Aperture
Of a camera, the adjustable opening through which light passes to reach the film. The diameter of the aperture determines the intensity of light admitted. The pupil of a human eye is a self-adjusting aperture.

Apoptosis
Programmed cell death, the body's normal method of disposing of damaged, unwanted, or unneeded cells. See also: cell

A site
Holds the incoming tRNA molecule charged with the next amino acid; the tRNA are held so that the anticodons form base pairs with adjacent complementary codons of the mRNA ll10ving through the ribosome.

Asteroid
A small rocky or metallic body orbitting the Sun. About 20,000 have been observed, ranging in size from several hundred kilometers across down 10 dust particles.

Archeology
The study of human history and prehistory through the excavation of sites and the analysis of physical remains, such as graves, tools, pottery, and other artifacts.

Archetype
The original form or body plan from which a group of organisms develops.

Arrayed library
Individual primary recombinant clones (hosted in phage, cosmid, YAC, or other vector) that are placed in two-­dimensional arrays in micro titer dishes. Each primary clone can be identified by the identity of the plate and the clone location (row and column) on that plate

Asexual Reproduction
A type of reproduction involving only one parent that ususally produces genetically identical offspring. Asexual reptoduction occurs without fertilization or genetic recombination, and may occur by budding, by division of a single cell, or by the breakup of a whole organism into two or more new individuals.

Association
The occurrence together in a population of two or more phenotypic characteristics more often than expected by chance.