initiator tRNA Special tRNA that intiates translation. It always carries the amino acid methionine, forming the complex MettRNAi. (Figure 6–70) innate immune response   An early immune response in all organisms to a pathogen, which includes the production of antimicrobial molecules and the activation of phagocytic cells. Such a response is not specific for the pathogen, in contrast to an adaptive immune response.
inner membrane Mitochondrial membrane that encloses the matrix space and forms extensive invaginations called cristae.
inner mitochondrial membrane Mitochondrial membrane that encloses the matrix space and forms extensive invaginations called cristae.
inner nuclear membrane One of two concentric membranes comprising the nuclear envelope; contnuous with the outer nuclear membrane; contains specific proteins as anchoring sites for chromatin and the nuclear lamina.
inositol 1,4,5-trisphosphate (IP3) Small intracellular signaling molecule produced during activation of the inositol phospholipid signaling pathway. Acts to release Ca2+ from the endoplasmic reticulum. (Figures 15–28 and 15–29) inositol phospholipid signaling pathway Intracellular signaling pathway that starts with the activation of phospholipase C and the generation of IP3 and diacylglycerol (DAG) from inositol phospholipids in the plasma membrane. The DAG helps to activate protein kinase C. (Figures 15–28 and 15–29) integrin   Transmembrane adhesion protein that is involved in the attachment of cells to the extracellular matrix and to each other. (Figure 19–3 and Table 19–1, p. 1037) interaction domain   Compact protein module, found in many intracellular signaling proteins, that binds to a particular structural motif (e.g., a short peptide sequence, a covalent modification, or another protein domain) in another protein or lipid.
interferon-α (IFNα) and interferon-β (IFNβ) Cytokines (type I interferons) produced by mammalian cells as a general response to a viral infection.
intermembrane space Compartment of mitochondrion between by the outer and inner mitochondrial membranes.
internal ribosome entry site (IRES) Specific site in a eukaryotic mRNA, other than at the 5′ end, at which translation can be initiated. (Figure 7–68) interphase Long period of the cell cycle between one mitosis and the next. Includes G1 phase, S phase, and G2 phase. (Figure 17–4) interpolar microtubule In the mitotic or meiotic spindle, a microtubule interdigitating at the equator with the microtubules emanating from the other pole. (Figure 17–23) intracellular pathogens Pathogens, including all viruses and many bacteria and protozoa, that enter and replicate inside host cells to cause disease.
intrinsic pathway (mitochondrial pathway) Pathway of apoptosis activated from inside the cell in response to stress or developmental signals; depends on the release into the cytosol of mitochondrial proteins normally resident in the mitochondrial intermembrane space.
intron   Noncoding region of a eukaryotic gene that is transcribed into an RNA molecule but is then excised by RNA splicing during production of the mRNA or other functional RNA. (Figure 4–15) invadopodia   Actin-rich protrusions extending in three-dimensions that are important for cells to cross tissue barriers by degrading the extracellular matrix.