nuclear export signal Sorting signal contained in the structure of molecules and complexes, such as nuclear RNPs and new ribosomal subunits, that are transported from the nucleus to the cytosol through nuclear pore complexes.  (Figure 12–13) nuclear import receptors Recognize nuclear localization signals to initiate nuclear import of proteins containing the appropriate nuclear localization signal.
nuclear lamin   Protein subunit of the intermediate filaments that form 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 sequence or signal patch found in proteins destined for the nucleus that enables their selective transport into the nucleus from the cytosol through the nuclear pore complexes. (Figures 12–9 and 12–13) nuclear magnetic resonance (NMR) spectroscopy NMR is the 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. NMR is used widely to determine the three-dimensional structure of small proteins and other small molecules. The principles of NMR are also used for medical diagnostic purposes in magnetic resonance imaging (MRI). (Figure 8–22) nuclear pore complex (NPC) Large multiprotein structure forming an aqueous channel (the nuclear pore) through the nuclear envelope that allows selected molecules to move between nucleus and cytoplasm. (Figure 12–8) nuclear receptor superfamily   Intracellular receptors for hydrophobic signal molecules such as steroid and thyroid hormones and retinoic acid. The receptor-ligand complex acts as a transcription factor in the nucleus. (Figure 15–65) nuclear transport receptor (karyopherin) Protein that escorts macromolecules either into or out of the nucleus: nuclear import receptor or nuclear export receptor. (Figure 12–13) nucleolus   A prominent structure in the nucleus where rRNA is transcribed and ribosomal subunits are assembled. (Figure 4–9) nucleoporin   Any of a number of different proteins that make up nuclear pore complexes.
nucleosome   Beadlike structure in eukaryotic chromatin, composed of a short length of DNA wrapped around an octameric core of histone proteins. The fundamental structural unit of chromatin. (Figures 4–22 and 4–23) nucleotide   Nucleoside with one or more phosphate groups joined in ester linkages to the sugar moiety. DNA and RNA are polymers of nucleotides. (Panel 2–6, pp. 100–101) nucleotide excision repair   Type of DNA repair that corrects damage of the DNA double helix, such as that caused by chemicals or UV light, by cutting out the damaged region on one strand and resynthesizing it using the undamaged strand as template. Compare base excision repair. (Figure 5–41)
O-linked glycosylation   Addition of one or more sugars to a hydroxyl group on a protein.
obligate pathogens   Bacteria that can only replicate inside their host.
olfactory receptors   G-protein-coupled receptors on the modified cilia of olfactory receptor neurons that recognize odors. The receptors activate adenylyl cyclase via an olfactoryspecific G protein (Golf) and resultant increases in cAMP open cyclic-AMP-gated cation channels, allowing Na+ influx and depolarization and initiation of a nerve impulse.
oligodendrocyte Glial cell in the vertebrate central nervous system that forms a myelin sheath around axons. Compare Schwann cell.