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.
NADP (nicotinamide adenine dinucleotide phosphate) • Like NAD, but possessing another phosphate group; plays similar roles but is used by different enzymes.
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) • 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.
Nekton [Gr. nekhein: to swim] • Animals, such as fish, that can swim against currents of water. (Contrast with plankton.)
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 basic component of the kidney, which is made up of numerous nephrons. Its form varies in detail, but it always has at one end a device for receiving a filtrate of blood, and then 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, sinew] • A cell derived from embryonic ectoderm and characterized by a membrane potential that can change in response to stimuli, generating action potentials. Action potentials are generated along an extension of the cell (the axon), which makes junctions (synapses) with other neurons, muscle cells, or gland cells.
Neurotransmitter • A substance, produced in and released by one neuron, that diffuses across a synapse and excites or inhibits the postsynaptic neuron.
Neurula (nure´ you la) [Gr. neuron: nerve] • Embryonic stage during formation of the dorsal nerve cord by 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) [E.: neutral] • One of the three most fundamental particles of matter, with mass approximately 1 amu and no electrical charge.
Nicotinamide adenine dinucleotide • (See NAD.)
Nicotinamide adenine dinucleotide phosphate • (See NADP.)
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 axons, 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 (chain-terminating) mutation • Mutations that change a codon for an amino acid to one of the codons (UAG, UAA, or UGA) that signal termination of translation. The resulting gene product is a shortened polypeptide that begins normally at the amino-terminal end and ends at the position of the altered codon. (Contrast with frame-shift mutation, missense mutation, synonymous mutation.)
Nonspecific defenses • Immunologic responses directed against most or all pathogens, generally without reference to the pathogens’ antigens. These defenses include the skin, normal flora, lysozyme, the acidic stomach, interferon, and the inflammatory response.
Nonsynonymous mutation • A nucleotide substitution that that changes the amino acid specified (i.e., AGC Ć AGA, or serine Ć arginine). (Compare with frame-shift mutation, missense mutation, nonsense mutation.)
Nonsynonymous substitution • The situation when a nonsynonymous mutation becomes widespread in a population. Typically influenced by natural selection. (Contrast with synonymous substitution.)
Nontracheophytes • Those plants lacking well-developed vascular tissue; the liverworts, hornworts, and mosses. (Contrast with tracheophytes.)
Normal flora • The bacteria and fungi that live on animal body surfaces without causing disease.
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.
Nucleic acid (new klay´ ik) [E.: nucleus of a cell] • 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) [from L. diminutive of nux: little kernel or little nut] • 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 (monomer) 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.
Nucleus (new´ klee us) [from L. diminutive of nux: kernel or nut] • (1) In chemistry, the dense central portion of an atom, made up of protons and neutrons, with a positive charge. Surrounded by a cloud of negatively charged electrons. (2) In cells, the centrally located chamber of eukaryotic cells that is bounded by a double membrane and contains the chromosomes. The information center of the cell.
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.