Human Mitochondrial DNA:
What is mtDNA all about?
Mitochondiral DNA (mtDNA) is located in cell organelles called Mitochondira, structures within Eukaryotic cells that convert the chemical energy from food into a form that cells can use, ATP. Most other DNA present in Eukaryotic organisms is found in the cell nucleus. mtDNA can be regarded as the smallest chromosome, and was the first significant part of Human Genome to be sequenced. In most species, including humans, mtDNA is inherited solely from the mother. The DNA sequence of mtDNA has been determined from a large number of organisms and individual (including some organisms that are extinct), and the comparison of those DNA Sequences represents a mainstay in Phylogenetics, in that it allows biologist to elucidate the evolutionary relationships among species. It also permits an examination of the relatedness of populations, and so has become important in Anthropology and Field Biology.
Mitochondrial Inheritance
In most multicellular organisms, mtDNA is inherited from mother (maternally inherited). Mechanisms for this includes simple dilution (an egg contains 100,000 to 1,000,000 mtDNA molecules, whereas a sperm contains only 100 to 1000), degradation of sperm mtDNA in the fertilized egg, and, at least in a few organisms, failure of sperm mtDNA to enter the egg. Whatever the mechanism, this single parent (uniparental) pattern of mtDNA inheritance is found in most animals, most plants and in fungi as well.
Female Inheritance
In sexual reproduction, mitochondria are normally inherited exclusively from the mother. The mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Also, the most mitochondria are present at the base of the sperm's tail, which is used for propelling the sperm cells. Sometomes the tail is lost during fertilization.
In 1999 it was reported that parental sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo. Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling. Ubiquitin can be attached to protein and label them for destruction. The ubiquitin tag directs protein to proteasome, which is a large protein complex in the cell that degrades and recycles unneeded proteins. This discovery won the Nobel Prize for Chemistry in 2004. Ubiquitin Tags can also direct proteins to other locations in the cell, where they control other protein and cell mechanisms. Ubiquitin is encoded in mammals by 4 different genes. UBA52 and RPS27A genes code for a single copy of ubiquitin fused to the ribosomal proteins L40 and S27a, respectively. The UBB and UBC genes code for polyubiquitin precursor proteins.
Peter Sutovsky Et. el [1], published a Nature Journal "Development: Ubiquitin Tag for Sperm Mitochondria" (25 November 1999). Like other mammals, humans inherit mitochondria from mother only, even though the sperm contributes nearly one hundred mitochondria to the fertilized egg. In support of the idea that this strictly maternal inheritance of mtDNA arises from selective destruction of sperm mtDNA. The journal shows the sperm mtDNA inside fertilized Cow and Monkey eggs are tagged by the recycling marker protein ubiquitin. This imprint is a death sentence that is written during spermatogenesis and executed after the sperm mitochondria encounter the egg's cytoplasmic destruction machinery.
The fact that mtDNA is maternally inherited enables researchers to trace maternal lineage far back in time (Y-Chromosome DNA, paternally inherited, is used in an analogous way to trace the Antage Lineage). This is accomplished in humans by sequencing one or more of the Hypervariable Control Regions (HVR1 or HVR2) of the mtDNA, as with a genealogical DNA Test. HVR1 consists of about 440 base pairs. These 440 base pairs are then compared to the control regions of the other individual to determine maternal lineage.
Because mtDNA is not highly conserved and has a rapid mutation rate, it is useful for studying the evolutionary relationships - phylogeny - of organisms. Biologists can determine and then compare mtDNA sequences among different species and use the comparisons to build an evolutionary tree for the species examined. Since mtDNA is transmitted from mother to child (both male and female), it can be useful tool in geneological research into a person's maternal line.
References:
[1] Peter Sutovsky, Ricardo D. Moreno, João Ramalho-Santos, Tanja Dominko, Calvin Simerly & Gerald Schatten. "Development: Ubiquitin Tag for Sperm Mitochondria". Link: http://www.nature.com/nature/journal/v402/n6760/full/402371a0.html
Mitochondrial Inheritance
In most multicellular organisms, mtDNA is inherited from mother (maternally inherited). Mechanisms for this includes simple dilution (an egg contains 100,000 to 1,000,000 mtDNA molecules, whereas a sperm contains only 100 to 1000), degradation of sperm mtDNA in the fertilized egg, and, at least in a few organisms, failure of sperm mtDNA to enter the egg. Whatever the mechanism, this single parent (uniparental) pattern of mtDNA inheritance is found in most animals, most plants and in fungi as well.
Female Inheritance
In sexual reproduction, mitochondria are normally inherited exclusively from the mother. The mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Also, the most mitochondria are present at the base of the sperm's tail, which is used for propelling the sperm cells. Sometomes the tail is lost during fertilization.
In 1999 it was reported that parental sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo. Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling. Ubiquitin can be attached to protein and label them for destruction. The ubiquitin tag directs protein to proteasome, which is a large protein complex in the cell that degrades and recycles unneeded proteins. This discovery won the Nobel Prize for Chemistry in 2004. Ubiquitin Tags can also direct proteins to other locations in the cell, where they control other protein and cell mechanisms. Ubiquitin is encoded in mammals by 4 different genes. UBA52 and RPS27A genes code for a single copy of ubiquitin fused to the ribosomal proteins L40 and S27a, respectively. The UBB and UBC genes code for polyubiquitin precursor proteins.
Peter Sutovsky Et. el [1], published a Nature Journal "Development: Ubiquitin Tag for Sperm Mitochondria" (25 November 1999). Like other mammals, humans inherit mitochondria from mother only, even though the sperm contributes nearly one hundred mitochondria to the fertilized egg. In support of the idea that this strictly maternal inheritance of mtDNA arises from selective destruction of sperm mtDNA. The journal shows the sperm mtDNA inside fertilized Cow and Monkey eggs are tagged by the recycling marker protein ubiquitin. This imprint is a death sentence that is written during spermatogenesis and executed after the sperm mitochondria encounter the egg's cytoplasmic destruction machinery.
The fact that mtDNA is maternally inherited enables researchers to trace maternal lineage far back in time (Y-Chromosome DNA, paternally inherited, is used in an analogous way to trace the Antage Lineage). This is accomplished in humans by sequencing one or more of the Hypervariable Control Regions (HVR1 or HVR2) of the mtDNA, as with a genealogical DNA Test. HVR1 consists of about 440 base pairs. These 440 base pairs are then compared to the control regions of the other individual to determine maternal lineage.
Because mtDNA is not highly conserved and has a rapid mutation rate, it is useful for studying the evolutionary relationships - phylogeny - of organisms. Biologists can determine and then compare mtDNA sequences among different species and use the comparisons to build an evolutionary tree for the species examined. Since mtDNA is transmitted from mother to child (both male and female), it can be useful tool in geneological research into a person's maternal line.
References:
[1] Peter Sutovsky, Ricardo D. Moreno, João Ramalho-Santos, Tanja Dominko, Calvin Simerly & Gerald Schatten. "Development: Ubiquitin Tag for Sperm Mitochondria". Link: http://www.nature.com/nature/journal/v402/n6760/full/402371a0.html
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