The widespread occurrence of linear mitochondrial genomes evokes intriguing questions concerning the evolutionary origin and mechanisms leading to the emergence and stabilization of linear DNA genophores. The study of their replication strategies opens a unique possibility of discovering alternative solutions to the end-replication problem and of elucidating how these mechanisms have appeared in evolution. The analysis of linear mitochondrial genomes in organisms belonging to different phylogenetic lines indicates that their evolutionary emergence was accompanied by the generation of various types of terminal structures, the adaptation of existing replication machinery and by the application of different strategies of the telomere replication. This scenario is illustrated by the molecular anatomy and replication of the linear mitochondrial genome in the opportunist yeast pathogen Candida parapsilosis. Recent studies have revealed the existence of extragenomic minicircular molecules derived from the telomere repeats that seem to participate in the novel pathway of telomere maintenance. Importantly, several lines of evidence indicate that a similar mechanism may also be involved in the alternative, telomerase-independent, maintenance of nuclear telomeres in higher eukaryotes, including human telomerase-negative tumor cells.
end-replication problem; linear mitochondrial DNA; telomere; replication; evolution
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