Na+-K+ pump (Na+-K+ ATPase) Transmembrane carrier protein found in the
plasma membrane of most animal cells that pumps Na+ out of and K+ into the
cell, using energy derived from ATP hydrolysis.
NAD+ (nicotine adenine dinucleotide) Activated carrier that participates in
an oxidation reaction by accepting a hydride ion (H ) from a donor molecule.
The NADH formed is an important carrier of electrons for oxidative
phosphorylation.
NADH dehydrogenase complex First of the three electron-driven proton
pumps in the mitochondrial respiratory chain. It accepts electrons from NADH.
NADP+ (nicotine adenine dinucleotide phosphate) Activated carrier closely
related to NAD+ that is used extensively in biosynthetic, rather than catabolic,
pathways. The reduced form is NADPH.
nano- Prefix denoting 10 9.
nanometer (nm) Unit of length commonly used to measure molecules and cell
organelles. 1 nm = 10 3 micrometer (mm) = 10 9 meter.
natural killer cell (NK cell) Cytotoxic cell of the innate immune system that
can kill virus-infected cells.
N-CAM see neural cell adhesion molecule
negative control Type of control of gene expression in which the active DNAbinding
form of the regulatory protein turns the gene off.
negative staining Staining technique for use in the electron electron
microscope in which a reverse, or negative, image of the object is created.
Nernst equation Quantitative expression that relates the equilibrium ratio of
concentrations of an ion on either side of a permeable membrane to the voltage
difference across the membrane. (See Panel 11 2, p. 634.)
nerve cell see neuron
neural cell adhesion molecule (N-CAM) Cell adhesion molecule of the
immunoglobulin superfamily, expressed by many cell types including most
nerve cells. It mediates Ca2+-independent cell-cell attachment in vertebrates.
neural tube Tube of ectoderm that will form the brain and spinal cord in a
vertebrate embryo.
neurite Long process growing from a nerve cell in culture. A generic term that
does not specify whether the process is an axon or a dendrite.
neurofilament Type of intermediate filament found in nerve cells.
neuromuscular junction Specialized chemical synapse between an axon
terminal of a motor neuron and a skeletal muscle cell.
neuron (nerve cell) Cell with long processes specialized to receive, conduct,
and transmit signals in the nervous system.
neuropeptide Peptide secreted by neurons as a signaling molecule either at
synapses or elsewhere.
neurotransmitter Small signal molecule secreted by the presynaptic nerve
cell at a chemical synapse to relay the signal to the postsynaptic cell. Examples
include acetylcholine, glutamate, GABA, glycine, and many neuropeptides.
neutron Uncharged subatomic particle that forms part of an atomic nucleus.
neutrophil White blood cell that is specialized for the uptake of particulate
material by phagocytosis and which enters tissues that become infected or
inflamed.
nicotine adenine dinucleotide phosphate see NADP+
nicotine adenine dinucleotide see NAD+
nitric oxide (NO) Gaseous signal molecule in both animals and plants. In
animals it regulates smooth muscle contraction, for example; in plants it is
involved in responses to injury or infection.
nitrogen cycle The natural circulation of nitrogen between molecular nitrogen
in the atmosphere, inorganic molecules in the soil, and organic molecules in
living organisms.
nitrogen fixation Biochemical process carried out by certain bacteria that
reduces atmospheric nitrogen (N2) to ammonia, leading eventually to various
nitrogen-containing metabolites.
nitrogenase complex Complex of enzymes in nitrogen-fixing bacteria that
catalyzes the reduction of atmospheric N2 to ammonia.
NK cell see natural killer cell
N-linked oligosaccharide Chain of sugars attached to a protein through the
NH2 group of the side chain of an asparagine residue.
NMR (nuclear magnetic resonance) Resonant absorption of electromagnetic
radiation at a specific frequency by atomic nuclei in a magnetic field, due to
flipping of the orientation of their magnetic dipole moments. The NMR
spectrum provides information about the chemical environment of the nuclei.
