barrier for lipid flipping. the mam may also be part of the secretory pathway, in addition to its role in intracellular lipid trafficking. in particular, the mam appears to be an intermediate destination between the rough er and the golgi in the pathway that leads to very-low-density lipoprotein, or vldl, assembly and secretion. the mam thus serves as a critical metabolic and trafficking hub in lipid metabolism. calcium signaling a critical role for the er in calcium signaling was acknowledged before such a role for the mitochondria was widely accepted, in part because the low affinity of ca2+ channels localized to the outer mitochondrial membrane seemed to contradict this organelle's purported responsiveness to changes in intracellular ca2+ flux. but the presence of the mam resolves this apparent contradiction: the close physical association between the two organelles results in ca2+ microdomains at contact points that facilitate efficient ca2+ transmission from the er to the mitochondria. transmission occurs in response to so-called "ca2+ puffs" generated by spontaneous clustering and activation of ip3r, a canonical er membrane ca2+ channel. the fate of these puffs—in particular, whether they remain restricted to isolated locales or integrated into ca2+ waves for propagation throughout the cell—is determined in large part by mam dynamics. although reuptake of ca2+ by the er (concomitant with its release) modulates the intensity of the puffs, thus insulating mitochondria to a certain degree from high ca2+ exposure, the mam often serves as a firewall that essentially buffers ca2+ puffs by acting as a sink into which free ions released into the cytosol can be funneled. this ca2+ tunneling occurs through the low-affinity ca2+ receptor vdac1, which recently has been shown to be physically tethered to the ip3r clusters on the er membrane and enriched at the mam. the ability of mitochondria to serve as a ca2+ sink is a result of the electrochemical gradient generated during oxidative phosphorylation, which makes tunneling of the cation an exergonic process. normal, mild calcium influx from cytosol into the mitochondrial matrix causes transient depolarization that is corrected by pumping out protons. but transmission of ca2+ is not unidirectional; rather, it is a two-way street. the properties of the ca2+ pump serca and the channel ip3r present on the er membrane facilitate feedback regulation coordinated by mam function. in particular, the clearance of ca2+ by the mam allows for spatio-temporal patterning of ca2+ signaling because ca2+ alters ip3r activity in a biphasic manner. serca is likewise affected by mitochondrial feedback: uptake of ca2+ by the mam stimulates atp production, thus providing energy that enables serca to reload the er with ca2+ for continued ca2+ efflux at the mam. thus, the mam is not a passive buffer for ca2+ puffs; rather it helps modulate further ca2+ signaling through feedback loops that affect er dynamics. regulating er release of ca2+ at the mam is especially critical because only a certain window of ca2+ uptake sustains the mitochondria, and consequently the cell, at homeostasis. sufficient intraorganelle ca2+ signaling is required to stimulate metabolism by activating dehydrogenase enzymes critical to flux through the citric acid cycle. however, once ca2+ signaling in the mitochondria passes a certain threshold, it stimulates the intrinsic pathway of apoptosis in part by collapsing the mitochondrial membrane potential required for metabolism. studies examining the role of pro- and anti-apoptotic factors support this model; for example, the anti-apoptotic factor bcl-2 has been shown to interact with ip3rs to reduce ca2+ filling of the er, leading to reduced efflux at the mam and preventing collapse of the mitochondrial membrane potential post-apoptotic stimuli. given the need for such fine regulation of ca2+ signaling, it is perhaps unsurprising that dysregulated mitochondrial ca2+ has been implicated in several neurodegenerative diseases, while the catalogue of tumor suppressors includes a few that are enriched at the mam. molecular basis for tethering recent advances in the identification of the tethers between the mitochondrial and er membranes suggest that the scaffolding function of the molecular elements involved is secondary to other, non-structural functions. in yeast, ermes, a multiprotein complex of interacting er- and mitochondrial-resident membrane proteins, is required for lipid transfer at the mam and exemplifies this principle. one of its components, for example, is also a constituent of the protein complex required for insertion of transmembrane beta-barrel proteins into the lipid bilayer. however, a homologue of the ermes complex has not yet been identified in mammalian cells. other proteins implicated in scaffolding likewise have functions independent of structural tethering at the mam; for example, er-resident and mitochondrial-resident mitofusins form heterocomplexes that regulate the number of inter-organelle contact sites, although mitofusins were first identified for their role in fission and fusion events between individual mitochondria. glucose-related protein 75 (grp75) is another dual-function protein. in addition to the matrix pool of grp75, a portion serves as a chaperone that physically links the mitochondrial and er ca2+ channels vdac and ip3r for efficient ca2+ transmission at the mam. another potential tether is sigma-1r, a non-opioid receptor whose stabilization of er-resident ip3r may preserve communication at the mam during the metabolic stress response. model of the yeast multimeric