Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

targeting sequence A relatively short amino acid sequence within a protein that directs the protein to a specific location within the cell; also called signal peptide, signal sequence, and uptake-targeting sequence. (Table 13-1)

Tat An HIV-encoded protein that prevents Pol II termination when synthesizing the polycistronic HIV mRNA, thereby allowing transcription of the complete proviral DNA genome.

TATA box A conserved sequence in the promoter of many eukaryotic protein-coding genes where the transcription-initiation complex assembles. (Figure 9-16)

T cell A lymphocyte that matures in the thymus and expresses antigen-specific receptors that bind antigenic peptides complexed to MHC molecules. There are two major classes: cytotoxic T cells (CD8 surface marker, class I MHC restricted, kill virus-infected and tumor cells) and helper T cells (CD4 marker, class II MHC restricted, produce cytokines, required for activation of B cells). (Figures 23-34 and 23-37)

T-cell receptor A heterodimeric antigen-binding transmembrane protein containing variable and constant regions and associated with the signal-transducing multimeric CD3 complex. (Figure 23-27)

telomere Region at each end of a eukaryotic chromosome containing multiple tandem repeats of a short telomeric (TEL) sequence. Telomeres are required for proper chromosome segregation and are replicated by a special process that prevents shortening of chromosomes during DNA replication. (Figure 8-44)

telophase Final mitotic stage, during which the nuclear envelope re-forms around the two sets of separated chromosomes, the chromosomes decondense, and division of the cytoplasm (cytokinesis) is completed. (Figure 18-37)

temperature-sensitive (ts) mutation A mutation that produces a wild-type phenotype at one temperature (the permissive temperature) but a mutant phenotype at another temperature (the nonpermissive temperature). This type of mutation is especially useful in identification of genes essential for life. (Figure 6-6)

termination, transcription Cessation of the synthesis of an RNA chain. (Figure 5-11)

tertiary structure In proteins, overall three-dimensional form of a polypeptide chain, which is stabilized by multiple noncovalent interactions between side chains. (Figure 3-11a)

thylakoids Flattened membranous sacs in a chloroplast that can be arranged in stacks and contain the photosynthetic pigments and photosystems. (Figure 12-37)

thrombopoietin A cytokine that stimulates the development of megakaryocytes, cells that form the platelets involved in blood clotting.

tight junction A type of cell-cell junction between the plasma membranes of adjacent epithelial cells that prevents diffusion of macromolecules and many small molecules and ions in the spaces between cells and diffusion of membrane components between the apical and basolateral regions of the plasma membrane. (Figure 20-17)

tolerance The absence of an immune response to a particular antigen or set of antigens.

Toll-like receptor (TLR) Member of a class of cell-surface and intracellular receptors that recognize a variety of microbial products. Ligand binding initiates a signaling pathway that induces various responses depending on the cell type. (Figure 23-35)

topogenic sequences Segments within a protein whose sequence, number, and arrangement direct the insertion and orientation of various classes of transmembrane proteins into the endoplasmic reticulum membrane. (Figure 13-14)

transcription Process in which one strand of a DNA molecule is used as a template for synthesis of a complementary RNA by RNA polymerase. (Figures 5-10 and 5-11)

transcription-control region Collective term for all the DNA regulatory sequences that regulate transcription of a particular gene.

transcription factor (TF) General term for any protein, other than RNA polymerase, required to initiate or regulate transcription in eukaryotic cells. General factors, required for transcription of all genes, participate in formation of the transcription-preinitiation complex near the start site. Specific factors stimulate (activators) or inhibit (repressors) transcription of particular genes by binding to their regulatory sequences.

transcription unit A region in DNA, bounded by an initiation (start) site and termination site, that is transcribed into a single primary transcript.

transcytosis Mechanism for transporting certain substances across an epithelial sheet that combines receptor-mediated endocytosis and exocytosis. (Figures 14-25 and 23-11)

transfection Experimental introduction of foreign DNA into cells in culture, usually followed by expression of genes in the introduced DNA. (Figure 6-29)

transfer RNA See tRNA.

transformation (1) Permanent, heritable alteration in a cell resulting from the uptake and incorporation of a foreign DNA into the host-cell genome; also called stable transfection. (2) Conversion of a “normal” mammalian cell into a cell with cancer-like properties usually induced by treatment with a virus or other cancer-causing agent.

