Molecular evolution and biology of human mitochondrial DNA
Nucleotide sequences of the major noncoding region of human mitochondrial DNA (mtDNA) from 95 human placentas have been determined. These sequences include 482 base-pair (bp) long region encompassing most part of the D-loop forming region. Comparisons of these sequences with those previously determined have revealed a remarkable features of nucleotide substitutions and insertion/deletion events. Average nucleotide diversity among the sequences is estimated as 1.45%, which is three to four-fold higher than the corresponding value estimated from restriction-enzyme analysis of whole mtDNA genome. This large variability in the noncoding region is consistent with the weak functional constraints on the region. A hypervariable domain has also been defined. In a 14 bp stretch, at least 17 different sequences were detected in this hypervariable domain. More than 97% of the base changes are transitions. Length differences have occurred exclusively as insertions of cytosine(s) or deletions of adenine(s) within stretch of adenines or cytocines, respectively. A significantly nonrandom distribution of nucleotide substitutions and sequence length variations were also noted. The results in the present study confirmed several features of the noncoding region of mtDNA which previously reported by Greenberg et al. (1) and Aquadro and Greenberg (2), though only seven sequences were determined and compared by these authors.
The phylogenetic analysis indicates that diversity among the Negroids is much larger than that among the Caucasoids or the Mongoloids. In fact, part of Negroids first diverged from the rest of individuals in the phylogenetic tree. A striking finding in the phylogenetic analysis is that the Mongoloids can be separated into two distinct groups. Divergence of part of the Mongoloids follows the earliest divergence of the Negroids. Remainder of the Mongoloids subsequently diverged together with the Caucasoids. This observation confirmed our earlier study which clearly demonstrated existence of two distinct groups in the Japanese by the restriction enzyme analysis (3).
KeywordsNucleotide Substitution Racial Group Polymorphic Site Nucleotide Difference Noncoding Region
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