As you can imagine, putting this together requires coordination between the two protein synthesis systems.