<P> Mitochondria contain their own genome, an indication that they are derived from bacteria through endosymbiosis . However, the ancestral endosymbiont genome has lost most of its genes so that the mitochondrial genome is one of the most reduced genomes across organisms . </P> <P> The human mitochondrial genome is a circular DNA molecule of about 16 kilobases . It encodes 37 genes: 13 for subunits of respiratory complexes I, III, IV and V, 22 for mitochondrial tRNA (for the 20 standard amino acids, plus an extra gene for leucine and serine), and 2 for rRNA . One mitochondrion can contain two to ten copies of its DNA . </P> <P> As in prokaryotes, there is a very high proportion of coding DNA and an absence of repeats . Mitochondrial genes are transcribed as multigenic transcripts, which are cleaved and polyadenylated to yield mature mRNAs . Not all proteins necessary for mitochondrial function are encoded by the mitochondrial genome; most are coded by genes in the cell nucleus and the corresponding proteins are imported into the mitochondrion . The exact number of genes encoded by the nucleus and the mitochondrial genome differs between species . Most mitochondrial genomes are circular, although exceptions have been reported . In general, mitochondrial DNA lacks introns, as is the case in the human mitochondrial genome; however, introns have been observed in some eukaryotic mitochondrial DNA, such as that of yeast and protists, including Dictyostelium discoideum . Between protein - coding regions, tRNAs are present . During transcription, the tRNAs acquire their characteristic L - shape that gets recognized and cleaved by specific enzymes . Mitochondrial tRNA genes have different sequences from the nuclear tRNAs but lookalikes of mitochondrial tRNAs have been found in the nuclear chromosomes with high sequence similarity . </P> <P> In animals, the mitochondrial genome is typically a single circular chromosome that is approximately 16 kb long and has 37 genes . The genes, while highly conserved, may vary in location . Curiously, this pattern is not found in the human body louse (Pediculus humanus). Instead, this mitochondrial genome is arranged in 18 minicircular chromosomes, each of which is 3--4 kb long and has one to three genes . This pattern is also found in other sucking lice, but not in chewing lice . Recombination has been shown to occur between the minichromosomes . The reason for this difference is not known . </P>

Where is the mitochondrion located in a plant cell