Distribution of nucleotide substitutions in human mitochondrial DNA genes

To analyze the distribution pattern of nucleotide substitutions in human mitochondrial DNA (mtDNA), mutational spectra of the mitochondrial genes were reconstructed. The reconstruction procedure is based on the mutation distribution data for 47 monophyletic mtDNA clusters, to which 794 examined mtDNA sequences encoding for tRNAs, rRNAs, and mitochondrial proteins are attributed. One of specific features of mitochondrial mutational spectra revealed was homoplasy of the mutations (the mean mutation number per variable nucleotide site in the coding region varied from 1.09 to 1.43). It was established that in the mtDNA genes maximum mutational constraint fell onto the guanine bases, albeit the content of these bases in the mtDNA L-chains was minimal. Maximal bias towards parallel G to A transitions was observed for rRNA genes, with the protein-and tRNA-encoding genes ranking next. Despite the fact that the differences in the average G-nucleotides content and variability between the genes of two mtDNA segments located between the OriH and OriL were statistically significant, the results did not provide the conclusion that the G-nucleotide instability observed in the mtDNA L-spectra was determined by the mechanism of asynchronous mtDNA replication, along with the deamination of cytosines in the H-chain regions, which remained single-stranded during replication.