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concerning the electronic motions which dynam- meiosis The reductive cell division which
ically interconvert successive species along the results in daughter cells containing one copy of
reaction path (as represented by curved arrows, each of the chromosomes of the parent. The
for example) are often included in the description entire meiotic process involves two separate
of a mechanism. divisions(meiosisIandmeiosisII).Thefirstdivi-
It should be noted that for many reactions sion is a true reductive division with the chromo-
all this information is not available and the sug- some number being halved, whereas the second
gested mechanism is based on incomplete exper- division resembles mitosis in many ways. Thus,
imental data. It is not appropriate to use the term a diploid parental cell will give rise to haploid
mechanism to describe a statement of the proba- daughter cells (cf. ploidy).
ble sequence in a set of stepwise reactions. That
memory A memory is a storage device of
should be referred to as a reaction sequence, and
fixed half-life for data and programs that per-
not a mechanism.
mit any combination of reading from and writing
to it.
mechanistic equation A balanced elemen-
tary reaction equation, bimolecular or less on
mesh width The mesh width of a triangula-
one side, which describes the ligand binding,
tion ; is the maximal diameter of its cells.
h
organic chemical, and conformational rearrange-
ment reactions which occur on an unambigu- mesomeric effect The effect (on reaction
ously identified entity, whose net effect in the rates, ionization, equilibria, etc.) attributed to a
biochemical reaction is catalytic, to produce an substituent due to overlap of its p-or π-orbitals
overall biochemical reaction. Equally, a set of with the p-or π-orbits of the rest of the molecu-
such reaction equations which can be combined lar entity. Delocalization is thereby introduced
by summation or logical operators to produce the or extended, and electronic charge may flow to
overall biochemical reaction. or from the substituent. The effect is symbolized
Comment: The requirement that the equation by M.
be elementary ensures that its order will be the Strictly understood, the mesomeric effect
sum of the molecularities of the reactants. All operates in the ground electronic state of the
types of interactions are included. Since the aim molecule. When the molecule undergoes elec-
of the equations is to describe interactions at the tronic excitation or its energy is increased on the
catalytic species, it follows that that molecule waytothe transition state of a chemical reac-
will not appear in these equations as a catalyst. tion, the mesomeric effect may be enhanced by
The requirement for clearly identified species the electromeric effect, but this term is not much
distinguishes among different catalysts carrying used, and the mesomeric and electronic effects
out the “same” reactions at different rates. The tend to be subsumed in the term resonance effect
definition provides that they can be combined by of a substituent.
summation or logical operators to represent alter-
mesomorphic transition A transition that
native paths through the space of possible reac-
occurs between a fully ordered crystalline solid
tions, depending on the consequences of prior
and an isotropic liquid. Mesomorphic transitions
reactions in the set.
can occur
mechanistic motif A repeated pattern over (i.) from a crystal to a liquid crystal,
mechanistic equations. (ii.) from a liquid crystal to another liquid
Comment: See also biochemical, chemical, crystal, and
dynamical, functional, kinetic, phylogenetic,
(iii.) from a liquid crystal to an isotropic liq-
regulatory, thermodynamic, and topological
uid.
motives.
mesopause (in atmospheric chemistry) That
medium The phase (and composition of the region of the atmosphere between the meso-
phase) in which chemical species and their reac- sphere and the thermosphere at which the tem-
tions are studied in a particular investigation. perature is a minimum.

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