The Cell Cooper Glossary.txt

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The Cell Cooper Glossary.txt
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2011-07-30 23:32:52
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RVU Molecular Cellular Mechanisms Terms
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  1. ��
    • � helix
    • A coiled secondary structure of a polypeptide chain formed by hydrogen bonding between amino acids separated by four residues.
  2. ABC transporters
    A large family of membrane transport proteins characterized by a highly conserved ATP binding domain.
  3. actin
    An abundant 43-kd protein that polymerizes to form cytoskeletal filaments.
  4. actin bundle
    Actin filaments that are crosslinked into closely packed arrays.
  5. actin network
    Actin filaments that are crosslinked into loose three-dimensional meshworks.
  6. activation energy
    The energy required to raise a molecule to its transition state to undergo a chemical reaction.
  7. active site
    The region of an enzyme that binds substrates and catalyzes an enzymatic reaction.
  8. 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.
  9. adaptin
    A protein that binds to membrane receptors and mediates the formation of clathrin-coated vesicles.
  10. adenine
    A purine that base-pairs with either thymine or uracil.
  11. adenoma
    A benign tumor arising from glandular epithelium.
  12. adenylyl cyclase
    An enzyme that catalyzes the formation of cyclic AMP from ATP.
  13. adherens junction
    A region of cell-cell adhesion at which the actin cytoskeleton is anchored to the plasma membrane.
  14. Akt
    A protein-serine/threonine kinase that is activated by PIP3 and plays a key role in signaling cell survival.
  15. allele
    One copy of a gene.
  16. 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.
  17. alternative splicing
    The generation of different mRNAs by varying the pattern of pre-mRNA splicing.
  18. 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.
  19. aminoacyl tRNA synthetase
    An enzyme that joins a specific amino acid to a tRNA molecule carrying the correct anticodon sequence.
  20. amphipathic
    A molecule that has both hydrophobic and hydrophilic regions.
  21. anaphase
    The phase of mitosis during which sister chromatids separate and move to opposite poles of the spindle.
  22. anaphase A
    The movement of daughter chromosomes toward the spindle poles during mitosis.
  23. anaphase B
    The separation of the spindle poles during mitosis.
  24. anaphase-promoting complex
    A ubiquitin ligase that triggers progression from metaphase to anaphase by signaling the degradation of cyclin B and cohesins.
  25. angiogenesis
    The formation of new blood vessels.
  26. antibody
    A protein produced by B lymphocytes that binds to a foreign molecule.
  27. anticodon
    The nucleotide sequence of transfer RNA that forms complementary base pairs with a codon sequence on messenger RNA.
  28. antigen
    A molecule against which an antibody is directed.
  29. antiport
    The transport of two molecules in opposite directions across a membrane.
  30. AP endonuclease
    A DNA repair enzyme that cleaves next to apyrimidinic or apurinic sites in DNA.
  31. apical domain
    The exposed free surface of a polarized epithelial cell.
  32. 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.
  33. Arabidopsis thaliana
    A small flowering plant used as a model for plant molecular biology and development.
  34. archaebacteria
    One of two major groups of prokaryotes; many species of archaebacteria live in extreme conditions similar to those prevalent on primitive Earth.
  35. ARF
    A GTP-binding protein required for vesicle budding from the trans-Golgi network.
  36. astral microtubules
    Microtubules of the mitotic spindle that extend to the cell periphery.
  37. ATP (adenosine 5-triphosphate)
    An adenine-containing nucleoside triphosphate that serves as a store of free energy in the cell.
  38. ATP synthase
    A membrane spanning protein complex that couples the energetically favorable transport of protons across a membrane to the synthesis of ATP.
  39. autocrine signaling
    A type of cell signaling in which a cell produces a growth factor to which it also responds.
  40. autophagy
    The degradation of cytoplasmic proteins and organelles by their enclosure in vesicles from the endoplasmic reticulum that fuse with lysosomes.
  41. autoradiography
    The detection of radioisotopically labeled molecules by exposure to X-ray film.
  42. axonemal dynein
    The type of dynein found in cilia and flagella.
  43. axoneme
    The fundamental structure of cilia and flagella composed of a central pair of microtubules surrounded by nine microtubule doublets.
  44. � sheet
    A sheetlike secondary structure of a polypeptide chain, formed by hydrogen bonding between amino acids located in different regions of the polypeptide.
  45. bacteriophage
    A bacterial virus.
  46. basal body
    A structure similar to a centriole that initiates the growth of axonemal microtubules and anchors cilia and flagella to the surface of the cell.
