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Page 2 of 11 Agresti et al. J Transl Genet Genom 2018;2:9 I http://dx.doi.org/10.20517/jtgg.2018.05

INTRODUCTION TO MITOCHONDRIAL BIOLOGY
Mitochondrial structure and function
[1]
The scientist, Ivan Wallin, stated that mitochondria had originated as endosymbionts in 1927 . Since each
component of a mitochondrion is unique onto itself, its ancestral roots are likely to have originated from
an “alpha-proteobacteria (gender: Rickettsia) that would have been engulfed by a heterotrophic host cell,
probably an archeobacterium, 1.5 billion years ago” as proposed by Lynn Margulis . In Margulis’ book,
[2]
Origin of Eukaryotic Cells, “symbiosis usually means two or more organisms with distinct evolutionary
histories derive benefit from highly specific associations with each other” . While this notion may have been
[3]
initially controversial, it has since been postulated that symbiosis is a necessary component of evolution.
Moreover, scientific evidence has emerged to support the endosymbiotic origin of mitochondria including:
free-living form of early mitochondria, mitochondrial DNA and structure, as well as the relegation of
mitochondrial metabolic processes to nuclear regulation .
[3]

The structure of the mitochondrion is quite unique from the remaining membrane-bounded eukaryotic
organelles. Its composition consists of an outer mitochondrial membrane, an inter-membrane space, an
inner mitochondrial membrane, and a mitochondrial matrix (containing the mtDNA and ribosomes). Due
to the four hydrocarbon tailed phospholipid, cardiolipin, the inner mitochondrial membrane is able to form
cristae structures that are resultant from membrane in-folding . In 1988, Robin and Wong had determined
[5]
[4]
that mitochondrial statistical information (number of mitochondria/cell and number of mtDNA/cell) varied
between the following mammalian cell lines: rabbit lung macrophages, rabbit peritoneal macrophages, mouse
LA9 fibroblasts, human lung fibroblasts and rat L-8 skeletal muscle cells. As per their analysis, the following
statistical measures were obtained: the amount of mitochondrial DNA per mitochondrion remained the
same in all cell varieties, the amount of mitochondrial DNA molecules per cell experienced an eight-fold
difference between the different cell types, while the virtual number of mitochondria per cell also varied .
[5]
Therefore, dependent on the cell type, the morphology of mitochondria can vary greatly, appearing as either
distinct entities ranging from 1 to 4 µm in length, or as part of an organized network . As an interesting
[6,7]
point, the extent of mitochondrial morphology is dependent on the balance between mitochondrial fusion
and fission . Furthermore, these events have the potential to be influenced by the mitochondrial membrane
[6]
[8]
mechanical characteristics .
Resultant of its unique structure, the mitochondrion serves as a crucial cellular metabolism component.
Mitochondria are well-defined as the powerhouse of the eukaryotic cell, but in addition to its famous
oxidative phosphorylation role, they participate in: the creation of iron-sulfur (Fe-S) clusters, β-oxidation
of fatty acids, synthesis of heme prosthetic groups, steroidogenesis (dependent on the cell variety), the
urea cycle, as well as the homeostatic maintenance of calcium . Mitochondria contribute greatly to the
[2]
generation of free radicals, and have a participatory role in inflammation and innate immunity . Moreover,
[4]
mitochondria serve as a component for critical cellular signals relating to either its survival or death . A
[2]
summary of mitochondrial functions, with their associated purpose, appears in Table 1 [2,4,9-17] .

Mitochondrial DNA
While studying chick embryo mitochondria, mtDNA was discovered in 1963 by Nass and Nass . Located
[18]
inside of the matrix, mtDNA is a critical component of mitochondrial processes. Seemingly, the circular
nature of the mitochondrial genome is reminiscent of its bacterial origins, and interestingly, its composition
varies according to the nucleotide densities of its component strands. Therefore, each strand of the circular
molecule is denoted as either the heavy (predominantly composed of guanine) or the light (predominantly
composed of cytosine) strand .
[2]
Unlike nuclear DNA, mtDNA follows a non-mendelian transmission pattern and is inherited maternally via
the oocyte (despite the postulation of a possible chance of paternal inheritance). In 1981, Anderson et al. had
[19]