Online Biology Dictionary - N

n (1) nano-; (2) nasal.

N (1) asparagine; (2) nitrogen; (3) normal (in reference to a solution); (4) Avogadro's number.

Na Sodium.

NAD Nicotinamide adenine dinucleotide.

NADH Reduced nicotinamide adenine dinucleotide.

NADP Nicotinamide adenine dinucleotide phosphate.

NADPH Reduced nicotinamide adenine dinucleotide phosphate.

Distinct stages in the life cyle of Naegleria fowleri

Naegleria A genus of amoebas much used to study the genetics of undulipodium development; Naegleria fowleri is the causative agent of amoebic meningoencephalitis.

NAG N-acetylglucosamine.

NAM N-acetylmuramic acid.

NANA N-acetylneuraminic acid.

Nannippus An extinct genus of diminutive, horselike animals that existed in North America for about 11 million years (Miocene-Pliocene, ~13.3–3.3 mya).

nanoliter (nl) /NAN-uh-LEE-ter/ One billionth of a liter (10^-9 L).

nanometer (nm) /NAN-uh-MEE-ter/ One billionth (10^-9) of a meter.

nasal /NAY-suhl/ Pertaining to the nose.

nasitis /nay-ZIGHT-uhs/ Inflammation of the nose.

nasopalatine /NAY-zoh-PAL-uh-teen/ Pertaining to the nose and palate.

nasopharynx /NAY-zoh-FAIR-uhnks/ The portion of the pharynx above the soft palate.

natal /NAY-tuhl/ Pertaining to birth.

natality /nay-TAL-uh-tee, nuh-/ Birthrate, number of births per unit of time.

nates /NAY-teez/ Medical term for the buttocks.

natural selection The differential survival and reproductive success of individuals, groups, or types of organisms. Conventional evolutionary theory describes evolution primarily as a matter of selection among individuals (and, sometimes, among groups) within reproductively isolated populations. The result under this scenario is that the individuals better suited to the environmental conditions in question contribute more genes to the next generation so that genes of that sort become more common in the population. This process is thought to result in the spread and perpetuation of those traits that are well-suited to the environment and the elimination of those that are not. In contrast, the theoretical portion of this website argues for the opposite view, that evolution is primarily a process that selects between distinct, stable forms or types of organisms and that selection between individuals has a negligible evolutionary effect. Under this view, the various stable forms are produced by non-Mendelian stabilization processes. READ MORE ABOUT THIS TOPIC

necrosis (also gangrene, mortification) /nuh-CROW-suhs/ Tissue death.

necrotic /nuh-CRAWT-uhk/ Dead or dying (said of tissue).

negative interference In genetics: a phenomenon where the occurrence of one crossover event increases the likelihood of a second crossover event occurring in the same vicinity. Compare: positive interference.

nematocyst /NEM-uh-duh-SIST, nuh-MAWD-/ See: cnidocyst.

nematode /NEEM-uh-toad/ A member of the Phylum Nematoda, composed of the true round worms, many of which are parasitic.

nematodiasis /NEEM-uh-toad-EYE-uh-suhs/ Infestation with nematodes.

nematoid /NEEM-uh-toid/ (1) threadlike; (2) of or relating to nematodes.

neoplasia /NEE-oh-PLAY-zhuh/ Development of neoplasms.

neoplasm /nee-oh-PLAZ-uhm/ Neoplastic tissue serving no useful function.

neoplastic /nee-oh-PLAST-ick/ Newly formed and abnormal.

neotropical /nee-oh-TRAWP-ik-uhl/ Of or pertaining to the tropical regions of the New World.

nephric (also nephritic) /NEF-rik/ Relating to the kidney(s).

nephridium (pl nephridia) /nuh-FRID-ee-uhm/ Excretory tube of an earthworm.

nephritis /nuh-FRIGHT-uhs/ Inflammation of the kidney.

nephron /NEFF-rawn/ Excretory tubule within the kidney.

nephropathy /neff-RAWP-uh-thee/ A general term for disease of kidney(s).

neritic zone /neh-RIT-ick/ The shallow ocean region defined by the continental shelves.

nerve A bundle of neurons connecting the central nervous system, with some other part of the body.

nerve fiber An elongated portion of a neuron, usually an axon or dendrite.

nervomuscular Pertaining to the nerves and muscles.

nervous system The network of neurons present in animals.

nervous tissue Tissue composed of neurons and glial cells. Nervous tissue is one the four classes to which tissue has traditionally been assigned, the other three being muscle, epithelium, and connective tissue.

neuralgia /noo-RAL-zhuh/ Sharp, shooting pain along the length of a nerve.

neurocysticercosis /N(Y)OOR-oh-sist-uh-sir-KOH-suhs/ A parasitic infection of the central nervous system resulting from ingestion of eggs from the adult tapeworm, Taenia solium. Infection occurs via ingestion of food contaminated with feces of a T. solium tapeworm carrier (tapeworm eggs are shed in the stool and contaminate food through poor hygiene). When the parasites form cysts in the brain seizures and epilepsy result. Neurocysticercosis has long been prevalent in the developing world and is now occurring at increasing rates in the U.S.

