NAD (nicotinamide adenine dinucleotide) • A compound found in all living cells, existing in two interconvertible forms: the oxidizing agent NAD⁺ and the reducing agent NADH + H⁺.

NADP (nicotinamide adenine dinucleotide phosphate) • A compound similar to NAD, but possessing another phosphate group; plays similar roles but is used by different enzymes.

Natural killer cells • A nonspecific defensive cell (lymphocyte) that attacks tumor cells and virus infected cells.

Natural selection • The differential contribution of offspring to the next generation by various genetic types belonging to the same population. The mechanism of evolution proposed by Charles Darwin.

Necrosis (nec roh´ sis) [Gk. nekros: death] • Tissue damage resulting from cell death.

Negative control • The situation in which a regulatory macromolecule (generally a repressor) functions to turn off transcription. In the absence of a regulatory macromolecule, the structural genes are turned on.

Nematocyst (ne mat´ o sist) [Gr. nema: thread + kystis: cell] • An elaborate, threadlike structure produced by cells of jellyfish and other cnidarians, used chiefly to paralyze and capture prey.

Nephridium (nef rid´ ee um) [Gr. nephros: kidney] • An organ which is involved in excretion, and often in water balance, involving a tube that opens to the exterior at one end.

Nephron (nef´ ron) [Gr. nephros: kidney] • The functional unit of the kidney, consisting of a structure for receiving a filtrate of blood, and a tubule that absorbs selected parts of the filtrate back into the bloodstream.

Nephrostome (nef´ ro stome) [Gr. nephros: kidney + stoma: opening] • An opening in a nephridium through which body fluids can enter.

Nerve • A structure consisting of many neuronal axons and connective tissue.

Net primary production • Total photosynthesis minus respiration by plants.

Neural plate • A thickened strip of ectoderm along the dorsal side of the early vertebrate embryo; gives rise to the central nervous system.

Neural tube • An early stage in the development of the vertebrate nervous system consisting of a hollow tube created by two opposing folds of the dorsal ectoderm along the anterior-posterior body axis.

Neuromuscular junction • The region where a motor neuron contacts a muscle fiber, creating a synapse.

Neuron (noor´ on) [Gr. neuron: nerve] • A nervous system cell that can generate and conduct action potentials along an axon to a synapse with another cell.

Neurotransmitter • A substance produced in and released by one a neuron (the presynaptic cell) that diffuses across a synapse and excites or inhibits another cell (the postsynaptic cell).

Neurula (nure´ you la) • Embryonic stage during the dorsal nerve cord forms from two ectodermal ridges.

Neutral allele • An allele that does not alter the functioning of the proteins for which it codes.

Neutral theory • A view of molecular evolution that postulates that most mutations do not affect the amino acid being coded for, and that such mutations accumulate in a population at rates driven by genetic drift and mutation rates.

Neutron (new´ tron) • One of the three most fundamental particles of matter, with mass approximately 1 amu and no electrical charge.

Neutrophils • Abundant, short-lived phagocytic leukocytes that attack antibody-coated antigens.

Niche • See ecological niche.

Nicotinamide adenine dinucleotide • (See NAD.)

Nicotinamide adenine dinucleotide phosphate • (See NADP.)

Nitrate reduction • The process by which nitrate (NO₃^-) is reduced to ammonia (NH₃).

Nitric oxide (NO) • An unstable molecule (a gas) that serves as a second messenger causing smooth muscle to relax. In the nervous system it operates as a neurotransmitter.

Nitrification • The oxidation of ammonia to nitrite and nitrate ions, performed by certain soil bacteria.

Nitrogenase • In nitrogen-fixing organisms, an enzyme complex that mediates the stepwise reduction of atmospheric N₂ to ammonia.

Nitrogen fixation • Conversion of nitrogen gas to ammonia, which makes nitrogen available to living things. Carried out by certain prokaryotes, some of them free-living and others living within plant roots.

Node [L. nodus: knob, knot] • In plants, a (sometimes enlarged) point on a stem where a leaf is or was attached.

Node of Ranvier • A gap in the myelin sheath covering an axon; the point where the axonal membrane can fire action potentials.

Noncompetitive inhibitor • An inhibitor that binds the enzyme at a site other than the active site. (Contrast with competitive inhibitor.)

Nondisjunction • Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.

Nonpolar molecule • A molecule whose electric charge is evenly balanced from one end of the molecule to the other.

Nonsense mutation • Mutations that prematurely terminate a polypeptide by changing a codon for an amino acid to one of the codons (UAG, UAA, or UGA) that signal termination of translation. (Contrast with frame-shift mutation, missense mutation, synonymous mutation.)

Nonspecific defenses • Immunologic responses directed against any invading agent without reacting to apecific antigens.

Nonsynonymous mutation • A nucleotide substitution that changes the amino acid specified (i.e., AGC -> AGA, or serine -> arginine). (Contrast with synonymous mutation.)

Nonsynonymous substitution • The situation when a nonsynonymous mutation becomes dominant in a population. (Contrast with synonymous substitution.)

Nontracheophytes • Those plants lacking well-developed vascular tissue; the liverworts, hornworts, and mosses. (Contrast with tracheophytes.)

Norepinephrine • A neurotransmitter found in the central nervous system and also at the postganglionic nerve endings of the sympathetic nervous system. Also called noradrenaline.

Notochord (no´ tow kord) [Gr. notos: back + chorde: string] • A flexible rod of gelatinous material serving as a support in the embryos of all chordates and in the adults of tunicates and lancelets.

Nuclear envelope • The surface, consisting of two layers of membrane, that encloses the nucleus of eukaryotic cells.

Nuclear pore complex • Protein structure situated in nuclear pores through which RNA and proteins enter and leave the nucleus.

Nucleic acid (new klay´ ik) • A long-chain alternating polymer of deoxyribose or ribose and phosphate groups, with nitrogenous bases—adenine, thymine, uracil, guanine, or cytosine (A, T, U, G, or C)—as side chains. DNA and RNA are nucleic acids.

Nucleoid (new´ klee oid) • The region that harbors the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not bounded by a membrane.

Nucleolar organizer (new klee´ o lar) • A region on a chromosome that is associated with the formation of a new nucleolus following nuclear division. The site of the genes that code for ribosomal RNA.

Nucleolus (new klee´ oh lus) • A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.

Nucleoplasm (new´ klee o plazm) • The fluid material within the nuclear envelope of a cell, as opposed to the chromosomes, nucleoli, and other particulate constituents.

Nucleosome • A portion of a eukaryotic chromosome, consisting of part of the DNA molecule wrapped around a group of histone molecules, and held together by another type of histone molecule. The chromosome is made up of many nucleosomes.

Nucleotide • The basic chemical unit in a nucleic acid. A nucleotide in RNA consists of one of four nitrogenous bases linked to ribose, which in turn is linked to phosphate. In DNA, deoxyribose is present instead of ribose.

Nucleoside • A nucleotide without the phosphate group.

Nucleus (new´ klee us) [L. nux: kernel or nut] • In cells, the centrally located compartment of eukaryotic cells that is bounded by a double membrane and contains the chromosomes.

Null hypothesis • The assertion that an effect proposed by its companion hypothesis does not in fact exist.

Nutrient • A food substance; or, in the case of mineral nutrients, an inorganic element required for completion of the life cycle of an organism.