dendritic cell   The most potent type of antigen-presenting cell, which takes up antigen and processes it for presentation to T cells. It is required for activating naïve T cells.  (Figure 24–11) deoxyribonucleic acid (DNA) Polynucleotide formed from covalently linked deoxyribonucleotide units. The store of hereditary information within a cell and the carrier of this information from generation to generation. (Figure 4–3 and Panel 2–6, pp. 100–101) depolarization   Deviation in the electric potential across the plasma membrane towards a positive value. A depolarized cell has a potential that is positive outside and negative inside.
desmosome Anchoring cell–cell junction, usually formed between two epithelial cells. Characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert. (Figure 19–2) detergent   Small amphiphilic molecule, more soluble in water than lipids, that disrupts hydrophobic associations and destroys the lipid bilayer thereby solubilizing membrane proteins.
D gene segment A short DNA sequence that encodes a part of the variable region of an immunoglobulin heavy chain or the β chain of a T cell receptor (TCR).
diacylglycerol (DAG) Lipid produced by the cleavage of inositol phospholipids in response to extracellular signals. Composed of two fatty acid chains linked to glycerol, it serves as a small signaling molecule to help activate protein kinase C (PKC). (Figure 15–28) dideoxy sequencing   The standard enzymatic method of DNA sequencing. (Panel 8–1, p. 478) differential-interference-contrast microscope   Type of light microscope that exploits the interference effects that occur when light passes through parts of a cell of different refractive indices. Used to view unstained living cells.
differentiation   Process by which a cell undergoes a change to an overtly specialized cell type.
diffusion   The net drift of molecules through space due to random thermal movements.
Dishevelled   Scaffold protein recruited to the Frizzled family of cell-surface receptors upon their activation by Wnt binding that helps relay the signal to other signaling molecules.
DNA cloning (1) The act of making many identical copies (typically billions) of a DNA molecule—the amplification of a particular DNA sequence. (2) Also, the isolation of a particular stretch of DNA (often a particular gene) from the rest of the cell’s genome.
DNA helicase Enzyme that is involved in opening the DNA helix into its single strands for DNA replication.
DNA library Collection of cloned DNA molecules, representing either an entire genome (genomic library) or complementary DNA copies of the mRNA produced by a cell (cDNA library).
DNA ligase Enzyme that joins the ends of two strands of DNA together with a covalent bond to make a continuous DNA strand.
DNA methylation   Addition of methyl groups to DNA.
Extensive methylation of the cytosine base in CG sequences is used in plants and animals to help keep genes in an inactive state.
DNA microarray A large array of short DNA molecules (each of known sequence) bound to a glass microscope slide or other suitable support. Used to monitor expression of thousands of genes simultaneously: mRNA isolated from test cells is converted to cDNA, which in turn is hybridized to the microarray. (Figure 8–64)
DNA polymerase Enzyme that synthesizes DNA by joining nucleotides together using a DNA template as a guide.
DNA primase Enzyme that synthesizes a short strand of RNA on a DNA template, producing a primer for DNA synthesis. (Figure 5–10)
DNA repair   A set of processes for repairing the many accidental lesions that occur continually in DNA.
DNA replication   Process by which a copy of a DNA molecule is made.