mtDNA D-loop is a non-coding locus actively used as an individualizing marker in molecular genetic research. Uneven distribution of SNP in D-loop suggests about irregular functional load within this region. The structural-functional role of various sites in D-loop of single individual's mtDNA and the degree of its (functional) importance from the point of phylogenetic conservatism are considered based on the analysis of nucleotide sequences. The role of duplication of various (functional) elements of the mtDNA D-loop (TAS, ETAS, CSB-elements) affecting the increase in functional reliability of the initiation and termination systems of the mtDNA replication is discussed.