Neuron (click to enlarge)

neurons /N(Y)OOR-awnz/ Nerve cells (see diagram above); these gray or reddish cells are the fundamental units of the nervous system.

Neurospora /n(y)oor-AWSP-ur-uh/ Pink bread mold, an ascomycete fungus; used in many genetics and cell biology experiments.

neurotoxin /N(Y)OOR-oh-TAWK-sin/ A toxin that interferes with nerve function.

neurotransmitter /N(Y)OOR-oh-TRANS-mit-er/ A messenger molecule released from one of the synaptic knobs of a neuron; after their release, which occurs at a synaptic cleft (see neuron diagram), neurotransmitters diffuse across the cleft and bind to the post synaptic cell, which results in its stimulation.

neurotropic /N(Y)OOR-uh-TRAWP-ik, N(Y)URR-uh-TROPE-ik/ Attracted to or residing in nerves <~ bacteria>.

neutrophils /N(Y)OO-truh-fills/ The most abundant type of leukocyte in mammals, and the first to respond to inflammation; stains pink; targets bacteria and fungi.

N-formylmethionine (fMet) The initial residue in protein synthesis in prokaryotes, mitochondria, and chloroplasts. It is not, however, used by eukaryotes in the synthesis of proteins from nuclear mRNAs. The name N-formylmethionine refers to the position of fMet at the N-terminus of the polypeptide.

niche Ecological niche.

nicotinamide adenine dinucleotide (NAD) A coenzyme electron carrier that occurs in two states, an oxidized form (NAD⁺) and a reduced one (NADH). In metabolism, NAD is active in redox reactions. As NAD⁺ it accepts electrons and becomes reduced (NADH), then, as NADH, it acts as a reducing agent, donating electrons.

nicotinamide adenine dinucleotide phosphate (NADP) A coenzyme electron carrier that occurs in two states, an oxidized form (NADP⁺) and a reduced one (NADPH). In metabolism, NADP is active in reactions such as lipid and nucleic acid synthesis that need NADPH as a reducing agent. As NADP⁺ it accepts electrons and becomes reduced (NADPH), then, as NADPH, it acts as a reducing agent, donating electrons.

nidifugous /nide-DIF-yuh-guhs/ Leaving the nest soon after hatching. Compare: precocial.

nidology /nide-DAWL-uh-jee/ The scientific study of nests.

nigricans /NIG-ruh-kanz/ In taxonomic names: black, blackened.

nitrite /NIGHT-rite/ Any salt or ester of nitrous acid.

Nitrobacter /NIGHT-roe-BACK-ter/ A genus of bacteria playing an important role in the nitrogen cycle by oxidizing nitrite into nitrate in soil.

nitrogen (N) /NIGH-truh-jen/ Chemical element; atomic number 7, atomic mass 14.00674; colorless, odorless, tasteless; makes up 78% of earth's atmosphere by volume. Because of its strong chemical bond, elemental nitrogen (N₂) is rather inert and therefore difficult for most organisms to utilize. However, nitrogen is fundamental to life because it is a component of all nucleotides. This problem is overcome by the few organisms that actually can introduce it into organic compounds via nitrogen fixation. PERIODIC TABLE

nitrogen fixation The conversion of atmospheric nitrogen, by various prokaryotes, into compounds (such as ammonia, nitrate and nitrogen dioxide) that can be utilized by other, more complex organisms. SEE DIAGRAM OF NITROGEN CYCLE

nitrogenous /nigh-TRAWJ-uh-nuhs/ Containing nitrogen.

nitrous acid /NIGH-truhs/ The unstable acid NHO₂.

nival /NIGH-vuhl/ Living in or under snow; living in a snowy habitat.

nl Nanoliter.

nm Nanometer.

NMN Nicotinamide mononucleotide.

NMR Nuclear magnetic resonance.

nociceptor Sensory neurons that can detect potentially dangerous stimuli; such detection is perceived as pain.

nocturnal Occurring or active at night. Compare: diurnal.

nodes The points on a plant's stem where the leaves are attached.

nodule A small node.

nonahydrate /NAWN-uh-HIGH-drayt/ A compound containing nine water molecules.

nondisjunction /nawn-dis-JUNK-shun/ There are two types of nondisjunction: (1) primary nondisjunction, failure of homologs to separate during meiosis I, and (2) secondary nondisjunction, failure of sister chromatids to separate during meiosis II. Nondisjunction results in some daughter cells receiving no copy, and others receiving two copies, of the chromosome in question.

NOR See: nucleolus organizing region.

norm of reaction The range of phenotypes that a single genotype can produce under the influence of a range of environments.

normocytosis /NORM-oh-SIGHT-oh-suhs/ The state in which blood cell counts are normal.

northern blot A gel-based laboratory procedure that locates mRNA sequences on a gel that are complementary to a piece of DNA used as a probe. See also: DNA, library.

nosocomial /NO-so-COAM-ee-uhl/ Pertaining to hospitals.

nosography /no-SAWG-ruff-ee/ The medical description of disease.

