Chargaff’s rule The principle that
in any sample of DNA the amount of
adenine equals the amount of
thymine and the amount of guanine
equals the amount of cytosine. It is a
consequence of *base pairing. The
rule was published in 1950 by the
Austrian–American biochemist Erwin
Chargaff (1905–2002).
chargeA property of some *ele-
mentary particles that gives rise to
an interaction between them and
consequently to the host of material
phenomena described as electrical.
Charge occurs in nature in two
forms, conventionally described as
positiveand negativein order to dis-
tinguish between the two kinds of in-
teraction between particles. Two
particles that have similar charges
(both negative or both positive) inter-
act by repelling each other; two par-
ticles that have dissimilar charges
(one positive, one negative) interact
by attracting each other.
The natural unit of negative charge
is the charge on an *electron, which
is equal but opposite in effect to the
positive charge on the proton. Large-
scale matter that consists of equal
numbers of electrons and protons is
electrically neutral. If there is an ex-
cess of electrons the body is nega-
tively charged; an excess of protons
results in a positive charge. AÛow of
charged particles, especially aÛow
of electrons, constitutes an electric
current. Charge is measured in
coulombs, the charge on an electron
being 1.602 × 10 –19coulombs.charge carrierThe entity that
transports electric charge in an elec-
tric current. The nature of the carrier
depends on the type of conductor: in
metals, the charge carriers are elec-
trons; in *semiconductors the carri-
ers are electrons (n-type) or positive
*holes (p-type); in gases the carriers
are positive ions and electrons; inelectrolytes they are positive and
negative ions.charge density1.The electric
charge per unit volume of a medium
or body (volume charge density).
2.The electric charge per unit sur-
face area of a body (surface charge
density).
charge-transfer complexA
chemical compound in which there
is weak coordination involving the
transfer of charge between two mol-
ecules. An example is phenoquinone,
in which the phenol and quinone
molecules are not held together by
formal chemical bonds but are asso-
ciated by transfer of charge between
the compounds’ aromatic ring sys-
tems.Charles, Jacques Alexandre
César(1746–1823) French chemist
and physicist, who became professor
of physics at the Paris Conservatoire
des Arts et Métiers. He is best re-
membered for discovering *Charles’
law (1787), relating to the volume
and temperature of a gas. In 1783 he
became theÜrst person to make an
ascent in a hydrogen balloon.Charles’ lawThe volume of aÜxed
mass of gas at constant pressure ex-
pands by a constant fraction of its
volume at 0°C for each Celsius de-
gree or kelvin its temperature is
raised. For any *ideal gas the fraction
is approximately 1/273. This can be
expressed by the equation V = V 0 (1 +
t/273), where V 0 is the volume at 0°C
and V is its volume at t°C. This is
equivalent to the statement that the
volume of aÜxed mass of gas at con-
stant pressure is proportional to its
thermodynamic temperature, V = kT,
where k is a constant. The law re-
sulted from experiments begun
around 1787 by Jacques *Charles but
was properly established only by the
more accurate results published inChargaff’s rule 114c