We review the main features of human mitochondrial function and structure, and 
in particular mitochondrial transcription, translation, and replication cycles. 
Furthermore, some pecularities such as mitochondria's high polymorphism, the 
existence of mitochondrial pseudogenes, and the various considerations to take 
into account when studying mitochondrial diseases will also be mentioned. 
Mitochondrial syndromes mostly affecting the nervous system have, during the 
past few years, been associated with mitochondrial DNA (mt DNA) alterations such 
as deletions, duplications, mutations and depletions. We suggest a possible 
classification of mitochondrial diseases according to the kind of mt DNA 
mutations: structural mitochondrial gene mutation as in LHON (Leber's Hereditary 
Optic Neuropathy) and NARP (Neurogenic muscle weakness, Ataxia and Retinitis 
Pigmentosa) as well as some cases of Leigh's syndrome; transfer RNA and 
ribosomal RNA mitochondrial gene mutation as in MELAS (Mitochondrial 
Encephalomyopathy, Lactic Acidosis and Strokelike Episodes) or MERRF (Myoclonic 
Epilepsy with Ragged Red Fibers) or deafness with aminoglycoside; structural 
with transfer RNA mitochondrial gene mutations as observed in large-scale 
deletions or duplications in Kearns-Sayre syndrome, Pearson's syndrome, diabetes 
mellitus with deafness, and CPEO (Chronic Progressive External Ophtalmoplegia). 
Depletions of the mt DNA may also be classified in this category. Even though 
mutations are generally maternally inherited, most of the deletions are 
sporadic. However, multiple deletions or depletions may be transmitted in a 
mendelan trait which suggests that nuclear gene products play a primary role in 
these processes. The relationship between a mutation and a particular phenotype 
is far from being fully understood. Gene dosage and energic threshold, which are 
tissue-specific, appear to be the best indicators. However, the recessive or 
dominant behavior of both the wild type or the mutated genome appears to play a 
significant role, which can be verified with in vitro studies.