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

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

DAG See diacylglycerol.

dalton Unit of molecular mass approximately equal to the mass of a hydrogen atom (1.66 × 10⁻²⁴ g).

denaturation Drastic alteration in the conformation of a protein or nucleic acid due to disruption of various noncovalent interactions caused by heating or exposure to certain chemicals; usually results in loss of biological function.

dendrite Process extending from the cell body of a neuron that is relatively short and typically branched and receives signals from axons of other neurons. (Figure 22-1)

dendritic cells Phagocytic professional antigen-presenting cells that reside in various tissues and can detect broad patterns of pathogen markers via their Toll-like receptors. After internalizing antigen at a site of tissue injury or infection, they migrate to lymph nodes and initiate activation of T cells. (Figure 23-7)

deoxyribonucleic acid See DNA.

depolarization Decrease in the cytosolic-face negative electric potential that normally exists across the plasma membrane of a cell at rest, resulting in a less inside-negative or an inside-positive membrane potential.

destruction box Recognition motif in APC/C substrates.

determinant In the context of antibody recognition of an antigen, a region on a protein to which the antibody binds. In this context, it is synonymous with epitope.

deuterostomes A group of bilaterally symmetric animals whose anus develops close to the blastopore and has a dorsal nerve cord. This group includes all chordates (fish, amphibians, reptiles, birds, and mammals) and echinoderms (sea stars, sea urchins).

diacylglycerol (DAG) Membrane-bound second messenger that can be produced by cleavage of phosphoinositides in response to stimulation of certain cell-surface receptors. (Figures 15-6 and 15-33)

diploid Referring to an organism or cell having two full sets of homologous chromosomes and hence two copies (alleles) of each gene or genetic locus. Somatic cells contain the diploid number of chromosomes (2n) characteristic of a species. See also haploid.

dipole A positive charge separated in space from an equal but opposite negative charge.

dipole moment A quantitative measure of the extent of charge separation, or strength, of a dipole, which for a chemical bond is the product of the partial charge on each atom and the distance between the two atoms.

G-8

disaccharide A small carbohydrate (sugar) composed of two monosaccharides covalently joined by a glycosidic bond. (Figure 2-19)

dissociation constant (K_d) See equilibrium constant.

disulfide bond (—S—S—) A common covalent linkage between the sulfur atoms on two cysteine residues in different polypeptides or in different parts of the same polypeptide.

diversity The entire set of antigen-specific receptors encoded by an immune system.

DNA (deoxyribonucleic acid) Long linear polymer, composed of four kinds of deoxyribose nucleotides, that is the carrier of genetic information. See also double helix, DNA. (Figure 5-3)

DNA-binding domain The domain of a transcription factor that binds specific, closely related DNA sequences.

DNA cloning Recombinant DNA technique in which specific cDNAs or fragments of genomic DNA are inserted into a cloning vector, which then is incorporated into cultured host cells and maintained during growth of the host cells; also called gene cloning. (Figure 6-14)

DNA damage response system Pathway that senses DNA damage and induces cell cycle arrest and DNA repair pathways.

DNA library Collection of cloned DNA molecules consisting of fragments of the entire genome (genomic library) or of DNA copies of all the mRNAs produced by a cell type (cDNA library) inserted into a suitable cloning vector.

DNA ligase An enzyme that links together the 3' end of one DNA fragment with the 5' end of another, forming a continuous strand.

DNA microarray An ordered set of thousands of different nucleotide sequences arrayed on a microscope slide or other solid surface; can be used to determine patterns of gene expression in different cell types or in a particular cell type at different developmental stages or under different conditions. (Figures 5-29 and 5-30)

DNA polymerase An enzyme that copies one strand of DNA (the template strand) to make the complementary strand, forming a new double-stranded DNA molecule. All DNA polymerases add deoxyribonucleotides one at a time in the 5' → 3' direction to the 3' end of a short preexisting primer strand of DNA or RNA.

DNA recombination The process by which two DNA molecules with similar sequences are subject to double-stranded breaks and then rejoined to generate two recombinant DNA molecules with sequences comprised of portions of each parent. (Figures 5-41 and 5-42)

domain A region of protein that has a distinct, and often independent, function or structure, or that has a distinct topology relative to the rest of the protein.

dominant In genetics, referring to that allele of a gene expressed in the phenotype of a heterozygote; the nonexpressed allele is recessive; also refers to the phenotype associated with a dominant allele. Mutations that produce dominant alleles generally result in a gain of function. (Figure 6-2)

dominant-negative In genetics, an allele that acts in a dominant manner but produces an effect similar to a loss of function; generally is an allele encoding a mutant protein that blocks the function of the normal protein by binding either to it or to a protein upstream or downstream of it in a pathway.

double helix, DNA The most common three-dimensional structure for cellular DNA in which the two polynucleotide strands are antiparallel and wound around each other with complementary bases hydrogen-bonded. (Figure 5-3)

double-strand break Form of DNA damage where both phosphate-sugar backbones of the DNA are severed.

downstream (1) For a gene, the direction RNA polymerase moves during transcription, which is toward the end of the template DNA strand with a 5'-hydroxyl group. Nucleotides downstream from the +1 position (the first transcribed a nucleotide) are designated +2, +3, etc. (2) Events that occur later in a cascade of steps (e.g., signaling pathway). See also upstream.

dyneins A class of motor proteins that use the energy released by ATP hydrolysis to move toward the (−) end of microtubules. Dyneins can transport vesicles and organelles, are responsible for the movement of cilia and flagella, and play a role in chromosome movement during mitosis. (Figures 18-24 and 18-25)