gated ion channel A specific ion channel that opens
and closes to allow the cell to alter its membrane
potential. An ion channel is a membrane protein that
forms an aqueous pore so that charged ions can cross
through the membrane. There are several types of ion
channels. For example, a LIGAND-gated ion channel is
where gating is controlled by binding of a chemical sig-
nal (the ligand) to a specific binding site on the channel
protein. Other ion channels are voltage gated and
mechanically gated.
GC-MS Gas chromatograph mass spectroscopy.
See alsoGAS CHROMATOGRAPHY; MASS SPECTROM-
ETER.
Geiger counter A radiation-detection and -measur-
ing instrument. Consists of a gas-filled tube that dis-
charges electrically when ionizing radiation passes
through it. It was named for Hans Geiger and W.
Mueller, who invented it in the 1920s.
gel A semisolid or highly viscous colloidal suspension
of a solid dispersed in a liquid.
gel electrophoresis The analytical laboratory pro-
cess to separate molecules according to their size. The
sample is put on an end of a slab of polymer gel, a
lyophilic colloid that has coagulated to a jelly. An elec-
tric field is then applied through the gel, which sepa-
rates the molecules; small molecules pass easily and
move toward the other end faster than corresponding
larger ones. Eventually all sizes get sorted, since
molecules with similar electric charge and density will
migrate together at the same rate. There are several
types of gel composition, and various chemicals can be
added to help separation.
gem-dimethyl group Two methyl groups of the
same carbon atom. A methyl group is the radical –3CH
that exists only in organic compounds.
geminate pair Pair of MOLECULAR ENTITIES INclose
proximity in solution within a solvent cage and result-
ing from reaction (e.g., bond scission, electron transfer,
group transfer) of a precursor that constitutes a single
kinetic entity.
See alsoION PAIR; RADICAL PAIR.geminate recombination The reaction with each
other of two transient molecular entities produced from
a common precursor in solution. If reaction occurs
before any separation by diffusion has occurred, this is
termed primary geminate recombination.If the mutu-
ally reactive entities have been separated, and come
together by diffusion, this is termed secondary gemi-
nate recombination.gene Structurally, a basic unit of hereditary material;
an ordered SEQUENCE of NUCLEOTIDE bases that
encodes one polypeptide chain (via MRNA). The gene
includes, however, regions preceding and following the
coding region (leader and trailer) as well as (in EUKARY-
OTEs) intervening sequences (INTRONs) between indi-
vidual coding segments (EXONs). Functionally, the gene
is defined by the cis–transtest that determines whether
independent MUTATIONs of the same phenotype occur
within a single gene or in several genes involved in the
same function.general acid catalysis The CATALYSISof a chemical
reaction by a series of BRONSTED ACIDs (which may
include the solvated hydrogen ion) so that the rate of
the catalyzed part of the reaction is given by ΣkHA[HA]
multiplied by some function of SUBSTRATEconcentra-
tions. (The acids HA are unchanged by the overall
reaction.) General catalysis by acids can be experimen-
tally distinguished from SPECIFIC CATALYSISby hydro-
gen cations (HYDRONs) by observation of the RATE OF
REACTIONas a function of buffer concentration.
See also CATALYSIS; CATALYTIC COEFFICIENT;
INTRAMOLECULAR CATALYSIS; PSEUDOCATALYSIS; SPE-
CIFIC CATALYSIS.116 gated ion channel