tethering complex, ermes perspective the mam is a critical signaling, metabolic, and trafficking hub in the cell that allows for the integration of er and mitochondrial physiology. coupling between these organelles is not simply structural but functional as well and critical for overall cellular physiology and homeostasis. the mam thus offers a perspective on mitochondria that diverges from the traditional view of this organelle as a static, isolated unit appropriated for its metabolic capacity by the cell. instead, this mitochondrial-er interface emphasizes the integration of the mitochondria, the product of an endosymbiotic event, into diverse cellular processes. recently it has also been shown, that mitochondria and mam-s in neurons are anchored to specialised intercellular communication sites (so called somatic-junctions). microglial processes monitor and protect neuronal functions at these sites, and mam-s are supposed to have an important role in this type of cellular quality-control. origin and evolution there are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous. the endosymbiotic hypothesis suggests that mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells; they became endosymbionts living inside the eukaryote. in the autogenous hypothesis, mitochondria were born by splitting off a portion of dna from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes; this dna portion would have been enclosed by membranes, which could not be crossed by proteins. since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is the more widely accepted of the two accounts. a mitochondrion contains dna, which is organized as several copies of a single, usually circular chromosome. this mitochondrial chromosome contains genes for redox proteins, such as those of the respiratory chain. the corr hypothesis proposes that this co-location is required for redox regulation. the mitochondrial genome codes for some rnas of ribosomes, and the 22 trnas necessary for the translation of mrnas into protein. the circular structure is also found in prokaryotes. the proto-mitochondrion was probably closely related to rickettsia. however, the exact relationship of the ancestor of mitochondria to the alphaproteobacteria and whether the mitochondrion was formed at the same time or after the nucleus, remains controversial. for example, it has been suggested that the sar11 clade of bacteria shares a relatively recent common ancestor with the mitochondria, while phylogenomic analyses indicate that mitochondria evolved from a pseudomonadota lineage that is closely related to or a member of alphaproteobacteria. some papers describe mitochondria as sister to the alphaproteobactera, together forming the sister the marineproteo1 group, together forming the sister to magnetococcidae. proteobacteria alphaproteobacteria s.l. magnetococcales marineproteo1 mitochondria alphaproteobacteria s.s. 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. the endosymbiotic relationship of mitochondria with their host cells was popularized by lynn margulis. the endosymbiotic hypothesis suggests that mitochondria descended from aerobic 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. evolution of mros 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). hydrogenosomes, mitosomes, and related organelles as found in some loricifera (e. g. spinoloricus) and myxozoa (e. g. henneguya zschokkei) are together classified as mros, mitochondrion-related organelles. monocercomonoides appear to have lost their mitochondria completely and at least some of the mitochondrial functions seem to be carried out by cytoplasmic proteins now. mitochondrial genetics the circular 16,569 bp human mitochondrial genome encoding 37 genes, i.e., 28 on the h-strand and 9 on the l-strand mitochondria contain their own genome. the human mitochondrial genome is a circular double-stranded 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 (12s and 16s rrna). one mitochondrion can contain two to ten copies of its dna. one of the two mitochondrial dna (mtdna) strands has a disproportionately higher ratio of the heavier nucleotides adenine and guanine, and this is termed the heavy strand (or h strand), whereas the other strand is termed the light strand (or l strand). the weight difference allows the two strands to be separated by centrifugation. mtdna has one long non-coding stretch known as the non-coding region (ncr), which contains the heavy strand promoter (hsp) and light strand promoter (lsp) for rna transcription, the origin of replication for the h strand (orih) localized on the l strand, three conserved sequence boxes (csbs 1–3), and a termination-associated sequence (tas). the origin of replication for the l strand (oril) is localized on the h strand 11,000 bp downstream of orih, located within a cluster of genes coding for trna. 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. most proteins necessary for mitochondrial function are encoded 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. 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. 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. 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. human population genetic studies the near-absence of genetic recombination in mitochondrial dna makes it a useful source of information for studying population genetics and evolutionary biology. because all the mitochondrial dna is inherited as a single unit, or haplotype, the relationships between mitochondrial dna from different individuals can be represented as a gene tree. patterns in these gene trees can be used to infer the evolutionary history