  47. basal lamina
    A sheetlike extracellular matrix that supports epithelial cells and surrounds muscle cells, adipose cells, and peripheral nerves.
  48. base-excision repair
    A mechanism of DNA repair in which single damaged bases are removed from a DNA molecule.
  49. basement membrane
    See basal lamina.
  50. basolateral domain
    The surface region of a polarized epithelial cell that is in contact with adjacent cells or the extracellular matrix.
  51. benign tumor
    A tumor that remains confined to its site of origin.
  52. cadherins
    A group of cell adhesion molecules that form stable cell-cell junctions at adherens junctions and desmosomes.
  53. Caenorhabditis elegans
    A nematode used as a simple multicellular model for development.
  54. calmodulin
    A calcium-binding protein.
  55. Calvin cycle
    A series of reactions by which six molecules of CO2 are converted into glucose.
  56. cAMP phosphodiesterase
    An enzyme that degrades cyclic AMP.
  57. cAMP-dependent protein kinase
    See protein kinase A.
  58. cancer
    A malignant tumor.
  59. carbohydrate
    A molecule with the formula (CH2O)n. Carbohydrates include both simple sugars and polysaccharides.
  60. carcinogen
    A cancer-inducing agent.
  61. carcinoma
    A cancer of epithelial cells.
  62. cardiolipin
    A phospholipid containing four hydrocarbon chains.
  63. carrier proteins
    Proteins that selectively bind and transport small molecules across a membrane.
  64. caspases
    A family of proteases that bring about programmed cell death.
  65. catalase
    An enzyme that decomposes hydrogen peroxide.
  66. caveolae
    Small invaginations of the plasma membrane that may be involved in endocytosis.
  67. Cdc2
    A protein-serine/threonine kinase that is a key regulator of mitosis in eukaryotic cells.
  68. Cdk inhibitor (CKI)
    A family of proteins that bind Cdks and inhibit their activity.
  69. Cdks
    Cyclin dependent protein kinases that control the cell cycle of eukaryotes. See also Cdc2.
  70. cDNA library
    A collection of recombinant cDNA clones.
  71. cell adhesion molecules
    Transmembrane proteins that mediate cell-cell interactions.
  72. cell cortex
    The actin network underlying the plasma membrane.
  73. cell wall
    A rigid, porous structure forming an external layer that provides structural support to bacteria, fungi, and plant cells.
  74. cellulose
    The principal structural component of the plant cell wall, a linear polymer of glucose residues linked by b(1�! 4) glycosidic bonds.
  75. central dogma
    The concept that genetic information flows from DNA to RNA to proteins.
  76. centriole
    A cylindrical structure consisting of nine triplets of microtubules in the centrosomes of most animal cells.
  77. centromere
    A specialized chromosomal region that connects sister chromatids and attaches them to the mitotic spindle.
  78. centrosome
    The microtubule-organizing center in animal cells.
  79. cGMP phosphodiesterase
    An enzyme that degrades cGMP.
  80. channel proteins
    Proteins that form pores through a membrane.
  81. chaperone
    A protein that facilitates the correct folding or assembly of other proteins.
  82. chaperonin
    A family of heat-shock proteins within which protein folding takes place.
  83. chemiosmotic coupling
    The generation of ATP from energy stored in a proton gradient across a membrane.
  84. chiasmata
    Sites of recombination that link homologous chromosomes during meiosis.
  85. chitin
    A polymer of N-acetylglucosamine residues that is the principal component of fungal cell walls.
  86. chlorophyll
    The major photosynthetic pigment of plant cells.
  87. chloroplast
    The organelle responsible for photosynthesis in the cells of plants and green algae.
  88. cholesterol
    A lipid consisting of four hydrocarbon rings. Cholesterol is a major constituent of animal cell plasma membranes and the precursor of steroid hormones.
  89. chromatin
    The fibrous complex of eukaryotic DNA and histone proteins. See histones, nucleosome, and chromatosome.
  90. chromatosome
    A chromatin subunit consisting of 166 base pairs of DNA wrapped around a histone core and held in place by a linker histone.
  91. chromosomes
    The carriers of genes, consisting of long DNA molecules and associated proteins.
  92. cilium
    A microtubule-based projection of the plasma membrane that moves a cell through fluid or fluid over a cell.
  93. cis-acting control element
    A regulatory DNA sequence that serves as a protein binding site and controls the transcription of adjacent genes.
  94. citric acid cycle
    A series of reactions in which acetyl CoA is oxidized to CO2. The central pathway of oxidative metabolism.
