Nanometer One billionth of a meter. What organisms are measured in nanometers? Find out in "HowBig?".
Natural Killer Cell A cytotoxic lymphocyte of the immune system that can target a virus-infected or tumor cell for destruction.
Necrosis Cell death triggered by injury or infection. Watch neutrophil necrosis triggered by Streptococcus.
Necrotizing Fasciitis An acute infection of the connective tissue caused by some Streptococci and other bacteria. The misnomer “flesh-eating bacteria” has been applied to these bacteria in sensational tabloid media. Watch these bacteria growing in culture.
Neutrophil The short term for polymorphonuclear neutrophil, a granule-filled white blood cell with a lobed nucleus that is a first line of defense against bacterial infection. The name derives from the fact that the granules have an affinity for dyes with a neutral pH. See the neutrophil ingesting Streptococci in the Gallery.
Nitroblue Tetrazolium, NBT A stain used to detect whether a white blood cell produces reactive oxygen species after phagocytosis of bacteria or other infectious organisms. If detected, the NBT produces a dark blue precipitate. The inability to produce reactive oxidative species as in a patient with chronic granulomatous disease of childhood, leads to a corresponding inability to kill certain infecting organisms. Watch NBT reduction in time-lapse.
Nuclear Envelope The double lipid bilayer enclosing the nucleus of the eukaryotic cell. Also called the nuclear membrane. More in the Eukaryotic Cell Model.
Nuclear Pore Any of a number of protein lined channels through the nuclear envelope that connect the nuclear contents with the cytoplasm. More in the Eukaryotic Cell Model.
Nucleic Acid A long macromolecule common to living cells and viruses that contains the genetic code for that cell or virus. Includes both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Nucleoid The region with the prokaryotic cell that contains the genophore (genetic material). Explore the structure of bacteria.
Nucleolus A densely-packed region of nucleic acids and proteins within the eukaryotic cell nucleus visible during interphase. Synthesis of ribosomal RNA (rRNA) important for the production of ribosomes takes place in this region. See the Eukaryotic Cell Model.
Nucleus That region in the eukaryotic cell in which the major portion of the genetic code resides. It is bounded by a double membrane.
Organelle Any one of the distinct structures found within the organized eukaryotic cell.
Oxidative Burst The rapid release of reactive oxygen species particularly by a phagocytic cell upon ingestion of a microbe. The reactive oxygen species include hydrogen peroxide (H2O2) and superoxide anion. See Nitroblue Tetrazolium (above).