Although all of the RNA of mitochondria are derived from mitochondrial DNA, only a few of the proteins of mitochondria are. Mitochondrial proteins are synthesized in the cytosol by free ribosomes. These proteins possess highly specific mitochondrial entry sequences on their amino termini specifying final destination, the outer membrane, the inner membrane, the intermembrane space and the matrix.

This is another episode in the important theme of protein targetting, or protein sorting. Some crucial points to remember for the MCAT is that the mechanisms of protein targeting are based on information contained in the proteins themselves. Furthermore, sorting targets can be the exterior of the cell by secretion, or it can be a location in the cell. A common fallacy among students is to think that the posession of a signal peptide must necessitate translation on the rough ER or that it is a definitive test of a secretory protein. Signal peptides target many other destinations besides secretion including mitochondria or into the nucleus.