Partial cross-section of a chick embryo showing notochord, neural tube, somite and other structures. (click to enlarge)

notochord /NO-tuh-cord/ In chordate embryos, the line of cells between the gut and nerve cord. It develops into the bones of the vertebral column.

novobiocin /NOH-vuh-BIGH-uh-sin/ An antimicrobial drug used in cases of resistant staphylococcic infection.

nucellus /n(y)oo-CELL-uhs/ The central, main portion of the ovule, which contains the embryo sac.

nuciform /N(Y)OO-suh-form/ Nut-shaped.

nuclear envelope (NE also nuclear membrane) /N(Y)OO-klee-ur/ The membrane enclosing the chromosomes in a eukaryotic cell. It is composed of an inner and outer membrane and is riddled with nuclear pores, holes facilitating and governing the passage of macromolecules into and out of the nucleus. Both membranes are lipid bilayers and are united at each of the nuclear pores. The rough endoplasmic reticulum is continuous with the outer membrane.

nuclear magnetic resonance (NMR) /N(Y)OO-klee-ur/ A technique used in analyzing the atomic composition of chemical compounds. NMR provides information about the structure of molecules because different types of atoms in a magnetic field resonate at different frequencies.

nuclear pore /N(Y)OO-klee-ur/ A perforation through the nuclear envelope facilitating and regulating macromolecules passing into and out of the nucleus.

nuclear transfer /N(Y)OO-klee-ur/ Removing a nucleus from a cell and inserting it into an oocyte that has had its own nucleus removed.

nucleic acid /n(y)oo-KLAY-ik/ A nucleotide polymer; nucleic acids are of two types called RNA and DNA.

nucleoid region (or simply nucleoid) /N(Y)OO-klee-oid/ The concentrated mass of DNA in a prokaryotic cell.

nucleolus /N(Y)OO-klee-OH-luhss/ A round body associated with a nucleolus organizing region. It is composed of the primary products of the rRNA genes, as well as their proteins and enzymes.

nucleolus organizing region (NOR) A chromosomal region containing ribosomal RNA (rRNA) genes.

nucleoplasm /N(Y)OO-klee-oh-PLAZ-uhm/ The fluid within the nucleus of a eukaryotic cell.

nucleoside /N(Y)OO-klee-oh-SIDE/ A purine or pyrimidine base attached to a ribose or a deoxyribose. The four nucleosides found in RNA (ribosides) are adenosine, cytidine, guanosine, and uridine. The four found in DNA (deoxyribosides) are adenine deoxyriboside, cytosine deoxyriboside, guanosine deoxyriboside, and thymidine (note: "deoxyriboside" generally is not appended to thymidine because there is no ambiguity since thymidine does not occur in RNA).

nucleosome /N(Y)OO-klee-oh-sohm/ The basic subunit of chromatin structure. Nucleosomes are visible. Nuclosomes are visible as beadlike regions along the chromosomes during meiotic prophase. Each is composed of DNA wound around a histone protein core, which itself consists of two copies each of four different types of histones.

Generic Nucleotide (click to enlarge)

nucleotide /N(Y)OO-klee-oh-tide/ One of the monomeric units of which DNA and RNA polymers are composed. A nucleotide is made up of a nitrogenous base and a phosphate group covalently bonded to a five-carbon sugar molecule (deoxyribose in DNA and ribose in RNA). purines and pyrimidines. In DNA the nitrogenous bases are the purines, adenine (A), guanine (G), and the pyrimidines, cytosine (C) and thymine (T). In RNA they are the same except that the pyrimidine uracil (U) is replaced for thymine. The nucleotides of DNA are deoxyadenylic acid, deoxycytidilic acid, deoxyguanylic acid, and thymidylic acid. The corresponding nucleotides of RNA are adenylic acid, cytidylic acid, guanylic acid, and uridylic acid. See also: base pair.

nucleotide pair substitution See: base-pair substitution.

nucleus (pl nuclei) /N(Y)OO-klee-uhs, N(Y)OO-klee-eye/ The organelle in eukaryotic cells that contains the chromosomes. SEE PICTURE

nudibranch /N(Y)OOD-uh-BRANK/ A member of the gastropod suborder Nudibranchia, marine opisthobranchs lacking shells. These, and the members of certain other gastropod groups, are informally called "sea slugs."

nullipara /nuhl-IP-uh-ruh/ A woman who has not borne offspring.

nulliparity /nuhl-uh-PAR-uh-tee/ The state of being a nullipara.

nummiform (also nummular) /NUHM-uh-form NUHM-yuh-ler/ Circular, oval, coin-shaped.

nutation /NOOT-ay-shun/ Nodding, especially involuntary nodding.

nyctalgia /nick-TAWL-zhuh/ Nocturnal pain.

nymph /NIMF/ The sexually immature larva of a hemimetabolous insect prior to the appearance of the first trace of wings. See: hemimetabolous, Hemimetabola.

nycturia /nick-TER-yuh or -ee-yuh/ Excessive nocturnal urination.

nyxis /NICK-suhs/ Piercing, puncturing.