distillation The creation of a purified liquid by con-
densing from a vapor produced during distilling (boiling).
distomer The ENANTIOMER of a chiral compound
that is the less potent for a particular action. This defi-
nition does not exclude the possibility of other effects
or side effects of the distomer.
See alsoCHIRALITY; EUTOMER.
distonic radical cation A radical cation in which
charge and radical sites are separated.DNA(deoxyribonucleic acid) A high-molecular-
mass linear polymer, composed of NUCLEOTIDEs con-
taining 2-deoxyribose and linked between positions 3′
and 5′ by phosphodiester groups. DNA contains the
GENETIC information of organisms. The double-
stranded form consists of a DOUBLE HELIXof two com-
plementary chains that run in opposite directions and
are held together by hydrogen bonds between pairs of
the complementary nucleotides. The way the helices are
constructed may differ and is usually designated as A,
B, Z, etc. Occasionally, alternative structures are found,
such as those with Hoogsteen BASE PAIRING.DNA ligase A linking enzyme involved in replicating
and repairing DNA molecules. It seals “nicks” in the
backbone of a single strand of a double-stranded DNA
molecule; connects Okazaki fragments, short, single-
stranded DNA fragments on the lagging strand during
DNA replication, producing a complementary strand of
DNA; and links two DNA molecules together by catalyz-
ing the formation of a (phosphodiester) bond between
the 5′and 3′ends of the nicked DNA backbone.DNA methylation A biochemical event that adds a
methyl group (–CH 3 ) to DNA, usually at the base cyto-
sine or adenosine, and may be a signal for a gene or
part of a chromosome to turn off gene expression and
become inactive.DNA polymerase An enzyme that catalyzes the syn-
thesis of new complementary DNA molecules from sin-
gle-stranded DNA templates and primers. Different
DNA polymerases are responsible for replication and
repair of DNA, and they extend the chain by adding
nucleotides to the 3′ end of the growing DNA; cat-
alyzes the formation of covalent bonds between the 3′
end of a new DNA fragment and the 5′end of the
growing strand. Used in DNA fingerprinting for
genealogical studies and forensics.DNA probe A single strand of DNA that is labeled
or tagged with a fluorescent or radioactive substance80 distillation
Artwork that represents the structure of deoxyribonucleic acid
(DNA). This is the structure that contains the inherited coded
instructions responsible for the development of an organism. A
DNA molecule is composed of two interwound strands twisted
into a helical shape that gives it great stability. Each strand alter-
nates a sugar group (pentagon) with a phosphate group (circle).
The two strands are linked by pairs of substances called bases, of
which there are four types, known as adenine, guanine, thymine,
and cytosine. The sequence of bases along a strand provides the
code for the activities of a cell.(Courtesy of Kairos, Latin
Stock/Science Photo Library)