Autogenesis model
According to this model, eukaryotic cells evolved by the specialization of internal
membranes originally derived from prokaryotic plasma membranes.
Autoimmune disease
An immunological disorder in which the immune system turns against itself.
Autonomic nervous system
A subdivision of the motor nervous system of vertebrates that regulates the internal
environment; consists of the sympathetic and parasympathetic divisions.
Autopolyploid
A type of polyploid species resulting from one species doubling its chromosome number to
become tetraploid, which may self-fertilize or mate with other tetraploids.
Autosomes
A chromosome that is not directly involved in determining sex, as opposed to the sex
chromosomes.
Autotroph
An organism that obtains organic food molecules without eating other organisms.
Autotrophs use energy from the sun or from the oxidation of inorganic substances to make
organic molecules from inorganic ones.
Autumn wood
The season of the year between summer and winter, lasting from the autumnal equinox to
the winter solstice and from September to December in the Northern Hemisphere; fall.
Auxin
A class of plant hormones, including indoleacetic acid (IAA), having a variety of effects, such
as phototropic response through the stimulation of cell elongation, stimulation of secondary
growth, and the development of leaf traces and fruit.
Auxotroph
A nutritional mutant that is unable to synthesize and that cannot grow on media lacking
certain essential molecules normally synthesized by wild-type strains of the same species.
Aves
The vertebrate class of birds, characterized by feathers and other flight adaptations.
Axial
Relating to, characterized by, or forming an axis.
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