Students can produce a page for an organelle catalog, conduct a mock radio interview, assess myths about Mitochondrial Eve… and more! Useful links for educators » Mitochondrial Control Region A how-to library which allows high school and college classes to isolate mtDNA, and to have it processed free of charge at the Cold Spring Harbor Laboratory.
Click on "sequencing service. An excellent resource listing all organelles, their functions, and complete with graphics. All rights reserved. Recently, he and his May Physical anthropologists have been providing an answer for over a hundred years by studying morphological characteristics, such as skull shape, of the fossilised remains of our human and proto-human ancestors.
For the last 15 years or so, molecular anthropologists have been comparing the DNA of living humans of diverse origins to build evolutionary trees. Mutations occur in our DNA at a regular rate and will often be passed along to our children. It is these differences polymorphisms that, on a genotypic level, make us all unique and analysis of these differences will show how closely we are related.
However, different approaches used by molecular and physical anthropologists have led to opposing views on how modern humans evolved from our archaic ancestors. Two main hypotheses The two main hypotheses agree that Homo erectus evolved in Africa and spread to the rest of the world around 1 - 2 million years ago; it is regarding our more recent history where they disagree.
Mitochondrial DNA DNA is present inside the nucleus of every cell of our body but it is the DNA of the cell's mitochondria that has been most commonly used to construct evolutionary trees. Mitochondria have their own genome of about 16, bp that exists outside of the cell nucleus. They are present in large numbers in each cell, so fewer samples is required. They have a higher rate of substitution mutations where one nucleotide is replaced with another than nuclear DNA making it easier to resolve differences between closely related individuals.
Random infrequent changes once again provide a way of estimating the number of generations back to a shared ancestor. The evidence of DNA reveals that all humans are very closely related.
A Scot, a Japanese and an Australian Aborigine are far more closely linked by family inheritance than any three chimpanzees from different African groups. DNA research suggests that all surviving humans are descended from one woman who lived perhaps , years ago. Research also shows that the story begins in Africa, home to the greatest variation in human DNA, and therefore the oldest location.
Accordingly the woman was promptly dubbed "the African Eve". Not surprisingly, people of the same ethnic and linguistic group turn out to be genetically more closely related to each other than to the rest of the planet, but the same research shows a great deal of mixing of populations as well. There are no distinct lines between one species and the next; everything works in shades of gray. The controversy over our origins will surely continue. Unlike many fields of study, human evolution is not something you can design experiments to test, Akey adds.
But then again, perhaps scientists need to rethink the debate entirely. All rights reserved. Share Tweet Email. Read This Next Wild parakeets have taken a liking to London. Animals Wild Cities Wild parakeets have taken a liking to London Love them or hate them, there's no denying their growing numbers have added an explosion of color to the city's streets.
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Animals This frog mysteriously re-evolved a full set of teeth. Animals Wild Cities Wild parakeets have taken a liking to London. Animals Wild Cities Morocco has 3 million stray dogs. The mtDNA of the cucumber Cucumis sativus is Kb and carries 65 genes, while that of the fungus Cryphonectria parasitica is only 1.
Some plants have smaller mtDNAs than that of the cucumber but carry more genes. The variation in size becomes even larger if we take into account organelles, such as mitosomes and hydrogenosomes, which derive from the mitochondrion but lack DNA and maintain only a double membrane [ 22 , 23 ].
The number of these chromosomes varies from two e. There are several exceptions to the rule that mtDNA is maternally transmitted. In some plants e. In animals, a well known and idiomorphic case of paternal transmission is that of doubly uniparental inheritance DUI that is found in several species of molluscan bivalves [ 32 , 33 ].
In this peculiar mode of inheritance, females transmit their mtDNA to both male and female offspring and males transmit their mtDNA only to male offspring. The result is the co-occurrence in the same species of two independently evolving mtDNA lineages, one that is transmitted through the eggs and another through the sperm. Males are heteroplasmic for both the maternal and the paternal mtDNA but produce sperm that contains only the latter.
The mechanisms that ensure maternal transmission of mtDNA vary in different organisms [ 34 ]. Maternal inheritance of mtDNA leads to homoplasmic individuals, i. Homoplasmy is further reinforced by pre- and a post- fertilization bottlenecks [ 40 ]. The pre-fertilization bottleneck occurs during oogenesis, where the number of mitochondria is severely reduced in the germ line, before maturation of the oocyte. The post-fertilization bottleneck occurs between the zygote formation and the blastocyst embryonic stages, during which there is intense cell division but suppression of mitochondrial proliferation, a mechanism that leads to a reduced number of mitochondria per cell.
The variation of the mechanisms that have been evolved to ensure maternal transmission of mtDNA and the ubiquity of this transmission mode among organisms indicate that there must be a strong evolutionary reason for the maintenance of maternal transmission. The most prominent hypothesis for maternal transmission of mtDNA is that uniparental maternal in the case of metazoans transmission of mtDNA prevents the spread of selfish fast replicating mutations in the population [ 41 — 43 ].
This hypothesis is supported by experiments in yeast, where yeast cells with a small, defective mtDNA molecule replicate faster than the normal mtDNA, but produce smaller colonies petit relative to the cells with normal mtDNA [ 44 ]. Recently, a second hypothesis—not necessarily mutually exclusive to the above—suggests that maternal transmission has evolved to prevent heteroplasmy [ 45 ].
Heteroplasmy has been involved in mitochondrial diseases [ 46 ]. Experimental evidence from mice have shown that heteroplasmy can cause severe physiological, cognitive and behavioral problems [ 47 ]. In Drosophila though, there are reports that heteroplasmy is adaptive. When both molecules contained a deleterious mutation, different in each molecule, the presence of both molecules in the same individual cancels out the deleterious effects caused when each molecule occurs alone [ 48 ].
