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- Mitochondria

ANT: Adenine nucleotide translocator. A transporter protein that exchanges ADP for ATP across the mitochondrial inner membrane. There are various tissue specific isoforms of ANT. Mutations in the nuclear gene ANT1 cause autosomal dominant chronic progressive external ophthalmoplegia with secondary mtDNA deletion formation (see table 1). There is a mouse knockout model for the cardiac isoform of ANT (see table 3).

Apoptosis: Programmed cell death. A critical process essential for normal development and cell turnover. The release of cytochrome c from the inner mitochondrial membrane is one mechanism that can trigger apoptosis. This is discussed in detail in other articles in this series.

ATP: Adenosine triphosphate. A high energy phosphate molecule produced by the respiratory chain from ADP. ATP is required for all active cellular processes.

Cybrid cell: Cultured cell generated by fusing the cytoplasm of one cell (cytoplast) with that of another. This technique can be used to generate cell lines with different levels of heteroplasmy on a specific nuclear genetic background.

Cytoplasmic transfer: Technique used to generate heteroplasmic mice. Cytoplasm containing mitochondria from one inbred mouse strain (for example, NZB) are transferred and fused with an early embryonic cell from another cell inbred mouse strain (for example, BALB).

Genetic bottleneck (mitochondrial): A restriction in the number of mitochondrial genomes during early development of the female germ line causes a "sampling effect" which results in great variation in the level of heteroplasmy among the offspring of a heteroplasmic mother.

Haplogroup (mtDNA): mtDNA is highly polymorphic within the general population. There are certain groups of polymorphisms that reflect the maternal ancestry of a particular individual. These large groups are called haplogroups, and they contain individual haplotypes.

Haplotype (mtDNA): A specific mitochondrial genotype defined by a characteristic collection of mtDNA polymorphisms

Heteroplasmy: Mammalian cells contain many copies of mtDNA. Patients with mtDNA disease often harbour a mixture of mutated and wild-type mtDNA—a situation known as heteroplasmy. The proportion of mutated mtDNA can vary between zero and 100%.

Homoplasmy: Mammalian cells contain many copies of mtDNA. Usually all of these copies are identical—a situation known as homoplasmy.

Knockout: Type of mouse model where a specific nuclear gene is removed.

Mitotic (vegetative) segregation: When a heteroplasmic cell divides, the daughter cells may receive different amounts of mutated mtDNA by chance.

mtDNA: Mitochondrial DNA. The 16 569 base pair circular mitochondrial genome. Each cell contains thousands of copies.

Oxidative phosphorylation (OXPHOS): Electrochemical process carried out by the respiratory chain. Electrons received from reduced cofactors are passed between the different respiratory chain complexes; protons are pumped from the mitochondrial matrix into the inter-membrane space generating the mitochondrial membrane potential. The influx of protons through complex V is used to generate ATP from ADP and inorganic phosphate.

POLG: Polymerase {gamma}—the only mitochondrial DNA polymerase. Mutations in the nuclear gene POLG1 are a common cause of autosomal dominant and recessive chronic progressive external ophthalmoplegia with secondary mtDNA deletions (see table 1).

Polymorphism: A phenotypically neutral natural variation in DNA code at a particular site. In strict terms a particular sequence change must be present in 1% of the population to be a polymorphism, but the term is often used to describe any phenotypically neutral sequence variant.

Relaxed replication: Unlike nuclear DNA which is only copied during cell division, mtDNA is continuously recycled within the cell (while the overall amount is maintained at roughly constant levels).

Respiratory chain: A group of four enzyme complexes (I–IV) situated on the inner mitochondrial membrane. Together with ATP synthase (complex V), the respiratory chain is the final common pathway for aerobic energy metabolism and the production of ATP

Satellite cell: Muscle cell precursor situated adjacent to the mature muscle fibre. Satellite cells proliferate in response to muscle stress and fuse with the mature muscle fibre.

Threshold effect: Most mtDNA mutations only cause a biochemical defect of the respiratory chain when the proportion exceeds a critical threshold level. This threshold varies from tissue to tissue, and from mutation to mutation.

Wild-type: Normal DNA found in the outbred population

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