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

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

NAD⁺ (nicotinamide adenine dinucleotide) A small organic molecule that functions as an electron carrier by accepting two electrons from a donor molecule and one H⁺ from the solution. (Figure 2-33a)

NADP⁺ (nicotinamide adenine dinucleotide phosphate) Phosphorylated form of NAD⁺ that is used extensively as an electron carrier in biosynthetic pathways and during photosynthesis.

Na⁺/K⁺ ATPase A P-class ATP-powered pump that couples hydrolysis of one ATP molecule to export of Na⁺ ions and import of K⁺ ions; is largely responsible for maintaining the normal intracellular concentrations of Na⁺ (low) and K⁺ (high) in animal cells; commonly called Na⁺/K⁺ pump. (Figure 11-13)

natural killer (NK) cells Components of the innate immune system that nonspecifically detect and kill virus-infected cells and tumor cells. (Figure 23-6)

necrosis Cell death resulting from tissue damage or other pathology; usually marked by swelling and bursting of cells with release of their contents. Contrast with apoptosis.

necroptosis A type of necrosis triggered by extracellular hormones that causes inflammation.

neoblasts Stem cells in planaria that can proliferate and regenerate many or all body cells.

negative feedback mechanism Process where the output of a pathway inhibits its own production.

neurofilaments (NFs) A group of intermediate filament proteins, found only in neurons, that contribute to axonal structure and rate of transmission of action potentials down axons. (Figure 18-2b)

neuron (nerve cell) Any of the impulse-conducting cells of the nervous system. A typical neuron contains a cell body; multiple short, branched processes (dendrites); and one long process (axon). (Figure 22-1)

neurotransmitter Extracellular signaling molecule that is released by the presynaptic neuron at a chemical synapse and relays the signal to the postsynaptic cell. The response elicited by a neurotransmitter, either excitatory or inhibitory, is determined by its receptor on the postsynaptic cell. (Figures 22-25 and 22-26)

neurotrophins Family of structurally and functionally related trophic factors that bind to receptors called Trks and are required for survival of neurons; include nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF).

neutrophils Phagocytic leukocytes that are attracted to sites of tissue damage and migrate into the tissue. Once activated, neutrophils secrete various chemokines, cytokines, bacteria-destroying enzymes (e.g., lysozyme), and other products that contribute to inflammation and help clear invading pathogens.

NF-κB A transcription factor that is sequestered in the cytosol until its inhibitory protein is degraded.

nicotinamide adenine dinucleotide See NAD⁺.

nicotinamide adenine dinucleotide phosphate See NADP⁺.

G-18

N-linked oligosaccharide A branched oligosaccharide chain attached to the side-chain amino group of an asparagine residue in a glycoprotein. See also O-linked oligosaccharide.

nociceptor Mechanosensor that responds to pain associated with injury to body tissues caused by mechanical trauma, heat, electricity, or toxic chemicals.

noncovalent interaction Any relatively weak chemical interaction that does not involve an intimate sharing of electrons. (Figures 2-6 and 2-12)

nondisjunction Mis-segregation of chromosomes during mitosis, resulting in aneuploidy or trisomy.

nonhomologous end joining (NHEJ) A pathway that repairs double-strand breaks in DNA in which the break ends are directly ligated without the need for a homologous template.

nonpolar Referring to a molecule or structure that lacks any net electric charge or asymmetric distribution of positive and negative charges. Nonpolar molecules generally are less soluble in water than polar molecules and are often water insoluble.

nonsense-mediated decay (NMD) The surveillance mechanism by which cells identify and destroy improperly processed or mutant mRNAs with a stop codon that occurs before the last exon-exon junction.

Northern blotting Technique for detecting specific RNAs separated by electrophoresis by hybridization to a labeled DNA probe. See also Southern blotting.

nuclear body Roughly spherical, functionally specialized region in the nucleus, containing specific proteins and RNAs; many function in the assembly of ribonucleoprotein (RNP) complexes. The most prominent type is the nucleolus.

nuclear envelope Double-membrane structure surrounding the nucleus; the outer membrane is continuous with the endoplasmic reticulum and the two membranes are perforated by nuclear pore complexes. (Figure 1-12a)

nuclear lamina Fibrous network on the inner surface of the nuclear envelope composed of lamin intermediate filaments. (Figure 19-19)

nuclear pore complex (NPC) Large, multiprotein structure, composed largely of nucleoporins, that extends across the nuclear envelope. Ions and small molecules freely diffuse through NPCs; large proteins and ribonucleoprotein particles are selectively transported through NPCs with the aid of soluble proteins. (Figure 13-33a and b)

nuclear receptor Member of a class of intracellular receptors that bind lipid-soluble molecules (e.g., steroid hormones), forming ligand-receptor complexes that activate transcription; also called steroid receptor superfamily. (Figure 9-45d)

nucleic acid A polymer of nucleotides linked by phosphodiester bonds. DNA and RNA are the primary nucleic acids in cells.

nucleic acid hybridization See hybridization, nucleic acid.

nucleocapsid A viral capsid plus the enclosed nucleic acid.

nucleolus Large structure in the nucleus of eukaryotic cells where rRNA synthesis and processing occurs and ribosome subunits are assembled. (Figure 8-28a)

nucleoporins Large group of proteins that make up the nuclear pore complex. One class (FG-nucleoporins) participates in nuclear import and export.

nucleoside A small molecule composed of a purine or pyrimidine base linked to a pentose (either ribose or deoxyribose). (Table 2-3)

nucleosome Structural unit of chromatin consisting of a disk-shaped core of histone proteins around which a 147-bp segment of DNA is wrapped. (Figure 8-24)

nucleotide A nucleoside with one or more phosphate groups linked via an ester bond to the sugar moiety, generally to the 5' carbon atom. DNA and RNA are polymers of nucleotides containing deoxyribose and ribose, respectively. (Figure 2-16 and Table 2-3)

nucleus Large membrane-bounded organelle in eukaryotic cells that contains DNA organized into chromosomes; synthesis and processing of RNA and ribosome assembly occur in the nucleus.