On-Line Biology Book: GLOSSARY

B

bacteriophages  Viruses that attack and kill bacterial cells; composed only of DNA and protein. PICTURE

bark  The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an inner layer of phloem. PICTURE

Barr body Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated prior to cell division and thus is passed on to all descendant cells of the embryonic cell that had one of its X-chromosomes inactivated. PICTURE 1 PICTURE 2

barriers to gene flow Factors, such as geographic, mechanical, and behavioral isolating mechanisms that restrict gene flow between populations, leading to populations with differing allele frequencies.

basal body   A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.

base  A substance that lowers the hydrogen ion concentration in a solution.

basidia  Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spores form (sing.: basidium).

basidiomycetes The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospores are produced. Includes mushrooms and toadstools. PICTURE

basidiospores  The spores formed on the basidia of club fungi (Basidiomycetes).

B cells  Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circulate in the circulatory and lymph systems where they transform into antibody-producing plasma cells when exposed to antigens.

benthic zone  One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling organisms.

beta decay Type of radioactive decay in which a radioisotope emits a small, negatively-charged and fast-moving particle from its nucleus. The beta particle is similar in size, charge, and speed to an electron and is formed when a neutron in the radioisotope's nucleus converts to a proton.

bicarbonate ions   A weak base present in saliva that helps to neutralize acids in food.

big bang theory   A model for the evolution of the universe that holds that all matter and energy in the universe were concentrated in one point, which suddenly exploded. Subsequently, matter condensed to form atoms, elements, and eventually galaxies and stars.

bilateral symmetry   In animals, refers to those that have a single axis of symmetry.

biliary system   The bile-producing system consisting of the liver, gallbladder, and associated ducts.