Page "Mitochondrial DNA" Paragraph 17

For human mitochondrial DNA ( and probably for that of metazoans in general ), 100-10, 000 separate copies of mtDNA are usually present per cell ( egg and sperm cells are exceptions ).

In mammals, each double-stranded circular mtDNA molecule consists of 15, 000-17, 000 base pairs.

The two strands of mtDNA are differentiated by their nucleotide content with the guanine rich strand referred to as the heavy strand, and the cytosine rich strand referred to as the light strand.

The heavy strand encodes 28 genes, and the light strand encodes 9 genes for a total of 37 genes.

Of the 37 genes, 13 are for proteins ( polypeptides ), 22 are for transfer RNA ( tRNA ) and two are for the small and large subunits of ribosomal RNA ( rRNA ).

This pattern is also seen among most metazoans, although in some cases one or more of the 37 genes is absent and the mtDNA size range is greater.

Even greater variation in mtDNA gene content and size exists among fungi and plants, although there appears to be a core subset of genes that are present in all eukaryotes ( except for the few that have no mitochondria at all ).

Some plant species have enormous mtDNAs ( as many as 2, 500, 000 base pairs per mtDNA molecule ) but, surprisingly, even those huge mtDNAs contain the same number and kinds of genes as related plants with much smaller mtDNAs.