T cell (T lymphocyte) Type of lymphocyte responsible for cell-mediated
immunity; includes both cytotoxic T cells and helper T cells.
TATA box Consensus sequence in the promoter region of many eucaryotic
genes that binds a general transcription factor and hence specifies the position
at which transcription is initiated.
TCA cycle see citric acid cycle
telomerase Enzyme that elongates telomere sequences in DNA.
telomere End of a chromosome, associated with a characteristic DNA
sequence that is replicated in a special way. Counteracts the tendency of the
chromosome otherwise to shorten with each round of replication. (From Greek
telos, end.)
telophase Final stage of mitosis in which the two sets of separated
chromosomes decondense and become enclosed by nuclear envelopes.
temperature-sensitive (ts) mutant Organism or cell carrying a genetically
altered protein (or RNA molecule) that performs normally at one temperature
but is abnormal at another (usually higher) temperature.
template A single strand of DNA or RNA whose nucleotide sequence acts as a
guide for the synthesis of a complementary strand.
terminator Signal in bacterial DNA that halts transcription.
tertiary structure Complex three-dimensional form of a folded polymer
chain, especially a protein or RNA molecule.
TGF-b superfamily see transforming growth factor-b superfamily
TGN see trans Golgi network (TGN)
thioester bond High-energy bond formed by a condensation reaction between
an acid (acyl) group and a thiol group ( SH); seen, for example, in acetyl CoA
and in many enzyme-substrate complexes.
thiol see sulfhydryl
thylakoid Flattened sac of membrane in a chloroplast that contains chlorophyll
and other pigments and carries out the light-trapping reactions of
photosynthesis. Stacks of thylakoids form the grana of chloroplasts.
tight junction Cell cell junction that seals adjacent epithelial cells together,
preventing the passage of most dissolved molecules from one side of the
epithelial sheet to the other.
TIM complexes Protein translocators in the mitochondrial inner membrane.
The TIM23 complex mediates the transport of proteins into the matrix and the
insertion of some proteins into the inner membrane; the TIM22 complex
mediates the insertion of a subgroup of proteins into the inner membrane.
Toll-like receptor family (TLR) Important family of mammalian pattern
recognition receptors abundant on macrophages, neutrophils and the epithelial
cells of the gut. They recognize pathogen-associated immunostimulants such
as lipopolysacharide and peptidoglycan.
TOM complex Multisubunit protein complex that transports proteins across
the mitochondrial outer membrane.
topoisomerase (DNA topoisomerase) Enzyme that makes reversible cuts in a
double-helical DNA molecule for the purpose of removing knots or unwinding
excessive twists.
tracer Molecule or atom that has been labeled either chemically or
radioactively so that it can be followed in a biochemical process or readily
located in a cell or tissue.
trans face Face of a Golgi stack at which material leaves the organelle for the
cell surface or another cell compartment. It is adjacent to the trans Golgi
network.
trans Golgi network (TGN) Network of interconnected cisternae and tubules
at the trans face of the Golgi apparatus, through which material is transferred
out of the Golgi.
transcellular transport Transport of solutes, such as nutrients, across an
epithelium, by means of membrane transport proteins in the apical and basal
faces of the epithelial cells.
transcript RNA product of DNA transcription.
transcription (DNA transcription) Copying of one strand of DNA into a
complementary RNA sequence by the enzyme RNA polymerase.
transcription attenuation Inhibition of gene expression in bacteria by the
premature termination of transcription.
transcription factor Term loosely applied to any protein required to initiate or
regulate transcription in eucaryotes. Includes both gene regulatory proteins as
well as the general transcription factors.
transcriptional control Control of of gene expression by controlling when
and how often the gene is transcribed.
transcytosis The uptake of material at one face of a cell by endocytosis, its
transfer across a cell in vesicles, and its discharge from another face by
exocytosis.
transfection Introduction of a foreign DNA molecule into a eucaryotic cell. It
is usually followed by expression of one or more genes in the newly
introduced DNA.
transfer RNA (tRNA) Set of small RNA molecules used in protein synthesis
as an interface (adaptor) between messenger RNA and amino acids. Each type
of tRNA molecule is covalently linked to a particular amino acid.
transforming growth factor-b superfamily (TGF-b superfamily) Large
family of structurally related, secreted proteins that act as hormones and local
mediators to control a wide range of functions in animals, including during
development. It includes TGF-bs, activins, and bone morphogenetic proteins
(BMPs).
transgenic organism Plant or animal that has stably incorporated one or more
genes from another cell or organism and can pass them on to successive
generations.
transition state Structure that forms transiently in the course of a chemical
reaction and has the highest free energy of any reaction intermediate. Its
formation is a rate-limiting step in the reaction.
translation (RNA translation) Process by which the sequence of nucleotides
in a messenger RNA molecule directs the incorporation of amino acids into
protein. It occurs on a ribosome.
translational control Control of gene expression by selection of which
mRNAs in the cytoplasm are translated by ribosomes.
translocation Type of mutation in which a portion of one chromosome is
broken off and attached to another.
transmembrane protein Membrane protein that extends through the lipid
bilayer, with part of its mass on either side of the membrane.
transmitter-gated ion channel Ion channel in the postsynaptic plasma
membranes of nerve and muscle cells that opens only in response to the
binding of a specific extracellular neurotransmitter. The resulting inflow of
ions leads to the generation of a local electrical signal in the postsynaptic cell.
transposable element Segment of DNA that can move from one position in a
genome to another. Also called a transposon.
transposition The movement of a DNA sequence from one site to another
within the genome. See also cut-and-paste transposition.
trans-splicing Type of RNA splicing present in a few eucaryotic organisms in
which exons from two separate RNA molecules are joined together to form an
mRNA.
treadmilling The process by which a polymeric protein filament is maintained
at constant length by addition of protein subunits at one end and loss of
subunits at the other. (See Panel 16 2, pp. 912 913.)
triacylglycerol Molecule composed of three fatty acids esterified to glycerol.
The main constituent of fat droplets in animal tissues (where the fatty acids are
saturated) and of vegetable oils (where the fatty acids are mainly unsaturated).
Also known as triglyceride. (See Panel 2 5, pp. 118 119.)
tricarboxylic acid (TCA) cycle see citric acid cycle
trimeric GTP-binding protein see GTP-binding protein
tRNA see transfer RNA
t-SNARE see SNAREs
tubulin The protein subunit of microtubules.
tumor progression The process by which an initial mildly disordered cell
behavior gradually evolves into a full-blown cancer.
tumor suppressor gene Gene that appears to prevent formation of a cancer.
Loss-of-function mutations in such genes enhance susceptibility to cancer.
two-dimensional gel electrophoresis Type of electrophoresis in which the
protein mixture is run first in one direction and then in a direction at right
angles to the first. It enables better separation of individual proteins.
two-hybrid system Technique for identifying interacting proteins using
genetically engineered yeast cells.
type III secretion system A bacterial system for delivering toxic proteins into
the cells of their host.