complementary DNA(cDNA) A laboratory-pro-
duced DNA section that is created by extracting a sin-
gle-stranded RNA from an organism as a template and
transcribing it back into a double-stranded DNA using
the enzyme reverse transcriptase. However, the cDNA
does not include introns, those portions of the DNA
that were spliced out while still in the cell. Complemen-
tary DNA is used for research purposes and can be
cloned into plasmids for storage.
complex A MOLECULAR ENTITY formed by loose
ASSOCIATION involving two or more component
molecular entities (ionic or uncharged), or the corre-
sponding CHEMICAL SPECIES. The bonding between the
components is normally weaker than in a covalent
BOND.
The term has also been used with a variety of
shades of meaning in different contexts; it is therefore
best avoided when a more explicit alternative is appli-
cable. In inorganic chemistry the term coordination
entityis recommended instead of complex.
See alsoACTIVATED COMPLEX; ADDUCT; CHARGE-
TRANSFER COMPLEX; ELECTRON-DONOR-ACCEPTOR
COMPLEX; ENCOUNTER COMPLEX; INCLUSION COM-
POUND;PI (π) ADDUCT; SIGMA(σ) ADDUCT, TRANSI-
TION STATE.
complex ions Ions composed of two or more ions or
radicals, both of which can exist alone.
composite reaction A CHEMICAL REACTION for
which the expression for the rate of disappearance of a
reactant (or rate of appearance of a product) involves
rate constants of more than a single ELEMENTARY REAC-
TION. Examples are opposing reactions (where rate
constants of two opposed chemical reactions are
involved), parallel reactions (for which the rate of dis-
appearance of any reactant is governed by the rate con-
stants relating to several simultaneous reactions to
form different respective products from a single set of
reactants), and STEPWISE REACTIONs.
composition stoichiometry Descibes the quantita-
tive (mass) relationships among elements in com-
pounds. Stoichiometry is the calculation of the quanti-
ties of reactants and products involved in a chemical
reaction.compost Fertilizer formed by decaying organic matter.compound The combination of two or more differ-
ent elements, held together by chemical bonds. The ele-
ments in a given compound are always combined in the
same proportion by mass (law of definite proportion).compressed gas A gas or mixture of gases having, in
a container, an absolute pressure exceeding 40 psi at
21.1°C (70°F).comproportionation Refers to a mixture of species
in different oxidation states that react and produce a
product that is in a different but more stable intermedi-
ate oxidation state. A type of redox reaction, e.g.,comproportionation 55