  95. clathrin
    A protein that coats the cytoplasmic surface of cell membranes and assembles into basketlike lattices that drive vesicle budding.
  96. clone
    See recombinant molecule.
  97. codon
    The basic unit of the genetic code; one of the 64 nucleotide triplets that code for an amino acid or stop sequence.
  98. coenzyme A (CoA)
    A coenzyme that functions as a carrier of acyl groups in metabolic reactions.
  99. coenzyme Q
    A small lipid-soluble molecule that carries electrons between protein complexes in the mitochondrial electron transport chain.
  100. coenzymes
    Low-molecular-weight organic molecules that work together with enzymes to catalyze biological reactions.
  101. cohesins
    A complex of proteins that maintain the connection between sister chromatids.
  102. colcemid
    A drug that inhibits the polymerization of microtubules.
  103. colchicine
    A drug that inhibits the polymerization of microtubules.
  104. collagen
    The major structural protein of the extracellular matrix.
  105. collenchyma
    Plant cells characterized by thick cell walls; they provide structural support to the plant.
  106. complementary DNA (cDNA)
    A DNA molecule that is complementary to an mRNA molecule, synthesized in vitro by reverse transcriptase.
  107. contact inhibition
    The inhibition of movement or proliferation of normal cells that results from cell-cell contact.
  108. contractile ring
    A structure of actin and myosin II that forms beneath the plasma membrane during mitosis and mediates cytokinesis.
  109. corticosteroids
    Steroid hormones produced by the adrenal gland.
  110. cosmid
    A vector that contains bacteriophage 5��� sequences, antibiotic resistance sequences, and an origin of replication. It can accommodate large DNA inserts of up to 45 kb.
  111. crista
    A fold in the inner mitochondrial membrane extending into the matrix.
  112. cyanobacteria
    The largest and most complex prokaryotes in which photosynthesis is believed to have evolved.
  113. cyclic AMP (cAMP)
    Adenosine monophosphate in which the phosphate group is covalently bound to both the 3� and 5� carbon atoms, forming a cyclic structure; an important second messenger in the response of cells to a variety of hormones.
  114. cyclic electron flow
    An electron transport pathway associated with photosystem I that produces ATP without the synthesis of NADPH.
  115. cyclic GMP (cGMP)
    Guanosine monophosphate in which the phosphate group is covalently bound to both the 3� and 5� carbon atoms, forming a cyclic structure; an important second messenger in the response of cells to a variety of hormones and in vision.
  116. cyclins
    A family of proteins that regulate the activity of Cdks and control progression through the cell cycle.
  117. cytochalasin
    A drug that blocks the elongation of actin filaments.
  118. cytochrome oxidase
    A protein complex in the electron transport chain that accepts electrons from cytochrome c and transfers them to O2.
  119. cytokines
    Growth factors that regulate blood cells and lymphocytes.
  120. cytokinesis
    Division of a cell following mitosis or meiosis.
  121. cytosine
    A pyrimidine that base-pairs with guanine.
  122. cytoskeleton
    A network of protein filaments that extends throughout the cytoplasm of eukaryotic cells. It provides the structural framework of the cell and is responsible for cell movements.
  123. density gradient centrifugation
    A method of separating particles by centrifugation through a gradient of a dense substance, such as sucrose or cesium chloride.
  124. deoxyribonucleic acid (DNA)
    The genetic material of the cell.
  125. desmosome
    A region of contact between epithelial cells at which keratin filaments are anchored to the plasma membrane. See also hemidesmosome.
  126. diacylglycerol
    A secondary messenger formed from the hydrolysis of PIP2 that activates protein kinase C.
  127. diakinesis
    The final stage of the prophase of meiosis I during which the chromosomes fully condense and the cell progresses to metaphase.
  128. Dictyostelium discoideum
    A unicellular eukaryote used for studies of cell movement and cell-cell signaling.
  129. diploid
    An organism or cell that carries two copies of each chromosome.
  130. diplotene
    The stage of mieosis I during which homologous chromosomes separate along their length but remain associated at chiasmata.
  131. DNA glycosylase
    A DNA repair enzyme that cleaves the bond linking a purine or pyrimidine to the deoxyribose of the backbone of a DNA molecule.
  132. DNA ligase
    An enzyme that seals breaks in DNA strands.
  133. DNA polymerase
    An enzyme catalyzing the synthesis of DNA.
  134. dolichol phosphate
    A lipid molecule in the endoplasmic reticulum upon which oligosaccharides are assembled for the glycosylation of proteins.
  135. domains
    Compact, globular regions of proteins that are the basic units of tertiary structure.