Two-dimensional NMR is used widely to determine the three-dimensional
structure of small proteins.
nm see nanometer
noncovalent attraction Chemical bond in which, in contrast to a covalent
bond, no electrons are shared. Noncovalent bonds are relatively weak, but they
can sum together to produce strong, highly specific interactions between
molecules.
noncyclic photophosphorylation Photosynthetic process that produces both
ATP and NADPH in plants and cyanobacteria.
nonenveloped virus Virus consisting of a nucleic acid core and protein capsid
only.
nonpolar (apolar) Lacking any asymmetric accumulation of positive and
negative charge. Nonpolar molecules are generally insoluble in water.
nonsense-mediated mRNA decay Mechanism for removing aberrant mRNAs
containing in-frame internal stop codons before they can be translated.
Northern blotting Technique in which RNA fragments separated by
electrophoresis are immobilized on a paper sheet. A specific RNA is then
detected by hybridization with a labeled nucleic acid probe.
Notch Receptor protein involved in many instances of choice of cell fate in
animal development, for example in the specification of nerve cells from
ectodermal epithelium. Its ligands are cell-surface proteins such as Delta and
Serrate.
notochord Stiff rod of mesoderm that runs along the back of all chordate
embryos. In vertebrates it does not persist and becomes incorporated into the
vertebral column.
NSF Protein with ATPase activity that disassembles a complex of a v-SNARE
and a t-SNARE.
nuclear envelope Double membrane surrounding the nucleus. Consists of an
outer and inner membrane and is perforated by nuclear pores.
nuclear export signal Sorting signal contained in the structure of molecules
and complexes, such as RNA and new ribosomal subunits, that are transported
from the nucleus to the cytosol through nuclear pore complexes.
nuclear lamin Protein subunit of the intermediate filaments of the nuclear
lamina.
nuclear lamina Fibrous meshwork of proteins on the inner surface of the inner
nuclear membrane. It is made up of a network of intermediate filaments
formed from nuclear lamins.
nuclear localization signal (NLS) Signal sequences or signal patches found in
proteins destined for the nucleus and which enable their selective transport into
the nucleus from the cytosol through the nuclear pore complexes.
nuclear magnetic resonance see NMR
nuclear pore complex Large multiprotein structure forming a channel (the
nuclear pore) through the nuclear envelope that allows selected molecules to
move between nucleus and cytoplasm.
nuclear receptor superfamily Intracellular receptors for hydrophobic signal
molecules such as steroids and retinoic acid. The receptor-ligand complex acts
as a transcription factor in the nucleus.
nuclear transport Movement of macromolecules into or out of the nucleus
mediated by nuclear transport receptors.
nucleation Critical stage in the assembly of a polymeric structure, such as a
microtubule, at which a small cluster of monomers aggregates in the correct
arrangement to initiate rapid polymerization. (See Panel 16 2, pp. 912 913.)
More generally, the rate-limiting step in an assembly process.
nucleic acid RNA or DNA, a macromolecule consisting of a chain of
nucleotides joined together by phosphodiester bonds.
nucleolar organizer Region of a chromosome containing a cluster of
ribosomal RNA genes that gives rise to a nucleolus.
nucleolus Structure in the nucleus where ribosomal RNA is transcribed and
ribosomal subunits are assembled.
nucleoporin Any of a number of different proteins that make up nuclear pore
complexes.
nucleoside Molecule composed of a purine or pyrimidine base covalently
linked to a ribose or deoxyribose sugar. (See Panel 2 6, pp. 120 121.)
nucleosome Beadlike structure in eucaryotic chromatin. It is composed of a
short length of DNA wrapped around a core of histone proteins, and is the
fundamental structural unit of chromatin.
nucleotide Nucleoside with one or more phosphate groups joined in ester
linkages to the sugar moiety. DNA and RNA are polymers of nucleotides. (See
Panel 2 6, pp. 120 121.)
nucleus Prominent membrane-bounded organelle in a eucaryotic cell,
containing DNA organized into chromosomes.
nurse cell Cell connected by cytoplasmic bridges to a developing oocyte and
which thereby supplies it with ribosomes, mRNAs, and proteins needed for the
development of the early embryo.