general base catalysis The catalysis of a chemical
reaction by a series of BRONSTED BASEs (which may
include the LYATE ION) so that the rate of the catalyzed
part of the reaction is given by ΣkB[B] multiplied by
some function of substrate concentration.
See alsoGENERAL ACID CATALYSIS.
genetic code The language of genetics. The instruc-
tions in a gene that tell the cell how to make a specific
protein. The code defines the series of NUCLEOTIDEs in
DNA, read as triplets called CODONs, that specifies the
sequence of AMINO ACIDs in a protein. The set comprises
64 nucleotide triplets (codons) that specify the 20 amino
acids and termination codons (UAA, UAG, UGA).
The code is made up of adenine (A), thymine (T),
guanine (G), and cytosine (C), the nucleotide bases of
DNA. Each gene’s code combines them in various ways
to spell out three-letter triplets (codons) that specify
which amino acid is needed at each step in making a
protein.
genome The complete assemblage of chromosomes
and extrachromosomal genes of a cell, organelle,
organism, or virus; the complete DNA portion of an
organism. The complete set of genes shared by mem-
bers of any reproductive body such as a population or
species.
geological time The span of time that has passed
since the creation of the Earth and its components; a
scale use to measure geological events millions of years
ago. Measured in chronostratic or relative terms, where
subdivisions of the Earth’s geology are set in an order
based on relative age relationships derived from fossil
composition and stratigraphic position, or in chrono-
metric or absolute time, where the use of radiometric
dating techniques gives numerical ages.
geometrical isomers (position isomers) Isomers dif-
fering in the way the atoms are oriented in space rela-
tive to each other.
geothermal energy Natural heat from within the
Earth.
geotropism A plant’s response to gravitational
effects. A plant’s roots grow downward toward the
gravitational pull, which is called positive geotropism,
while shoots grow upward against gravitational pull
(negative geotropism). Also called gravitropism.g-factor SeeELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.Gibbs energy diagram A diagram showing the rela-
tive standard Gibbs energies of reactants, TRANSITION
STATEs, reaction INTERMEDIATEs, and products in the
same sequence as they occur in a CHEMICAL REACTION.
These points are often connected by a smooth curve (a
“Gibbs energy profile,” still commonly referred to as a
“free energy profile”), but experimental observation
can provide information on relative standard Gibbs
energies only at the maxima and minima and not at the
configurations between them. The abscissa expresses
the sequence of reactants, products, reaction intermedi-
ates, and transition states and is usually undefined or
only vaguely defined by the REACTION COORDINATE
(extent of bond breaking or bond making). In some
adaptations, however, the abscissas are explicitly
defined as BOND ORDERs, Bronsted exponents, etc.
Contrary to statements in many textbooks, the highest
point on a Gibbs energy diagram does not necessarily
correspond to the transition state of the RATE-LIMITING
STEP. For example, in a STEPWISE REACTIONconsisting
of two reaction steps- A + B C
- C + D →E
one of the transition states of the two reaction steps
must (in general) have a higher standard Gibbs energy
than the other, whatever the concentration of D in the
Gibbs energy diagram 117