NFκB protein   Latent transcription regulator that is activated by various intracellular signaling pathways when cells are stimulated during immune, inflammatory, or stress responses. Also has important roles in animal development. (Figure 15–62) nitric oxide (NO) Gaseous signal molecule that is widely used in cell–cell communication in both animals and plants. (Figure 15–40) nitrogen fixation   Biochemical process carried out by certain bacteria that reduces atmospheric nitrogen (N2) to ammonia, leading eventually to various nitrogen-containing metabolites.
NMDA receptor Subclass of glutamate-gated ion channel in the mammalian central nervous system critical for longterm potentiation and long-term depression. NMDA-receptor channels are doubly gated, opening only when glutamate is bound to the receptor and, simultaneously, the membrane is strongly depolarized.
NO synthase (NOS) Enzyme that synthesizes nitric oxide (NO) by the deamination of arginine. (Figure 15–40B)
NOD-like receptors (NLRs) Large family of pattern recognition receptors (PRRs) with leucine-rich repeat motifs; they are exclusively cytoplasmic and recognize a distinct set of microbial molecules.
nonclassical cadherins   Large family of cadherins that are more distantly related in sequence than classical cadherins and include proteins involved in adhesion (including protocadherins, desmocollins, and desmogleins) and signaling.
noncoding RNA   An RNA molecule that is the final product of a gene and does not code for protein. These RNAs serve as enzymatic, structural, and regulatory components for a wide variety of processes in the cell.
nondisjunction Event occurring occasionally during meiosis in which a pair of homologous chromosomes fails to separate so that the resulting germ cell has either too many or too few chromosomes.
nonenveloped virus   Virus consisting of a nucleic acid core and a protein capsid only. (Figure 23–18C,D) nonhomologous end joining   A DNA repair mechanism for double-strand breaks in which the broken ends of DNA are brought together and rejoined by DNA ligation, generally with the loss of one or more nucleotides at the site of joining.
nonretroviral retrotransposons   Type of transposable element that moves by being first transcribed into an RNA copy that is converted to DNA by reverse transcriptase then inserted elsewhere in the genome. The mechanism of insertion differs from that of the retroviral-like transposons. (Table 5–4, p. 288) nonsense-mediated mRNA decay Mechanism for degrading aberrant mRNAs containing in-frame internal stop codons before they can be translated into protein. (Figure 6–76) normal flora   The human microbiota consisting of approximately 1014 bacterial, fungal, and protozoan cells, representing thousands of microbial species.
Notch   Transmembrane receptor protein (and latent transcription regulator) involved in many cell-fate choices 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. (Figure 15–59)
NSF   Hexameric ATPase that disassembles a complex of a v-SNARE and a t-SNARE. (Figure 13–20) nuclear envelope Double membrane (two bilayers) surrounding the nucleus. Consists of an outer and inner membrane and is perforated by nuclear pores. The outer membrane is continuous with the endoplasmic reticulum. (Figures 4–9 and 12–7) nuclear export receptors Bind to both the export signal and nuclear pore complex proteins to guide their cargo through the nuclear pore complex to the cytosol.