N

NAD+: Nicotinamide adenine dinucleotide, a coenzyme present in all cells that helps enzymes transfer electrons during the redox reactions of metabolism.
NADPH: Nicotine adenine dinucleotide phosphate (in its reduced form). It is used as an energy and redox carrier in all organisms.
Natural killer (NK) cell:  A type of white blood cell that can kill tumor cells and virus-infected cells; an important component of innate immunity.
Natural selection: The process by which genotypes with higher fitness increase in frequency in a population. Differential success  in the reproduction of different phenotypes resulting from the interaction of organisms with their environment.
Negative feedback: A primary mechanism of homeostasis, whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation.
Negative pressure breathing: A breathing system in which air is pulled into the lungs.
Nematocyst:A stinging, capsule-like organelle in a cnidocyte.
Nematode: Member of a major phylum within the Ecdysozoa of very diverse and abundant worm-like animals, including the round worms and thread worms (phylum Nemata).
Neocortex: In the mammalian brain, the outer most region of the cerebral cortex.
Neoproterozoic: Division of time from1000 Mya to the base of the Cambrian at 542 Mya.
Neoteny: Reproduction by juveniles. For example, axolotls reproduce without metamorphosis into adult salamanders.
Neprhon: The tubular excretory unit of the vertebrate kidney.
Neritic zone:The swallow region of the ocean overlying the continental shelve.
Nerve: A rope-like bundle of neuron fibers (axons and dendrites) tightly wrap in connective tissue.
Nerve cord: A rope-like arrangement of neurons, characteristic of animals with bilateral symmetry and cephalization.
Nerve net: A web-like system of neurons, characteristic of radially symmetric animals, such as hydra.
Nerve tissue: Tissue made up of neurons and supportive cells.
Net primary production (NPP): The gross primary production of an ecosystem minus the energy used by the producers for respiration.
Neural crest: A band of cells along the border where the neural tube pinches off from the ectoderm. The cells migrate to various parts of the embryo and form the pigment cells in the skin, bones of the skull, the teeth, the adrenal glands, and parts of the peripheral nervous system.
Neural tube: A tube of cells running along the dorsal axis of the body, just dorsal to the notochord. It will give rise to the central nervous system.
Neurogenic heart: A type of heart, such as in insects, in which the pacemaker originate in motor nerves arising from outside the heart.
Neurohypophysis: see Posterior pituitary.
Neuron: A nerve cell; the fundamental unit of the nervous system, having structures and properties that allows it to conduct signals by taking advantage of the electric charge across its cell membrane.
Neuropeptide: A relative short chain of amino acids that serves as a neurotrasmitter.
Neurosecretory cell: A specialized nerve cell that releases a hormone into the blood stream in response to signals from other nerve cells; located in the hypothalamus and adrenal medulla.
Neurotransmitter: A chemical messenger  released from the synaptic terminal of a neuron at the chemical synapse that diffuses across the synaptic cleft and binds and stimulates the postsynaptic cell.
Neutral: Having no effect on fitness.
Neutral evolution: Evolving without the influence of natural selection.
Neutral mutation: A mutation that does not affect fitness. Also called neutral variation.
Neutron: An electrical neutral particle (having no charge), found in the nucleus of an atom.
Neutrophil: The most abundant type of white blood cell. Neutrophils are phagocytic and tend to self-destruct as they destroy foreign invaders.
Niche: The set of ecological environments in which a species can survive and reproduce. A habitat supplying the necessary elements for the existence of an organism.
Nitric oxide (NO): A gas produced by many types of cells that functions as a local regulator, a neurotransmitter and an antibacterial agent.
Nitrogen fixation: The assimilation of atmospheric nitrogen  by certain prokaryotes into  nitrogenous compounds that can be  directly used by plants.
Nitrogenase: An enzyme complex. unique to certain prokaryotes,  that reduces N₂ to NH₃.
Nitrogen-fixing bacteria: Microorganisms that restock nitrogenous minerals in the soil by converting nitrogen to ammonia: N₂ to NH₃.
