Relaxed codon usage. Whereas 30 or more tRNAs specify amino acids in the cytosol and in chloroplasts, only 22 tRNAs are required for mitochondrial protein synthesis. The normal codon–anticodon pairing rules are relaxed in mitochondria, so that many tRNA molecules recognize any one of the four nucleotides in the third (wobble) position. Such “2 out of 3” pairing human mtDNA16,569 bporigin ofreplication cytochrome bdehydrogenase dehydrogenasecytochrome oxidasesubunitsATP synthase subunits
Figure 14–64 A model for mitochondrial fusion. the fusions of the outer and inner mitochondrial membranes are coordinated sequential events, each of which requires a separate set of protein factors. Outer membrane fusion is brought about by an outer-membrane Gtpase (purple), which forms an oligomeric complex that includes subunits anchored in the two membranes to be fused. Fusion of outer membranes requires Gtp and an h+ gradient across the inner membrane. For fusion of the inner membrane, a dynamin-related protein  forms an oligomeric tethering complex (blue) that includes subunits anchored in  the two inner membranes to be fused. Fusion of the inner membranes requires Gtp and the electrical component of the potential across the inner membrane. (adapted from S. hoppins, L. Lackner  and J. Nunnari, Annu. Rev. Biochem. 76:751–780, 2007.)
Figure 14–65 The organization of the human mitochondrial genome. the human mitochondrial genome of ≈16,600 nucleotide pairs contains 2 rrNa genes, 22 trNa genes, and 13 protein-coding sequences. there are two transcriptional promoters, one for each strand of the mitochondrial DNa (mtDNa). the DNas of many other animal mitochondrial genomes have been completely sequenced. Most of these animal mitochondrial DNas encode precisely the same genes as humans, with the gene order being identical for animals ranging from fish to mammals.
allows one tRNA to pair with any one of four codons and permits protein synthesis with fewer tRNA molecules.
3. Variant genetic code. Perhaps most surprising, comparisons of mitochondrial gene sequences and the amino acid sequences of the corresponding proteins indicate that the genetic code is different: 4 of the 64 codons have different “meanings” from those of the same codons in other genomes (Table 14–3).
The close similarity of the genetic code in all organisms provides strong evidence that they all have evolved from a common ancestor. How, then, do we explain the differences in the genetic code in many mitochondria? A hint comes from the finding that the mitochondrial genetic code in different organisms is not the same. In the mitochondrion with the largest number of genes in Figure 14–60, that of the protozoan Reclinomonas, the genetic code is unchanged from the standard genetic code of the cell nucleus. Yet UGA, which is a stop codon elsewhere, is read as tryptophan in the mitochondria of mammals, fungi, and invertebrates. Similarly, the codon AGG normally codes for arginine, but it codes for stop in the mitochondria of mammals and codes for serine in the mitochondria of Drosophila (see Table 14–3). Such variation suggests that a random drift can occur in the genetic code in mitochondria. Presumably, the unusually small number of proteins encoded by the mitochondrial genome makes an occasional change in the meaning of a rare codon tolerable, whereas such a change in a larger genome would alter the function of many proteins and thereby destroy the cell.