| 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. |
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| B memory cells |
| Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if the antigen is encountered again. |
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| bacterial artificial chromosome (BAC) |
| Large segments of DNA, 100,000 to 200,000 bases, from another species cloned into bacteria. Once the foreign DNA has been cloned into the host bacteria, many copies of it can be made.
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| bacteriophages |
| bacteriophagesViruses that attack and kill bacterial cells; composed only of DNA and protein. |
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| 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. |
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| 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. |
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| 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. |
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| base |
| A substance that lowers the hydrogen ion concentration in a solution. |
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| base pair |
| Two bases which form a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
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| basidia |
| Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spores form (sing.: basidium). |
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| basidiomycetes |
| The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospores are produced. Includes mushrooms and toadstools. |
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| basidiospores |
| The spores formed on the basidia of club fungi (Basidiomycetes). |
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| benthic zone |
| One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling organisms. |
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| bicarbonate ions |
| A weak base present in saliva that helps to neutralize acids in food. |
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| 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. |
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| bilateral symmetry |
| In animals, refers to those that have a single axis of symmetry. |
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| biliary system |
| The bile-producing system consisting of the liver, gallbladder, and associated ducts. |
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| binary fission |
| The method by which bacteria reproduce. The circular DNA molecule is replicated; then the cell splits into two identical cells, each containing an exact copy of the original cell's DNA. |
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| binding sites |
| Areas on the ribosome within which tRNA-amino acid complexes fit during protein synthesis. |
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| binomial system of nomenclature |
| A system of taxonomy developed by Linnaeus in the early eighteenth century. Each species of plant and animal receives a two-term name; the first term is the genus, and the second is the species. |
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| biochemical cycle |
| The flow of an element through the living tissue and physical environment of an ecosystem; e. g., the carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus cycles. |
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| biochemical reactions |
| Specific chemical processes that occur in living things. |
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| biochemistry |
| Chemical processes associated with living things. |
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| biodiversity |
| Biological diversity; can be measured in terms of genetic, species, or ecosystem diversity. |
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| biogeography |
| The study of the distribution of plants and animals across the Earth. |
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| bioluminescent |
| Refers to organisms that emit light under certain conditions. |
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| biomass |
| The total weight of living tissue in a community. |
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| biome |
| A large-scale grouping that includes many communities of a similar nature. |
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| biosphere |
| All ecosystems on Earth as well as the Earth's crust, waters, and atmosphere on and in which organisms exist; also, the sum of all living matter on Earth. |
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| birds |
| Taxonomic class of terrestrial vertebrates that are characterized by endothermy and feathers; descended from some group of reptiles (or possibly dinosaurs). |
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| birth defect |
| Any defect present at birth, whether caused by mutant genes or by prenatal events that are not genetic.
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| birth rate |
| The ratio between births and individuals in a specified population at a particular time. |
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| bladder |
| A hollow, distensible organ with muscular walls that stores urine and expels it through the urethra. |
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| blastocoel |
| The fluid-filled cavity at the center of a blastula. |
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| blastocyst |
| The developmental stage of the fertilized ovum by the time it is ready to implant; formed from the morula and consists of an inner cell mass, an internal cavity, and an outer layer of cells (the trophoblast). |
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| blastula |
| A ball of cells surrounding a fluid-filled cavity (the blastocoel) that is produced by the repeated cleavage of a zygote. |
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| blending |
| Term applied to 19th century belief that parental traits "blended" in their offspring; disproven by Mendel's work. |
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| blood group or type |
| One of the classes into which blood can be separated on the basis of the presence or absence of certain antigens; notably, the ABO types and the Rh blood group. |
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| body fossil |
| The actual remains (however permineralized, compressed or otherwise post-mortem altered) of an organism; includes bones, shells, and teeth. |
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| bone marrow transplantation |
| A medical procedure to replenish the soft tissue within bones that produces new blood cells. Bone marrow transplants are necessary when marrow has been destroyed by drug or radiation therapy for cancer, often leukemia. A bone marrow donor is usually a close relative of the patient.
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| bony fish |
| A term applied collectively to all groups of fish with bony (as opposed to cartilaginous) skeletons. |
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| bottlenecks |
| Drastic short-term reductions in population size caused by natural disasters, disease, or predators; can lead to random changes in the population's gene pool. |
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| brachiopods |
| A phylum of hinge-shelled animals that have left an excellent fossil record; brachiopods live on or in the ocean floor. |
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| brachydactly |
| Human genetic disorder that causes production of an extra digit; an autosomal dominant trait. Sometimes referred to as polydactly. |
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| brain |
| The most anterior, most highly developed portion of the central nervous system. |
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| brain stem |
| The portion of the brain that is continuous with the spinal cord and consists of the medulla oblongata and pons of the hindbrain and the midbrain. |
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| BRCA1/BRCA2 |
| The first breast cancer genes to be identified. Mutated forms of these genes are believed to be responsible for about half the cases of inherited breast cancer, especially those that occur in younger women. Both are tumor suppressor genes.
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| bronchi |
| Tubes that carry air from the trachea to the lungs (sing.: bronchus). |
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| bronchioles |
| Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli. |
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| bronchitis |
| A respiratory disorder characterized by excess mucus production and swelling of the bronchioles; caused by long-term exposure to irritants such as cigarette smoke and air pollutants. |
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| brown algae |
| Multicellular protistans placed in the Division Phaeophyta, includes kelp. |
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| brush border |
| The collection of microvilli forming a border on the intestinal side of the epithelial cells of the small intestine. |
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| bryophytes |
| The nonvascular plants, characterized by life cycles dominated by the gametophyte phase. This group includes the mosses, liverworts, and hornworts, which lack lignified conducting tissues. |
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| bud sports |
| Buds that produce fruit that is different from the rest of the fruit on the tree; vegetatively propagated by grafting cuttings onto another plant. |
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| budding |
| 1. Asexual production of new organisms; usually found in yeast; 2. the process by which HIV and similar viruses leave the cell (other than by lysing). |
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| buffers |
| Chemicals that maintain pH values within narrow limits by absorbing or releasing hydrogen ions. |
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| bulbourethral glands |
| Glands that secrete a mucus-like substance that is added to sperm and provides lubrication during intercourse. |
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