ODMR SeeOPTICALLY DETECTED MAGNETIC RESO-
NANCE.
OEC SeeOXYGEN-EVOLVING COMPLEX.
oil A slippery, viscous, or liquefiable substance not
miscible with water.
oil shale A dark-colored shale that contains a solid
substance, kerogen, which is partially formed oil and
can be extracted when crushed and heated to liberate
oil.
olefin Unsaturated aliphatic hydrocarbons (CnH 2 n)
characterized by relatively great chemical activity, e.g.,
ethylene, propylene, and butene.
olfaction The process of smell. In humans, chemore-
ceptors are located in a patch of tissue about the size of
a postage stamp high in the nasal cavity, called the
olfactory epithelium.
oligonucleotide Macromolecules composed of short
SEQUENCEs of NUCLEOTIDEs that are usually synthetically
prepared and used in SITE-DIRECTED MUTAGENESIS.
oligopeptide Four to 10 amino acids joined by pep-
tide bonds.
oligotrophic lake A condition of a lake that has low
concentrations of nutrients and algae, resulting in clear
blue conditions. Contrast with mesotrophic lakes that
have a moderate nutrient condition and EUTROPHIC
LAKEs that have excessive levels of nutrients.
onium ion (1) A cation (with its counterion) derived
by addition of a hydron (hydrogen ion) to a mononu-
clear parent hydride of the nitrogen, chalcogen, and
halogen family, e.g., H 4 N+ammonium ion.
(2) Derivatives formed by substitution of the above
parent ions by univalent groups, e.g., (CH 3 ) 2 S+H
dimethylsulfonium, (CH 3 CH 2 ) 4 N+ tetraethylammo-
nium.
(3) Derivatives formed by substitution of the above
parent ions by groups having two or three free valen-
cies on the same atom. Such derivatives are, whenever
possible, designated by a specific class name, e.g.,
R 2 CNH 2 +iminium ion.
See alsoCARBENIUM ION; CARBONIUM ION.open sextet Species that have six electrons in the
highest energy shell of the central element. Examples
include many LEWIS ACIDs.open system A system that can exchange both mat-
ter and energy with its surroundings.operon A functional unit consisting of a PROMOTER,
an operator, and a number of structural GENEs, found
mainly in PROKARYOTEs. An example is the operon NIF.
The structural genes commonly code for several func-
tionally related ENZYMEs, and although they are tran-
scribed as one (polycistronic) mRNA, each has its
separate TRANSLATION initiation site. In the typical
operon, the operator region acts as a controlling ele-
ment in switching on or off the synthesis of mRNA.
A group or sequence of closely linked genes that
function as a unit in synthesizing enzymes needed for
biosynthesis of a molecule and is controlled by operator
and repressor genes; common in bacteria and phages.
An operator gene is the region of the chromosome, next
to the operon, where a repressor protein binds to pre-
vent transcription of the operon. The repressor gene
protein binds to an operator adjacent to the structural
gene, preventing the transcription of the operon.
See alsoMESSENGERRNA.opposing reaction SeeCOMPOSITE REACTION.optical activity A material that rotates the plane of
polarization of any polarized light transmitted
through it.200 ODMR