ROLE OF MATERNAL INHERITANCE OF MITOCHONDRIA’Mitochondria’ produces energy currency of cells (ATP) through oxidative phosphorylation. It is one of the major organelle, which involves in various cellular works like cell signalling, cell-differentiation, cell death, cell cycle and cell growth.
Most cell contain their DNA in nucleus, but the mitochondria have its own protein, which transcribe by mitochondrial DNA or mtDNA. Mitochondria, plasmid/chloroplast are thought to be endosymbiotic organelles or the organelles who have their own DNA and the DNA inherited in a non-mendelian manner. Mitochondrial genes are not normally inherited as nuclear genes. In addition, this mitochondria are mainly inherited by one parent and which is maternal inheritance. In humans when a sperm fertilizes an egg cell, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote nucleus but when the sperm’s mitochondria enter the egg, do not contribute genetic information to the embryo.
In short, the mitochondria and therefore the mitochondrial DNA usually come from the egg only. This pattern of inheritance is generally refer to as “uniparental inheritance,” and specifically it is called “maternal inheritance.” Some human mitochondrial diseases caused by mtDNA mutations are maternally inherited. Uniparental inheritance of mtDNA, chloroplast DNA (cpDNA)/plastid DNA (ptDNA), is also found in plants.
The egg cell, which contains relatively few mitochondria, but these mitochondria that survive and divide into populate the cells of the adult organism. Paternal mitochondria are tagged with ubiquitin to select them for later destruction inside the embryo. Mitochondria are, therefore, in most cases inherited only from mothers, known as maternal inheritance.
This is seen in most organisms, including the majority of animals. However, mitochondria in some species sometimes be inherited paternally. This is the found among certain coniferous plants, although not in pine trees and yews. For Mytilids sp., paternal inheritance only occurs within males of the species.
It has been seen that it occurs in a very low level in humans. There is a recent research shows, that mitochondria shorten male lifespan stay in the system because they are inherited only through the mother. With contrast, if natural selection decreases mitochondria that reduce female survival as such mitochondria are less likely to be passed on to the next generation. Therefore, it is found that human females and female animals tend to live longer than male. The paternal mitochondria and their DNA are somehow eliminated and are never transmitted to offspring.
Two hypotheses are proposed to explain the mechanism of the maternal inheritance of mtDNA. Based to the “simple dilution model,” the paternal mtDNA, which is present at a much lower copy number, is simply diluted away by the excess of oocyte mtDNA and consequently it is hardly detectable in the offspring. On the other hand in the “active degradation model” where, the paternal mtDNA or mitochondria themselves are thought to be selectively degraded, either before or after fertilization, to actively prevent the transmission of paternal mtDNA to the next generation.
Recently several lines of evidence found supporting the latter model have been gathered in various animals. These results further suggest that different species are seems to be employ distinct mechanisms to prevent the inheritance of paternal mtDNA. Now we will discuss how and when the paternal mitochondria or their genomes are selectively eliminated.Ubiquitin-mediated degradation of paternal mitochondria in Mammals In mammals, paternal mitochondria are tightly packed into the midpiece, the structure connecting the head and flagellar tail of the sperm.
In Chinese hamsters (Cricetulus griseus) that have unusually giant sperm, the sperm midpiece including the paternal mitochondria and the flagellar tail remain outside the egg and do not enter the oocyte cytoplasm after fertilization. This had been thought to explain the mechanism of maternal inheritance of mtDNA