<Tr> <Td> Robust 16S + 23S phylogeny of Rickettsidae from Ferla et al . </Td> </Tr> <P> The ribosomes coded for by the mitochondrial DNA are similar to those from bacteria in size and structure . They closely resemble the bacterial 70S ribosome and not the 80S cytoplasmic ribosomes, which are coded for by nuclear DNA . </P> <P> The endosymbiotic relationship of mitochondria with their host cells was popularized by Lynn Margulis . The endosymbiotic hypothesis suggests that mitochondria descended from bacteria that somehow survived endocytosis by another cell, and became incorporated into the cytoplasm . The ability of these bacteria to conduct respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage . This symbiotic relationship probably developed 1.7 to 2 billion years ago . </P> <P> A few groups of unicellular eukaryotes have only vestigial mitochondria or derived structures: the microsporidians, metamonads, and archamoebae . These groups appear as the most primitive eukaryotes on phylogenetic trees constructed using rRNA information, which once suggested that they appeared before the origin of mitochondria . However, this is now known to be an artifact of long - branch attraction--they are derived groups and retain genes or organelles derived from mitochondria (e.g., mitosomes and hydrogenosomes). </P>

Where is the mitochondria located in a cell