Chemistry Reference
In-Depth Information
hydrolysis. Over time, radiation and chemical reactions damage the
DNA so that its sequence is undecipherable.
Two compartments in the cells of humans (and other animals) have
DNA—the nucleus, which contains most of the genes of the organism,
and mitochondria. Mitochondria are membrane-bound compartments
scattered throughout the cell and are involved in extracting energy from
food molecules by a series of complex chemical reactions. Capable of
dividing, mitochondria resemble bacteria and have their own DNA.
(Most biologists believe that mitochondria were once independent or-
ganisms that were captured and assimilated into ancestors of animal
cells. The Russian scientist Konstantin Mereschkowsky [1855-1921]
first proposed this theory in the early 20th century.) Genes in mitochon-
drial DNA (mtDNA) produce some but not all of the proteins needed
by mitochondria—the rest come from genes in nuclear DNA.
While each cell has only one nucleus, some cells require a lot of
energy and have up to several thousand mitochondria. Because of these
multiple copies, bits and pieces of mtDNA are more likely to survive
than nuclear DNA.
A few years after Ötzi was found, Svante Pääbo, then at the Uni-
versity of Munich in Germany, and his colleagues searched for DNA
in eight small tissue samples of muscle, connective tissue, and bone,
coming from the left hip area. Because the amount of DNA was so tiny,
the researchers had to amplify the DNA molecules by making many
copies of them, otherwise they would never have been able to find and
sequence them. DNA amplification can be done in a technique known
as polymerase chain reaction (PCR). This technique has become a stan-
dard procedure for forensics experts sifting through crime scene sam-
ples, researchers who sequence genomes (complete sets of genes for an
organism), and archaeological chemists hunting for ancient DNA. The
following sidebar describes PCR in more detail.
Although the Iceman lived a long time ago, his physique and his
biology seem identical to modern humans, so Ötzi's genes should be
quite similar to modern humans. his helped Pääbo and his colleagues
to choose the appropriate sequence for the primers, which as described
in the sidebar on PCR are needed to initiate the DNA amplification pro-
cess. The choice of primer is essential because this is the beginning point
of the copying, and it defines what DNA segments will be amplified. A
primer has to bind in order to provide the starting point, and only com-
plementary sequences will bind tightly enough. Pääbo doubted that any
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