basal lamina (plural basal laminae) Thin mat of extracellular matrix that separates epithelial sheets, and many other types of cells such as muscle or fat cells, from connective tissue. Sometimes called basement membrane. (Figure 19–51) base   (1) A substance that can reduce the number of protons in solution, either by accepting H+ ions directly, or by releasing OH– ions, which then combine with H+ to form H2O.
(2) The purines and pyrimidines in DNA and RNA are organic nitrogenous bases and are often referred to simply as bases. (Panel 2–2, pp. 92–93) base excision repair   DNA repair pathway in which single faulty bases are removed from the DNA helix and replaced. Compare nucleotide excision repair. (Figure 5–41) base pair   Two nucleotides in an RNA or DNA molecule that are held together by hydrogen bonds—for example, G paired with C, and A paired with T or U.
basement membrane Thin mat of extracellular matrix that separates epithelial sheets, and many other types of cells such as muscle or fat cells, from connective tissue. Also called basal lamina. (Figure 19–51)
Bax   A main effector Bcl2 family protein of the intrinsic pathway of apoptosis in mammalian cells; located mainly in the cytosol and translocates to the mitochondria only after activation, usually by activated pro-apoptotic BH3-only proteins.
B cell receptor (BCR) The transmembrane immunoglobulin protein on the surface of a B cell that serves as its receptor for antigen.
Bcl2   Anti-apoptotic Bcl2 family protein of the outer mitochondrial membrane that binds and inhibits pro-apoptotic Bcl2 family proteins and prevents inappropriate activation of the intrinsic pathway of apoptosis.
Bcl2 family   Family of intracellular proteins that either promote or inhibit apoptosis by regulating the release of cytochrome c and other mitochondrial proteins from the intermembrane space into the cytosol.
BclXL   Anti-apoptotic Bcl2 family protein of the outer mitochondrial membrane that binds and inhibits pro-apoptotic Bcl2 family proteins and prevents inappropriate activation of the intrinsic pathway of apoptosis.
benign   Of tumors: self-limiting in growth, and noninvasive.
beta sheet (β sheet) Common structural motif in proteins in which different sections of the polypeptide chain run alongside each other, joined together by hydrogen-bonding between atoms of the polypeptide backbone. Also known as a β pleated sheet. (Figure 3–7) beta-catenin (β-catenin) Multifunctional cytoplasmic protein involved in cadherin-mediated cell–cell adhesion, linking cadherins to the actin cytoskeleton. Can also act independently as a transcription regulatory protein. Has an important role in animal development as part of a Wnt signaling pathway.
BH3-only proteins The largest subclass of Bcl2 family proteins. Produced or activated in response to an apoptotic stimulus and promote apoptosis mainly by inhibiting antiapoptotic Bcl2 family proteins.
bi-orientation   The attachment of sister chromatids to opposite poles of the mitotic spindle, so that they move to opposite ends of the cell when they separate in anaphase.
binding site Region on the surface of one molecule (usually a protein or nucleic acid) that can interact with another molecule through noncovalent bonding.
BiP   Endoplasmic reticulum (ER)-resident chaperone protein member of the family of hsp70-type chaperone proteins.
Bithorax complex One of two gene clusters in Drosophila that contain Hox genes; genes in the Bithorax complex control the differences among the abdominal and thoracic segments of the body.