Biology Reference
In-Depth Information
Molecular interaction:
A molecular interaction involves physical
contact between molecules that can produce a more or less stable
bond. The bond may allow complexes of several molecules (macro-
molecular complexes) to be constructed which are incorporated into
the structure of the cells (membranes, chromatin etc.). It can also
trigger the biochemical reactions (enzyme/substrate bond) of
metabolism or induce regulation (a bond between a gene transcrip-
tion regulator protein and a DNA sequence). Competition exists
between molecules. Let us imagine red, black and white balls mov-
ing randomly in a space. Contacts between balls of different colours
occur randomly but their frequency relative to the whole set of balls
depends on their relative proportions. For example, the more red
balls there are, the more contacts there will be involving red balls.
Owing to the random character of the movement of the balls the
frequency of these contacts is permanently subjected to random
fluctuations. The same is true of molecules subjected to thermal
agitation.
Morphogen molecules
: Morphogens are chemical substances induc-
ing cell differentiation in the embryo. They often form gradients
and exert their effects at defined concentrations.
Morphogenetic gradients
: See
Morphogen molecules
.
Morphogens
: See
Morphogen molecules
.
Multicellular organisms
: Organisms having several cells (see
Cell
differentiation
).
Natural selection
: Evolution was acknowledged by many naturalists
from the 18th century onwards. Darwin gave an explanation that
has provided the general framework for modern theories of evolu-
tion. Organisms are subject to countless variations. Individuals
with advantageous variations (access to food, success in sexual
reproduction etc.) reproduce more than others and after several
generations become the dominant type. In this way populations of