Given the strictness of the mechanisms that prevent paternal leakage of mtDNA, heteroplasmy should be extremely rare in nature. However, more and more publications reported heteroplasmy in natural populations in several species such as anchovy [ 49 ], Drosophila [ 50 ], mice [ 51 ], oniscid crustaceans [ 52 ], frogs [ 53 ] and humans [ 54 , 55 ]. Despite the increased number of reports of heteroplasmy, the proportion of species in which heteroplasmy has been observed remains low, compared to the large number of species that sequences for their mtDNA have been deposited in GenBank.
One possibility is that heteroplasmy is common but it cannot be easily detected with the techniques used. The most commonly used technique involves PCR-amplification of a specific segment of the mtDNA molecule and subsequent sequencing, either directly or after cloning.
If the targeted mtDNA pool contains a predominant molecule and some other types in low frequencies, the traditional Sanger sequencing will not reveal the presence of the rare molecules.
Only the design of specific primers for the rare molecules could detect them when using direct sequencing. Alternatively, a large number of clones from the PCR product need to be sequenced, or next generation sequencing NGS should be applied, to reveal the presence of rare molecules. The accuracy of next generation sequencing allows not only to detect heteroplasmy when it is present [e.
In a recent study, paternal transmission was excluded from four human parents-offspring trios [ 56 ]. Heteroplasmy may result through different routes, not mutually exclusive. First, the egg may be heteroplasmic, containing two or more different types; this is the case of mother-inherited heteroplasmy. Second, somatic mutations may occur during mtDNA replication in somatic cells.
Given the tremendous amount of mtDNA copies per individual a diploid organism may contain billions of cells and each cell contains two copies of nuclear genes but hundreds or thousands of copies of mtDNA and the elevated mutation rate of mtDNA in animals, each individual contains unavoidably many mutated forms of the mtDNA which it inherited from its mother.
Finally, leakage of paternal mtDNA can be a significant source of heteroplasmy. The mitochondria of the sperm may escape destruction in the egg during fertilization so that the embryo will be heteroplasmic for a maternal and a paternal mtDNA molecule. Some authors suggested that leakage of paternal mtDNA occurs accidentally because the mechanism that supervises uniparental transmission is not infallible [ 57 ]. The chance of paternal leakage in offspring will be higher when the genetic distance between the two parents is high.
The reason for this is failure of the egg-sperm mitochondrial recognition mechanism. Basically, the mechanism consists of a factor that is coded by the maternal nuclear genome and occurs in the eggs, and of a signal that is coded by the paternal genome and occurs in the outer surface of sperm mitochondria. When the latter enter the egg their signal is recognized by the factor and destruction of sperm mitochondria ensues.
A sequel of this hypothesis is that the mechanism will be less efficient the more divergent are the maternal and paternal species. The hypothesis suggests that heteroplasmy will be more common in progeny from heterospecific than from homospecific crosses. Evidence from Drosophila and frogs supports this hypothesis [ 53 , 58 , 59 ], but further support is needed.
Other authors have suggested that paternal leakage might be under the control of natural selection [ 59 ]. The overall evidence suggests that heteroplasmy is common to the point that strictly maternal inheritance cannot be held as a rule. But it might be more useful to consider maternal transmission as a quantitative characteristic [ 60 ]. If we do so, then the answer is that in animals an overwhelming amount of mtDNA is maternally transmitted and that paternal leakage is restricted to very low amounts.
Recombination in plants and fungi mtDNA has been reported in the early 80s of past century [ 61 , 62 ], but animal mtDNA was considered for decades as a non-recombining genome [ 63 ]. The view that there is no recombination in the animal mtDNA was based on the assumption of homoplasmy, itself a result of the assumption of strict maternal transmission. The view was supported by the persistent lack of evidence for recombination.
But experiments showed that animal mitochondria contain the enzymatic apparatus for recombination [ 64 ]. The first direct evidence for recombination was obtained by Ladoukakis and Zouros [ 65 ] in mussels. This was followed by a long list of mtDNA recombination in other organisms, including human [ 66 ] and Drosophila [ 67 ], using either direct sequencing or utilizing data deposited in GenBank [ 68 — 71 ]. Like heteroplasmy, the detection of recombination is not easy given the rarity of recombinant molecules in an individual.
However, next generation sequencing NGS techniques promise to be a powerful tool for the detection of recombinants, given their ability to detect molecules in a DNA pool that occur in very low amounts.
But NGS can also produce artificial recombinants chimeric sequences in a low frequency [ 72 ]. Evidence for real recombination requires, therefore, that the detected recombinants exceed the error threshold of the used technology.
Using these sensitive techniques Kraytsberg et al. The evolutionary consequences of recombination of mtDNA are far reaching. The elevated mutation rate and the low effective population size of animal mtDNA, which is estimated to be one quarter of that of nuclear autosomes [ 78 ] makes the mtDNA more prone to accumulation of deleterious mutations. That the mtDNA molecule has not collapsed may be due to recombination. The presence of mtDNA recombination may have had an undesirable effect on its use as a genetic marker.
Simulations have shown that a phylogenetic tree reconstructed with mtDNA sequences that were allowed to recombine had, in comparison with a tree in which mtDNA recombination was not allowed, longer terminal branches, larger total branch lengths and shorter times to the most recent common ancestor [ 81 ]. However, more research is needed to appreciate the effects of mtDNA recombination on phylogeny reconstruction, particularly with the extensive use of Bayesian methods on phylogeny.
The history of our understanding of animal mtDNA is itself a lesson of how we gain knowledge of the complexity in biology. The result was an explosion of our knowledge of the tree of life from its roots and big branches to tiny bifurcations at the top.
Gradually the molecule presented us its true face.
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