ARCHAEOLOGICAL CHEMISTRY—EXPLORING HISTORY WITH CHEMISTRY - Chemistry: Notable Research and Discoveries - page 176

Further information about a person's background is contained in
the hereditary material known as deoxyribonucleic acid (DNA). DNA
consists of long sequences of covalently bonded molecules known as
nucleotides, or bases—adenine (A), thymine (T), cytosine (C), and gua-
nine (G)—that provide the blueprint for the body and all of its meta-
bolic activities. Genes are inherited from parents, but because of the
large number of genes in humans—about 25,000—and the mixing that
occurs during reproduction, each person has a unique sequence. (Ex-
ceptions are identical twins, whose DNA sequences are the same be-
cause identical twins come from the same fertilized egg cell.) Forensics
experts often match DNA sequences found in blood or tissue samples
from a crime scene to that of a suspect, placing him or her at the scene
of the crime. In principle, DNA from the Iceman and other ancient
sources could reveal a great deal of information. The problem with the
use of DNA in archaeological chemistry is that it rapidly deteriorates.
AnCIEnTdnA
A DNA molecule is normally a double-stranded helix, with the strands
held together by relatively weak hydrogen bonds. The structure and ge-
ometry of the nucleotide bases are such that A on one strand only pairs
with T on the opposite strand, and C pairs with G. This arrangement is
known as complementary binding—A and T are complementary, as are
C and G. A DNA helix is stable, but the strands are not joined so tightly
that they cannot be “unzipped,” which must occur when genes are read
or the DNA is copied during cell division.
A cell's DNA is contained and protected by little compartments
that are surrounded by membranes. Enzymes known as nucleases chop
up any free DNA floating around the cell; these enzymes act as cellular
sentinels, guarding against invading viruses that try to sneak their genes
into the cell. When an organism dies, the cellular membranes break
down and nucleases destroy much of the cell's own DNA. Finding DNA
in a body that has been around for a while is unlikely.
But certain conditions such as freezing or desiccation (removal
of water) can greatly reduce the activity of these enzymes. This means
that mummies such as Iceman, despite its great age, may contain some
quantity of DNA in good enough shape to determine its sequence. There
are other enemies of ancient DNA, though, including radiation, which
breaks apart the strands, and chemical reactions such as oxidation and