  136. dominant
    The allele that determines the phenotype of an organism when more than one allele is present.
  137. dominant inhibitory mutant
    A mutant that interferes with the function of the normal allele of the gene.
  138. Drosophila melanogaster
    A species of fruit fly commonly used for studies of animal genetics and development.
  139. dynein
    A motor protein that moves along microtubules towards the minus end.
  140. E. coli (Escherichia coli)
    A species of bacteria used as a model for biochemistry and molecular biology.
  141. E2F
    A family of transcription factors that regulate the expression of genes involved in cell cycle progression and DNA replication.
  142. ecdysone
    An insect steroid hormone that triggers metamorphosis.
  143. ectoderm
    The outer germ layer; gives rise to tissues that include the skin and nervous system.
  144. eicosanoid
    A class of lipids, including prostaglandins, prostacyclins, thromboxanes, and leukotrienes, that act in autocrine and paracrine signaling.
  145. electrochemical gradient
    A difference in chemical concentration and electric potential across a membrane.
  146. electron microscopy
    A type of microscopy that uses an electron beam to form an image. In transmission electron microscopy, a beam of electrons is passed through a specimen stained with heavy metals. In scanning electron microscopy, electrons scattered from the surface of a specimen are analyzed to generate a three-dimensional image.
  147. electron transport chain
    A series of carriers through which electrons are transported from a higher to a lower energy state.
  148. embryonal stem (ES) cells
    Cells cultured from early embryos.
  149. endocrine signaling
    A type of cell-cell signaling in which endocrine cells secrete hormones that are carried by the circulation to distant target cells.
  150. endocytosis
    The uptake of extracellular material in vesicles formed from the plasma membrane.
  151. endoderm
    The inner germ layer; gives rise to internal organs.
  152. endoplasmic reticulum (ER)
    An extensive network of membrane-enclosed tubules and sacs involved in protein sorting and processing as well as in lipid synthesis.
  153. endosome
    A vesicular compartment involved in the sorting and transport to lysosomes of material taken up by endocytosis.
  154. endosymbiosis
    A symbiotic relationship in which one cell resides within a larger cell.
  155. enhancer
    A transcriptional regulatory sequence that can be located at a site distant from the promoter.
  156. enzymes
    Proteins or RNAs that catalyze biological reactions.
  157. epidermal cells
    Cells forming a protective layer on the surfaces of plants and animals.
  158. epidermal growth factor (EGF)
    A growth factor that stimulates cell proliferation.
  159. epithelial cells
    Cells forming sheets (epithelial tissue) that cover the surface of the body and line internal organs.
  160. equilibrium centrifugation
    The separation of particles on the basis of density by centrifugation to equilibrium in a gradient of a dense substance.
  161. erythrocytes
    Red blood cells.
  162. estrogen
    A steroid hormone produced by the ovaries.
  163. eubacteria
    One of two major groups of prokaryotes, including most common species of bacteria.
  164. euchromatin
    Decondensed, transcriptionally active interphase chromatin.
  165. eukaryotic cells
    Cells that have a nuclear envelope, cytoplasmic organelles, and a cytoskeleton.
  166. exon
    A segment of a gene that contains a coding sequence.
  167. exonuclease
    An enzyme that hydrolyzes DNA molecules in either the 5� to 3� or 3� to 5� direction.
  168. exportin
    A receptor protein that recognizes nuclear export signals and directs transport from the nucleus to the cytosol.
  169. expression vector
    A vector used to direct expression of a cloned DNA fragment in a host cell.
  170. extracellular matrix
    Secreted proteins and polysaccharides that fill spaces between cells and bind cells and tissues together.
  171. facilitated diffusion
    The transport of molecules across a membrane by carrier or channel proteins.
  172. FAK (focal adhesion kinase)
    A nonreceptor protein-tyrosine kinase that plays a key role in integrin signaling.
  173. fats
    See triacylglycerols.
  174. fatty acids
    Long hydrocarbon chains usually linked to a carboxyl group (COO-).
  175. feedback inhibition
    A type of allosteric regulation in which the product of a metabolic pathway inhibits the activity of an enzyme involved in its synthesis.
  176. fibroblast
    A cell type found in connective tissue.
  177. fibronectin
    The principal adhesion protein of the extracellular matrix.
  178. filopodium
    A thin projection of the plasma membrane supported by actin bundles.
  179. flagellum
    A microtubule-based projection of the plasma membrane that is responsible for cell movement.
  180. flavin adenine dinucleotide (FADH2)
    A coenzyme that functions as an electron carrier in oxidation/reduction reactions.
