Mitochondrial gene

Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DNA is only a small portion of the DNA in a eukaryotic cell; most of the DNA can be found in the cell nucleus and, in plants and algae, also in plastids such as chloroplasts.

Human mitochondrial DNA was the first significant part of the human genome to be sequenced. This sequencing revealed that the human mtDNA includes 16,569 base pairs and encodes 13 proteins.

Since animal mtDNA evolves faster than nuclear genetic markers, it represents a mainstay of phylogenetics and evolutionary biology. It also permits an examination of the relatedness of populations, and so has become important in anthropology and biogeography.

Origin

Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of bacteria engulfed by the early ancestors of today's eukaryotic cells. This theory is called the endosymbiotic theory. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in mammals) are coded for by nuclear DNA, but the genes for some, if not most, of them are thought to have originally been of bacterial origin, having since been transferred to the eukaryotic nucleus during evolution.

The reasons mitochondria have retained some genes are debated. The existence in some species of mitochondrion-derived organelles lacking a genome suggests that complete gene loss is possible, and transferring mitochondrial genes to the nucleus has several advantages. The difficulty of targeting remotely-produced hydrophobic protein products to the mitochondrion is one hypothesis for why some genes are retained in mtDNA; colocalisation for redox regulation is another, citing the desirability of localised control over mitochondrial machinery. Recent analysis of a wide range of mtDNA genomes suggests that both these features may dictate mitochondrial gene retention.

Genome structure and diversity

Across all organisms, there are six main genome types found in mitochondrial genomes, classified by their structure (i.e. circular versus linear), size, presence of introns or plasmid like structures, and whether the genetic material is a singular molecule or collection of homogeneous or heterogeneous molecules.

In many unicellular organisms (e.g., the ciliate ''Tetrahymena'' and the green alga ''Chlamydomonas reinhardtii''), and in rare cases also in multicellular organisms (e.g. in some species of Cnidaria), the mtDNA is found as linearly organized DNA. Most of these linear mtDNAs possess telomerase-independent telomeres (i.e., the ends of the linear DNA) with different modes of replication, which have made them interesting objects of research because many of these unicellular organisms with linear mtDNA are known pathogens.

Animals

Most animals, specifically bilaterian animals, have a circular mitochondrial genome. Medusozoa and calcarea clades however have species with linear mitochondrial chromosomes.

In terms of base pairs, the anemone ''Isarachnanthus nocturnus'' has the largest mitochondrial genome of any animal at 80,923 bp.

In February 2020, a jellyfish-related parasite – '' Henneguya salminicola'' – was discovered that lacks mitochondrial genome but retains structures deemed mitochondrion-related organelles. Moreover, nuclear DNA genes involved in aerobic respiration and in mitochondrial DNA replication and transcription were either absent or present only as pseudogenes. This is the first multicellular organism known to have this absence of aerobic respiration and lives completely free of oxygen dependency.

Plants and fungi

There are three different mitochondrial genome types found in plants and fungi. The first type is a circular genome that has introns (type 2) and may range from 19 to 1000 kbp in length. The second genome type is a circular genome (about 20–1000 kbp) that also has a plasmid-like structure (1 kb) (type 3). The final genome type that can be found in plants and fungi is a linear genome made up of homogeneous DNA molecules (type 5).

Great variation in mtDNA gene content and size exists among fungi and plants, although there appears to be a core subset of genes that are present in all eukaryotes (except for the few that have no mitochondria at all). In Fungi, however, there is no single gene shared among all mitogenomes.
Some plant species have enormous mitochondrial genomes, with '' Silene conica'' mtDNA containing as many as 11,300,000 base pairs. Surprisingly, even those huge mtDNAs contain the same number and kinds of genes as related plants with much smaller mtDNAs.
The genome of the mitochondrion of the cucumber (''Cucumis sativus

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