Mitochondria replicate their DNA and divide mainly in response to the energy needs of the cell; in other words their growth and division is not linked to the cell cycle. When the energy needs of a cell are high, mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive. At cell division, mitochondria are distributed to the daughter cells more or less randomly during the division of the cytoplasm. Mitochondria divide by binary fission similar to bacterial cell division. Unlike bacteria, however, mitochondria can also fuse with other mitochondria. Sometimes new mitochondria are synthesized in centers that are rich in proteins and polyribosomes needed for their synthesis.
Mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote nucleus. In contrast, the mitochondria, and therefore the mitochondrial DNA, usually comes from the egg only. At fertilization of an egg, a single sperm enters the egg along with the mitochondria that it uses to provide the energy needed for its swimming behavior. However, the mitochondria provided by the sperm are targeted for destruction very soon after entry into the egg. The egg itself contains relatively few mitochondria, but it is these mitochondria that survive and divide to populate the cells of the adult organism. This means that mitochondria are usually inherited purely down the female line.
This maternal inheritance of mitochondrial DNA is seen in most organisms, including all animals. However, mitochondria in some species can sometimes be inherited through the father. This is the norm amongst certain coniferous plants (although not in pines and yew trees). It has been suggested to occur at a very low level in humans.
Uniparental inheritance means that there is little opportunity for genetic recombination between different lineages of mitochondria. For this reason, mitochondrial DNA is usually thought of as reproducing clonally. However, there are several claims of recombination in mitochondrial DNA, most controversially in humans. If recombination does not occur, the whole mitochondrial DNA sequence represents a single haplotype, which makes it useful for studying the evolutionary history of populations.
Mitochondrial genomes have many fewer genes than do the related eubacteria from which they are thought to be descended. Although some have been lost altogether, many seem to have been transferred to the nucleus. This is thought to be relatively common over evolutionary time. A few organisms, such as Cryptosporidium, actually have mitochondria which lack any DNA, presumably because all their genes have either been lost or transferred.
The uniparental inheritance of mitochondria is thought to result in intragenomic conflict, such as seen in the petite mutant mitochondria of some yeast species. It is possible that the evolution of separate male and female sexes is a mechanism to resolve this organelle conflict.
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