The inheritance of mitochondria in animals and plants is quite different. In these organisms, the egg cell contributes much more cytoplasm to the zygote than does the male gamete (sperm in animals, pollen in plants). For example, a typical human oocyte contains about 100,000 copies of maternal mitochondrial DNA, whereas a sperm cell contains only a few. In addition, an active process ensures that the sperm mitochondria do not compete with those in the egg. As sperm mature, the DNA is degraded in their mitochondria. Sperm mitochondria are also specifically recognized then eliminated from the fertilized egg cell by autophagy in very much the same way that damaged mitochondria are removed (by ubiquitylation followed by delivery to lysosomes, as discussed in Chapter 13). Because of these two processes, the mitochondrial inheritance in both animals and plants is uniparental. More precisely, the mitochondrial DNA passes from one generation to the next by maternal inheritance.
In about two-thirds of higher plants, the chloroplast precursors from the male parent (contained in pollen grains) fail to enter the zygote, so that chloroplast as well as mitochondrial DNA is maternally inherited. In other plants, the chloroplast precursors from the pollen grains enter the zygote, making chloroplast inheritance biparental. In such plants, defective chloroplasts are a cause of variegation: a mixture of normal and defective chloroplasts in a zygote may sort out by mitotic segregation during plant growth and development, thereby producing alternating green and white patches in leaves. Leaf cells in the green patches contain normal chloroplasts, while those in the white patches contain defective chloroplasts (Figure 14–68).
In humans, as we have explained, all the mitochondrial DNA in a fertilized egg cell is inherited from the mother. Some mothers carry a mixed population of both mutant and normal mitochondrial genomes. Their daughters and sons will inherit this mixture of normal and mutant mitochondrial DNAs and be healthy unless the process of mitotic segregation results in a majority of defective mitochondria in a particular tissue. Muscle and the nervous system are most at risk. Because they need particularly large amounts of ATP, muscle and nerve cells are particularly dependent on fully functional mitochondria.
Numerous diseases in humans are caused by mutations in mitochondrial DNA. These diseases are recognized by their passage from affected mothers to both their daughters and their sons, with the daughters but not the sons producing children with the disease. As expected from the random nature of mitotic segregation, the symptoms of these diseases vary greatly between different family members—including not only the severity and age of onset, but also which tissue is affected. There are also mitochondrial diseases that are caused by mutations in nuclear-encoded mitochondrial proteins; these diseases are inherited in the regular, Mendelian fashion.
Figure 14–68 A variegated leaf. In the white patches, the plant cells have inherited a defective chloroplast. (Courtesy of John Innes Foundation.) the accumulation of Mitochondrial DNa Mutations Is a Contributor to aging
Mitochondria are marvels of efficiency in energy conversion, and they supply the cells of our body with a readily available source of energy in the form of ATP. But in highly developed, long-lived animals such as ourselves, the cells in our body age and eventually die. A factor in this inevitable process is the accumulation of deletions and point mutations in mitochondrial DNA. Oxidative damage to the cell by reactive oxygen species (ROS) such as H2O2, superoxide, or hydroxyl radicals also increases with age. The mitochondrial respiratory chain is the main source of ROS in animal cells, and animals in which mitochondrial superoxide dismutase—the main ROS scavenger—has been knocked out, die prematurely.