The cytoplasm of most eukaryotic cells contain organelles called mitochondria, where energy is extracted from food molecules and stored in ATP (adenosine triphosphate) for later use throughout the cell. Virtually all of the oxygen we use is consumed by our mitochondria.
Mitochondria contain their own DNA molecules (mitochondrial DNA, or mtDNA). These molecules carry a few dozen genes that are essential for energy metabolism. For example, the cob gene carries the instructions for making a protein, cytochrome b, which is an important component of the electron transport system in mitochondria. All the other proteins and RNAs encoded by mtDNA genes are also used in energy metabolism. However, many other key proteins for energy metabolism are encoded by nuclear genes. These are synthesized elsewhere in the cell and imported into the mitochondria. In fact, while the mtDNA genes are absolutely essential for the aerobic production of energy, the majority of all mitochondrial components derive from nuclear genes.
In addition to mitochondria, the cells of plants and algae also contain organelles called chloroplasts, in which photosynthesis takes place. Like the mitochondria, chloroplasts contain DNA molecules (chloroplast DNA, or cpDNA). The cpDNA molecules have genes that encode some of the proteins needed for photosynthesis. Also like the mitochondria, the majority of components needed for photosynthesis are made outside the chloroplast, using information from nuclear genes.
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