Two cytoplasmic organelles responsible for the production of energy are the mitochondria (present in nearly all eukaryotic cells) and chloroplasts (present only in photosynthetic organisms). Both contain small, circular DNA molecules that constitute the nonnuclear portion of a eukaryotic genome. These organelles are descended from formerly free-living bacteria that took up residence in the first eukaryotes.
The human mitochondrial genome contains 16,569 base pairs specifying 13 protein products and 24 RNA products. In both lower eukaryotes and especially plants, larger mitochondrial genomes are present. In extreme cases, mitochondrial genomes may be several hundred thousand or millions of base pairs. Chloroplast genomes contain between 100 and 200 kilobases. It is thought that each was once larger, but over time their genes have been moved to the nucleus.
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