transforming growth factor beta (TGFβ) A family of secreted signaling proteins that are used in the development of most tissues in most or all animals. Members of the TGFβ family more often inhibit growth than stimulate it. Mutations in TGFβ signal transduction components are implicated in human cancer, including breast cancer. (Figure 16-3)

G-27

transgene A cloned gene that is introduced and stably incorporated into a plant or animal and is passed on to successive generations.

trans-Golgi network (TGN) Complex network of membranes and vesicles that serves as a major branch point in the secretory pathway. Vesicles budding from this most-distal Golgi compartment carry membrane and soluble proteins to the cell surface or to lysosomes. (Figures 14-1 and 14-17)

transition state State of the reactants during a chemical reaction when the system is at its highest energy level; also called the transition-state intermediate.

translation The ribosome-mediated assembly of a polypeptide whose amino acid sequence is specified by the nucleotide sequence in an mRNA. (Figure 5-17)

translocon Multiprotein complex in the membrane of the rough endoplasmic reticulum through which a nascent secretory protein enters the ER lumen as it is being synthesized. (Figure 13-7)

transporters Membrane proteins that undergo conformational changes as they move a wide variety of ions and molecules across cell membranes at a slower rate than channels. See uniporter, symporter, and antiporter in Figure 11-3.

transport protein See membrane transport protein.

transport vesicle A small membrane-bounded compartment that carries soluble and membrane “cargo” proteins in the forward or reverse direction in the secretory pathway. Vesicles form by budding off from the donor organelle and release their contents by fusion with the target membrane.

transposable DNA element Any DNA sequence that is not present in the same chromosomal location in all individuals of a species and can move to a new position by transposition; also called mobile DNA element and interspersed repeat. (Table 8-1)

transposition Movement of a transposable DNA element within the genome; occurs by a cut-and-paste mechanism or copy-and-paste mechanism depending on the type of element. (Figure 8-8)

transposon, DNA A transposable DNA element present in prokaryotes and eukaryotes that moves in the genome by a mechanism involving DNA synthesis and transposition. See also retrotransposon. (Figures 8-9 and 8-10)

triacylglycerol See triglyceride.

triglyceride Major form in which fatty acids are stored and transported in animals; consists of three fatty acyl chains esterified to a glycerol molecule.

trimeric (large) G protein A regulatory membrane-associated GTPase consisting of a catalytic α subunit and a β and γ subunit. When the α subunit is bound to GTP, the β and γ subunits dissociate as a heterodimer. The free α subunit and the free β-γ heterodimer can then interact with other proteins and transduce a signal across the membrane. When the α subunit hydrolyzes the GTP to GDP and phosphate, the α-GDP associates with the β-γ heterodimer, terminating signaling. (Figure 15-14)

tRNA (transfer RNA) A group of small RNA molecules that function as amino acid donors during protein synthesis. Each tRNA becomes covalently linked to a particular amino acid, forming an aminoacyl-tRNA. (Figures 5-19 and 5-20)

trophic factor Any of numerous signaling proteins required for the survival of cells in multicellular organisms; in the absence of such signals, cells often undergo “suicide” by apoptosis.

trophectoderm (TE) The part of an early mammalian embryo that will form the extra-embryonic tissues, including the placenta but not the embryo proper.

t-SNAREs See SNAREs.

tubulin A family of globular cytoskeletal proteins that polymerize to form the cylindrical wall of microtubules. (Figure 18-3)

TUG A protein that tethers GLUT4 storage vesicles to the Golgi matrix.

tumor A mass of cells, generally derived from a single cell, that arises due to loss of the normal regulators of cell growth; may be benign or malignant.

tumor-suppressor gene Any gene whose encoded protein directly or indirectly inhibits progression through the cell cycle and in which a loss-of-function mutation is oncogenic. Inheritance of a single mutant allele of many tumor-suppressor genes (e.g., RB, APC, and BRCA1) greatly increases the risk for developing colorectal and other types of cancer. (Figures 24-12 and 24-15)

tunneling nanotubes Tubelike projections of the plasma membrane that form a continuous channel connecting the cytosols of animal cells and can transfer chemical and electrical signals between cells in a manner analogous to plasmodesmata in plants.