helix A coiled secondary structure of a polypeptide chain formed by hydrogen bonding between amino acids separated by four residues.
ABC transporters A large family of membrane transport proteins characterized by a highly conserved ATP binding domain.
actin An abundant 43-kd protein that polymerizes to form cytoskeletal filaments.
actin bundle Actin filaments that are crosslinked into closely packed arrays.
actin network Actin filaments that are crosslinked into loose three-dimensional meshworks.
activation energy The energy required to raise a molecule to its transition state to undergo a chemical reaction.
active site The region of an enzyme that binds substrates and catalyzes an enzymatic reaction.
active transport The transport of molecules in an energetically unfavorable direction across a membrane coupled to the hydrolysis of ATP or other source of energy.
adaptin A protein that binds to membrane receptors and mediates the formation of clathrin-coated vesicles.
adenine A purine that base-pairs with either thymine or uracil.
adenoma A benign tumor arising from glandular epithelium.
adenylyl cyclase An enzyme that catalyzes the formation of cyclic AMP from ATP.
adherens junction A region of cell-cell adhesion at which the actin cytoskeleton is anchored to the plasma membrane.
Akt A protein-serine/threonine kinase that is activated by PIP3 and plays a key role in signaling cell survival.
allele One copy of a gene.
allosteric regulation The regulation of enzymes by small molecules that bind to a site distinct from the active site, changing the conformation and catalytic activity of the enzyme.
alternative splicing The generation of different mRNAs by varying the pattern of pre-mRNA splicing.
amino acid Monomeric building blocks of proteins, consisting of a carbon atom bound to a carboxyl group, an amino group, a hydrogen atom, and a distinctive side chain.
aminoacyl tRNA synthetase An enzyme that joins a specific amino acid to a tRNA molecule carrying the correct anticodon sequence.
amphipathic A molecule that has both hydrophobic and hydrophilic regions.
anaphase The phase of mitosis during which sister chromatids separate and move to opposite poles of the spindle.
anaphase A The movement of daughter chromosomes toward the spindle poles during mitosis.
anaphase B The separation of the spindle poles during mitosis.
anaphase-promoting complex A ubiquitin ligase that triggers progression from metaphase to anaphase by signaling the degradation of cyclin B and cohesins.
angiogenesis The formation of new blood vessels.
antibody A protein produced by B lymphocytes that binds to a foreign molecule.
anticodon The nucleotide sequence of transfer RNA that forms complementary base pairs with a codon sequence on messenger RNA.
antigen A molecule against which an antibody is directed.
antiport The transport of two molecules in opposite directions across a membrane.
AP endonuclease A DNA repair enzyme that cleaves next to apyrimidinic or apurinic sites in DNA.
apical domain The exposed free surface of a polarized epithelial cell
apoptosis An active process of programmed cell death, characterized by cleavage of chromosomal DNA, chromatin condensation, and fragmentation of both the nucleus and the cell.
Arabidopsis thaliana A small flowering plant used as a model for plant molecular biology and development.
archaebacteria One of two major groups of prokaryotes; many species of archaebacteria live in extreme conditions similar to those prevalent on primitive Earth.
ARF A GTP-binding protein required for vesicle budding from the trans-Golgi network.
astral microtubules Microtubules of the mitotic spindle that extend to the cell periphery.
ATP (adenosine 5´-triphosphate) An adenine-containing nucleoside triphosphate that serves as a store of free energy in the cell.
ATP synthase A membrane spanning protein complex that couples the energetically favorable transport of protons across a membrane to the synthesis of ATP.
autocrine signaling A type of cell signaling in which a cell produces a growth factor to which it also responds.
autophagy The degradation of cytoplasmic proteins and organelles by their enclosure in vesicles from the endoplasmic reticulum that fuse with lysosomes.
autoradiography The detection of radioisotopically labeled molecules by exposure to X-ray film.
axonemal dynein The type of dynein found in cilia and flagella.
axoneme The fundamental structure of cilia and flagella composed of a central pair of microtubules surrounded by nine microtubule doublets.
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