The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

An Introduction to Mitochondria
Fig. 2.8. The chemiosmotic proton circuit.
laboratory at a particular time. The discovery was that mitochondria contained their own
DNA, and the best that can be said with fairness is that it spanned a period between 1959,
when Chèvremont and colleagues first reported evidence for it, 30 and 1963, when Nass and
Nass published a clear demonstration that a frequently isolated component of mitochondria
was indeed DNA. 31 No one had any reason to suppose that mitochondria needed DNA, so
there was initially a tendency to ascribe its presence in mitochondrial preparations to nuclear
contamination. Indeed, mitochondria are the only components of animal cells’ cytoplasm
that have any of their own DNA;* all the rest of our genetic material is stored in our
chromosomes, in the nucleus.
This reticence was somewhat compensated in the following years. There was still a
lingering suspicion that the mitochondrial DNA might be of nuclear origin after all, but this
was swept away by the discovery that it is a circular molecule, in contrast to the linear character
of all eukaryotic chromosomes. This discovery was made in 1966, and published with alacrity,
by three labs independently. 32-34
Each mitochondrion has several copies of the mtDNA. The exact number varies between
species; in humans it is typically about five.
2.4.2. Inheritance of mtDNA
Erythrocytes are the only mammalian cells with no mitochondria, but there is another
cell type which has very few: sperm. Mitochondria are passed on from one generation to the
next in egg cells, but almost none are contributed by sperm.** This is largely because sperm
* There is still some debate on this point with regard to centrioles, but the view that they have their own DNA
is not currently favoured.
** It is widely stated that no mtDNA whatever is paternally inherited, but this is incorrect: see Ref. 35 for
an eloquent dissection of this “meme.”
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