Nociceptor: A class of naked dendrites in the epidermis of the skin.
Node: (1) The point in a phylogenetic tree where one branch split into two. (2) A point along the stem of a plant at which leaves are attached.
Nodule: A swelling on the root of a legume. Nodules are composed of plant cells that contain nitrogen-fixing bacteria of the genus Rizhobium.
Noncompetitive inhibitor: A substance that reduce the activity of an enzyme by binding to a location remote from the active site, changing its conformation so that it no longer binds to the substrate.
Noncycling electron flow: A route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATP, NADPH, and oxygen. The net electron flow is from water to NADP⁺.
Nondisjunction: An error in meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
Nonequilibrium model: The model of communities that emphasizes that they are not stable in time but constantly changing after being buffeted by disturbance.
Nonhomologous gene displacement: A lateral gene transfer event in which a gene that carries out a particular function is replaced by a nonhomologous gene that carries out a similar function.
Non-Mendelian inheritance: Inheritance that does not follow Mendelian patterns.
Nonpolar covalent bond: A type of covalent bond in which electrons are shared equally between two atoms of similar electronegative.
Nonsense mutation: A point mutation in a protein-coding region that produces a stop codon, prematurely truncating the protein sequence.
Nonshivering thermogenesis (NST): The increased production of heat in some mammals by the action of certain hormones that cause mitochondria to increase their metabolic activity and produce heat instead of ATP.
Nonsynonymous mutation: A point mutation in a protein-coding region that changes a codon such that it alters the resulting amino acid sequence of the protein.
Norepinephrine: A hormone that is chemically and functionally similar to epinephrine.
Norm of reaction: The range of phenotypes produced by a single genotype, due to environmental influences.
Normal distribution:  The bell-shaped curve that describes the distribution of the sum of a large number of independent variables.
Notochord: A longitudinal, flexible rod that runs along the dorsal axis of an animal’s body in the future position of the vertebral column.
Nuclear envelop: The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
Nuclear lamina: A netlike array of protein filaments that maintains the shape of the nucleus.
Nuclease: An enzyme that hydrolyzes DNA and RNA into their components nucleotides.
Nuclei acid: A polymer (polynucleotide) consisting of many nucleotide monomers, serve as a blueprint for  proteins, and though the action of proteins, for all cellular activities. The two types are DNA and RNA.
Nucleic acid hybridization: Base pairing between a gene and a complimentary sequence on another nucleic acid molecule.
Nucleic acid probe: In DNA technology, a labeled single stranded nucleic acid molecule used to tag a specific nucleotide sequence in a nucleic acid sample.  Molecules of the probe hydrogen-bond to the complimentary sequence wherever it occurs; radioactive or other labelling of the probe, allow its location to be detected.
Nucleoid: A dense region of DNA in a prokaryotic cell.
Nucleoid region: The region in a prokaryotic cell consistent of a concentrated mass of DNA.
Nucleolus: Subcompartment within the nucleus that is involved primarily in making ribosome components.
Nucleomorph: A highly reduced relic of a nucleus. It is found in many eukaryotic species that have gone secondary symbioses in which one eukaryote became the endosymbiont of another eukaryote.
Nucleosome: The basic, bead-like unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.
Nucleotide: A nitrogenous base attached to a ribose or deoxyribose sugar and a phosphate molecule. It is the basic unit of a nuclei acid.
Nucleotide excision repair:  The process of removing and then correctly  replacing a damaged segment of DNA using the undamaged strand as a guide.
Nucleus: (1) An atomic central core, containing protons and neutrons. (2) The chromosome containing organelle of a eukaryotic cell. (3) A cluster of neurons.
Null allele: An allele of a gene in which function has been completely abolished. It is generally considered equivalent to the complete deletion of the gene.
Null hypothesis: A hypothesis that is presume true and against which alternative hypothesis are tested statistically.
Nurse cells: These cells transfer their cytoplasm (RNA and proteins) into the oocyte to support oocyte development in the Drosophila egg chamber.