The complete DNA sequences for more than 200 mitochondrial genomes have been determined. The lengths of a few of these mitochondrial DNAs are shown to scale as circles for circular genomes and lines for linear genomes. The largest circle represents the genome of Rickettsia prowazekii, a small pathogenic bacterium whose genome most closely resembles that of mitochondria. The size of mitochondrial genomes does not correlate well with the number of proteins encoded in them: while human mitochondrial DNA encodes 13 proteins, the 22-fold larger mitochondrial DNA of Arabidopsis encodes only 32 proteins—that is, about 2.5-fold as many as human mitochondrial DNA. The extra DNA that is found in Arabidopsis, Marchantia, and other plant mitochondria may be “junk DNA”. The mitochondrial DNA of the protozoan Reclinomonas americana has 97 genes, more than the mitochondrion of any other organism analyzed so far. (Adapted from M.W. Gray et al., Science 283:1476–1481, 1999.)