Na Symbol for: Sodium.
NaCl Chemical formula for: Sodium Chloride.
nacreous /NAK-ree-əs/ Having a pearl-like luster.
NAD abbr. Nicotinamide adenine dinucleotide.
NADH Reduced nicotinamide adenine dinucleotide.
NADP abbr. Nicotinamide adenine dinucleotide phosphate.
NADPH Reduced nicotinamide adenine dinucleotide phosphate.
NaF Chemical formula for: Sodium Fluoride.
Naegleria /nay-GLARE-ee-ə/ A genus of amoebas much used to study the genetics of undulipodium development; Naegleria fowleri is the causative agent of amoebic meningoencephalitis.
NAG abbr. N-acetylglucosamine.
NAM abbr. N-acetylmuramic acid.
NANA abbr. N-acetylneuraminic acid.
nanoliter (nl) /NAN-ə-LEE-ter/ One billionth of a liter (10⁻⁹ L).
nanometer (nm) /NAN-ə-MEE-ter/ One billionth (10⁻⁹) of a meter.
nanosecond (ns) /NAN-ə-SEC-ənd/ One billionth (10⁻⁹) of a second.
nasal /NAY-səl/ Pertaining to the nose.
nasitis /nay-ZĪT-əs/ Inflammation of the nose.
nasopalatine /NAY-zō-PAL-ə-teen/ Pertaining to the nose and palate.
nasopharynx /NAY-zō-FARE-ənks/ The portion of the pharynx above the soft palate.
natal /NAY-təl/ Pertaining to birth.
natality /nay-TAL-ə-tee, nə-/ Birthrate, number of births per unit time.
nates /NAY-teez/ Medical term for the buttocks.
natron /NAY-trən/ A naturally occurring salt, composed primarily of sodium carbonate and sodium bicarbonate, used by the ancient Egyptians in the process of mummification. It prevented putrefaction in the same way that ordinary salt preserves meat.
natural selection A natural evolutionary process in which different heritable traits have a differential effect on the ability to produce offspring. Those traits aiding in the production of more offspring become more common with time. They are naturally selected. MORE INFORMATION
nausea /NAW-see-ə/ The urge to vomit.
necrosis (also gangrene, mortification) /nə-CROW-səs/ Tissue death.
necrotic /nə-CRAWT-ək/ 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-MAWD-ə-sist/ A capsule, containing a venomous projectile, present in a cnidocyte. MORE INFORMATION.
nematode /NEEM-ə-tōd/ A member of the Phylum Nematoda, composed of the true round worms, many of which are parasitic.
nematodiasis /NEEM-ə-tōd-Ī-ə-səs/ Infestation with nematodes.
nematoid /NEEM-ə-toid/ (1) threadlike; (2) of or relating to nematodes.
neoplasia /nee-ō-PLAY-zhə/ Development of neoplasms.
neoplasm /nee-ə-PLAZ-əm/ Neoplastic tissue serving no useful function.
neoplastic /nee-ō-PLAST-ick/ Newly formed and abnormal.
neotropical /nee-ō-TRAWP-ik-əl/ Of or pertaining to the tropical regions of the New World.
nephric (also nephritic) /NEF-rik/ Relating to the kidney(s).
nephridium (pl nephridia) /nə-FRID-ee-əm, nə-FRID-ee-ə/ Excretory tube of an earthworm.
nephritis /nə-FRĪT-əs/ Inflammation of the kidney.
nephron /NEFF-rawn/ Excretory tubule within the kidney.
nephropathy /neff-RAWP-ə-thee/ A general term for disease of kidney(s).
neritic zone /neh-RIT-ick/ The shallow ocean region defined by the continental shelves.
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, epithelial tissue, and connective tissue.
neuralgia /noo-RAL-zhə/ Sharp, shooting pain along the length of a nerve.
neurocysticercosis /N(Y)OOR-ō-sist-ə-sir-KŌ-səs/ 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.
neurology /n(y)oo-RAWL-ə-jee/ The branch of biology that studies the nervous system and its diseases — neurological /n(y)oor-ə-LAWJ-ə-kəl/
neuropils /N(Y)OOR-ə-pilz/ Portions of the arthropod brain that serve particular functions, such as collecting and processing input from sensory organs.
Neurospora /n(y)oor-AWSP-ur-ə/ Pink bread mold, an ascomycete fungus; used in many genetics and cell biology experiments.
neurotransmitter /N(Y)OOR-ō-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-ə-TRAWP-ik, N(Y)URR-ə-TROPE-ik/ Attracted to or residing in nerves.
neutrophils /N(Y)OO-trə-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 See: 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-yə-gəs/ Leaving the nest soon after hatching. Compare: precocial.
nidology /nide-DAWL-ə-jee/ The scientific study of nests.
nigricans /NIG-rə-kanz/ In taxonomic names: black, blackened.
nitrogen (N) /NĪ-trə-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 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 | ABOUT THE NITROGEN CYCLE
nitrogen fixation The conversion of elemental nitrogen, by certain prokaryotes, into compounds (such as ammonia, nitrate and nitrite that can be utilized by other organisms that cannot assimilate nitrogen in its elemental state. DIAGRAM OF NITROGEN CYCLE
nitrogenous /nī-TRAWJ-ə-nəs/ Containing nitrogen.
nitrous acid /NĪ-trəs/ The unstable acid NHO₂.
nival /NĪ-vəl/ Living in or under snow; living in a snowy habitat.
nl abbr. Nanoliter.
nm abbr. Nanometer.
NMN abbr. Nicotinamide mononucleotide.
NMR abbr. 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-ə-HĪ-drayt/ A compound containing nine water molecules.
nondisjunction /nawn-dis-JUNK-shən/ 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.
normocytosis /NORM-ə-sī-TŌ-səs/ The condition of the blood in which blood cell counts are normal.
nosocomial /NAW-sə-COAM-ee-əl/ Pertaining to hospitals.
nosography /no-SAW-grə-fee/ The medical description of disease.
novobiocin /NŌ-və-BĪ-ə-sin/ An antimicrobial drug used in cases of resistant staphylococcic infection.
nuciform /N(Y)OO-sə-form/ Nut-shaped.
nuclear /N(Y)OO-klee-er/ Of, residing in, or pertaining to the nucleus.
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.
nucleolus organizing region (NOR) A chromosomal region containing ribosomal RNA (rRNA) genes.
nucleoside /N(Y)OO-klee-ō-sīd/ 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-ō-sōm/ 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.
nucleotide /N(Y)OO-klee-ə-tīd, -ō-tīd/ 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.
nudibranch /N(Y)OOD-ə-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 /nəl-IP-ə-rə/ A woman who has not borne offspring.
nulliparity /nəl-ə-PAR-ə-tee/ The state of being a nullipara.
nummiform (also nummular) /NƏM-ə-form, NƏM-yə-ler/ Circular, oval, coin-shaped.
nutation /NOOT-ay-shən/ Nodding, especially involuntary nodding.
nyctalgia /nick-TAWL-zhə/ Nocturnal pain.
nycturia /nick-TER-yə, -ee-yə/ Excessive nocturnal urination.
nyxis /NICK-səs/ Piercing, puncturing.
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