Inheritance, Extranuclear
Less than a decade after the rediscovery of Mendel's laws describing the inheritance of genes in the nucleus, hereditary traits were discovered that obey a different set of laws. The genes involved in this non-Mendelian pattern of inheritance reside outside the nucleus, in the cytoplasm of the cell. Specifically, they were found to reside in mitochondria, chloroplasts, or intracellular symbiotic bacteria. Those genes play important roles in the cell. Mutations in extranuclear genes are responsible for some hereditary diseases in humans and other organisms, are used in plant breeding, and are used to study population genetics and evolution.
Genes in Mitochondria and Chloroplasts
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.
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