Science / Biology Glossary

Archean - Proterozoic Era: The period of time beginning 4.6 billion years ago with the formation of the Earth and ending 570 million years ago.

Aridity: The condition of receiving sparse rainfall; associated with cooler climates because cool air can hold less water vapor than warm air. Many deserts occur in relatively warm climates, however, . . . View Full Definition

Arrector Pili: A muscle running from a hair follicle to the dermis. Contraction of the muscle causes the hair to rise perpendicular to the skin surface, forming goose pimples.

Arteries: Thick-walled vessels that carry blood away from the heart. Singular=artery.

Arterioles: The smallest arteries; usually branch into a capillary bed.

Artificial Selection: The process in which breeders choose the variants to be used to produce succeeding generations.

Ascomycetes: Division of fungi that contains the yeasts and morels; ascomycetes produce an ascus (or sac) in which ascospores are produced.

Ascus: Structure produced by sac fungi in which sexual ascospores develop.

Asexual Reproduction: A method of reproduction in which genetically identical offspring are produced from a single parent; occurs by many mechanisms, including fission, budding, and fragmentation.

Assortment: A way in which meiosis produces new combinations of genetic information. Paternal and maternal chromosomes line up randomly during synapsis, so each daughter cell is likely to receive an ass . . . View Full Definition

Aster: Short fibers produced by cells during mitosis and meiosis. These radiate from the centriole (if it is present).

Asteroid Impacts: Hypothesis that links certain mass extinction events with the impact of a comet or asteroid, most notably the mass extinction 65 million years that caused the disappearance of dinosaurs and . . . View Full Definition

Asthma: A respiratory disorder caused by allergies that constrict the bronchioles by inducing spasms in the muscles surrounding the lungs, by causing the bronchioles to swell, or by clogging the bro . . . View Full Definition

Asymmetrical: In animals, a term referring to organisms that lack a general body plan or axis of symmetry that divides the body into mirror-image halves.

Atmosphere: The envelope of gases that surrounds the Earth; consists largely of nitrogen (78%) and oxygen (21%).

Atom: The smallest indivisible particle of matter that can have an independent existence.

Atomic Number: The number of protons in the nucleus of an atom.

Atomic Weight: The sum of the weights of an atoms protons and neutrons, the atomic weight differs between isotopes of the same element.

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