Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

bacteria Class of prokaryotes that constitutes one of the three distinct evolutionary lineages of modern-day organisms; also called eubacteria. Phylogenetically distinct from archaea and eukaryotes. (Figure 1-1)

bacteriophage (phage) Any virus that infects bacterial cells. Some phages are widely used as vectors in DNA cloning.

basal See basolateral.

basal body Structure at the base of a cilium or flagellum from which microtubules forming the axoneme assemble; structurally similar to a centriole. (Figure 18-31)

basal lamina (pl. basal laminae) A thin sheet-like network of extracellular-matrix components that underlies most animal epithelia and other organized groups of cells (e.g., muscle), separating them from connective tissue or other cells. (Figures 20-21 and 20-22)

base Any compound, often containing nitrogen, that can accept a proton (H⁺) from an acid. Also, commonly used to denote the purines and pyrimidines in DNA and RNA.

base pair Association of two complementary nucleotides in a DNA or RNA molecule stabilized by hydrogen bonding between their base components. Adenine pairs with thymine or uracil (A · T, A · U) and guanine pairs with cytosine (G · C). (Figure 5-3b)

basic helix-loop-helix See helix-loop-helix, basic.

basolateral Referring to the base (basal) and side (lateral) of a polarized cell, organ, or other body structure. In the case of epithelial cells, the basolateral surface abuts adjacent cells and the underlying basal lamina. (Figure 20-10)

B cell A lymphocyte that matures in the bone marrow and expresses antigen-specific receptors (membrane-bound immunoglobulin). After interacting with antigen, a B cell proliferates and differentiates into antibody-secreting plasma cells.

G-4

B-cell receptor Complex composed of an antigen-specific membrane-bound immunoglobulin molecule and associated signal-transducing Igα and Igβ chains. (Figure 23-18)

benign Referring to a tumor containing cells that closely resemble normal cells. Benign tumors stay in the tissue where they originate but can be harmful due to continued growth. See also malignant.

beta (β)-adrenergic receptors Seven spanning G protein–coupled receptors that bind adrenaline and related molecules, leading to activation of adenylyl cyclase.

beta (β) sheet A flat secondary structure in proteins that is created by hydrogen bonding between the backbone atoms in two different polypeptide chains or segments of a single folded chain. (Figure 3-5)

beta (β) turn A short U-shaped secondary structure in proteins. (Figure 3-6)

bi-oriented Indicates that the kinetochores of sister chromatids have attached to microtubules emanating from opposite spindle poles.

blastocyst Stage of mammalian embryo composed of ≈64 cells that have separated into two cell types—trophectoderm, which will form extra-embryonic tissues, and the inner cell mass, which gives rise to the embryo proper; stage that implants in the uterine wall and corresponds to the blastula of other animal embryos. (Figure 21-3)

blastopore The first opening that forms during embryogenesis of bilaterally symmetric animals, which later becomes the gut. This opening may become either the mouth or the anus.

bromodomain A protein domain of ∼120 amino acids that binds acetylated lysine; found in chromosome-associated proteins involved in transcriptional activation.

buffer A solution of the acid (HA) and base (A⁻) form of a compound that undergoes little change in pH when small quantities of strong acid or base are added at pH values near the compound’s pK_a.