Mitochondrial inheritance is the study of how mitochondrial genes are inherited. Mitochondria are cellular organelles that contain their own DNA and RNA , allowing them to grow and replicate independent of the cell. Each cell has 10,000 mitochondria each containing two to ten copies of its genome. Because mitochondria are organelles that contain their own genome, they follow an inheritance pattern different from simple Mendelian inheritance, known as extranuclear or cytoplasmic inheritance. Although they posses their own genetic material, mitochondria are semi-autonomous organelles because the nuclear genome of cells still codes for some components of mitochondria.
Mitochondria are double membrane-bound organelles that function as the energy source of eukaryotic cells. Within the inner membrane of mitochondria are folds called cristae that enclose the matrix of the organelle. The DNA of mitochondria, located within the matrix, is organized into circular duplex chromosomes that lack histones and code for proteins, rRNA, and tRNA. A nucleoid, rather than a nuclear envelope, surrounds the genetic material of the organelle. Unlike the DNA of nuclear genes, the genetic material of mitochondria does not contain introns or repetitive sequences resulting in a relatively simple structure. Because the chromosomes of mitochondria are similar to those of prokaryotic cells, scientists hold that mitochondria evolved from free-living, aerobic bacteria more than a billion years ago. It is hypothesized that mitochondria were engulfed by eukaryotic cells to establish a symbiotic relationship providing metabolic advantages to each.
Mitochondria are able to divide independently without the aid of the cell. The chromosomes of mitochondria are replicated continuously by the enzyme DNA polymerase, with each strand of DNA having different points of origin. Initially, one of the parental strands of DNA is displaced while the other parental strand is being replicated. When the copying of the first strand of DNA is complete, the second strand is replicated in the opposite direction. Mutation rates of mitochondria are much greater than that of nuclear DNA allowing mitochondria to evolve more rapidly than nuclear genes. The resulting phenotype (cell death, inability to generate energy, or a silent mutation that has no phenotypic effect) is dependent on the number and severity of mutations within tissues.
During fertilization, mitochondria within the sperm are excluded from the zygote, resulting in mitochondria that come only from the egg. Thus, mitochondrial DNA is inherited through the maternal lineage exclusively without any recombination of genetic material. Therefore, any trait coded for by mitochondrial genes will be inherited from mother to all of her offspring. From an evolutionary standpoint, Mitochondrial Eve represents a single female ancestor from who our mitochondrial genes, not our nuclear genes, were inherited 200,000 years ago. Other women living at that time did not succeed in passing on their mitochondria because their offspring were only male. Although the living descendants of those other females were able to pass on their nuclear genes, only Mitochondrial Eve succeeded in passing on her mitochondrial genes to humans alive today.
See also Mitochondria and cellular energy; Mitochondrial DNA; Molecular biology and molecular genetics; Molecular biology, central dogma of