  181. flow cytometer
    An instrument that measures the fluoresence intensity of individual cells.
  182. fluid mosaic model
    A model of membrane structure in which proteins are inserted in a fluid phospholipid bilayer.
  183. fluid-phase endocytosis
    The nonselective uptake of extracellular fluids during endocytosis.
  184. fluorescence in situ hybridization (FISH)
    A method used to localize genes on chromosomes using fluorescent probes.
  185. fluorescence microscopy
    Type of microscopy in which molecules are detected based on the emission of fluorescent light.
  186. focal adhesion
    A site of attachment of cells to the extracellular matrix at which integrins are linked to bundles of actin filaments.
  187. freeze fracture
    Method of electron microscopy in which specimens are frozen in liquid nitrogen and then fractured to split the lipid bilayer, revealing the interior faces of cell membranes.
  188. G-protein
    A family of cell signaling proteins regulated by guanine nucleotide binding.
  189. G protein-coupled receptor
    A receptor characterized by seven membrane-spanning a helices. Ligand binding causes a conformational change that activates a G protein.
  190. G0
    A quiescent state in which cells remain metabolically active but do not proliferate.
  191. G1 phase
    The phase of the cell cycle between the end of mitosis and the begining of DNA synthesis.
  192. G2 phase
    The phase of the cell cycle between the end of S phase and the begining of mitosis.
  193. gap junction
    A plasma membrane channel forming a direct cytoplasmic connection between adjacent cells.
  194. gene
    A segment of DNA that encodes a polypeptide chain or an RNA molecule.
  195. gene amplification
    An increase in the number of copies of a gene resulting from the repeated replication of a region of DNA.
  196. gene transfer
    The introduction of foreign DNA into a cell.
  197. general homologous recombination
    The exchange of segments between DNA molecules that share extensive sequence homology.
  198. general transcription factors
    Transcription factors that are part of the general transcription machinery.
  199. genetic code
    The correspondence between nucleotide triplets and amino acids in proteins.
  200. genomic imprinting
    The regulation of genes whose expression depends on whether they are maternally or paternally inherited, apparently controlled by DNA methylation.
  201. genomic library
    A collection of recombinant DNA clones that collectively contain the genome of an organism.
  202. genotype
    The genetic composition of an organism.
  203. Gibbs free energy (G)
    The thermodynamic function that combines the effects of enthalpy and entropy to predict the energetically favorable direction of a chemical reaction.
  204. gluconeogenesis
    The synthesis of glucose.
  205. glycerol phospholipids
    Phospholipids consisting of two fatty acids bound to a glycerol molecule.
  206. glycocalyx
    A carbohydrate coat covering the cell surface.
  207. glycogen
    A polymer of glucose residues that is the principal storage form of carbohydrates in animals.
  208. glycolipid
    A lipid consisting of two hydrocarbon chains linked to a polar head group containing carbohydrates.
  209. glycolysis
    The anaerobic breakdown of glucose.
  210. glycoprotein
    A protein linked to oligosaccharides.
  211. glycosaminoglycan (GAG)
    A gel-forming polysaccharide of the extracellular matrix.
  212. glycosidic bond
    The bond formed between sugar residues in oligosaccharides or polysaccharides.
  213. glycosylation
    The addition of carbohydrates to proteins.
  214. glycosylphosphatidylinositol (GPI) anchor
    Glycolipids containing phosphatidylinositol that anchor proteins to the external face of the plasma membrane.
  215. glyoxylate cycle
    The conversion of fatty acids to carbohydrates in plants.
  216. Golgi apparatus
    A cytoplasmic organelle involved in the processing and sorting of proteins and lipids. In plant cells, it is also the site of the synthesis of cell wall polysaccharides.
  217. green fluorescent protein (GFP)
    A protein from jellyfish that is commonly used as a marker for fluorescence microscopy.
  218. growth factors
    Polypeptides that control animal cell growth and differentiation.
  219. guanine
    A purine that base-pairs with cytosine.
  220. guanylyl cyclase
    An enzyme that catalyzes the formation of cyclic GMP from GTP.
  221. haploid
    An organism or cell that has one copy of each chromosome.
  222. heat-shock proteins
    A highly conserved group of chaperone proteins expressed in cells exposed to elevated temperatures or other forms of environmental stress.
  223. helicase
    An enzyme that catalyzes the unwinding of DNA.
  224. hemicellulose
    A polysaccharide that crosslinks cellulose microfibrils in plant cell walls.
  225. hemidesmosome
    A region of contact between cells and the extracellular matrix at which keratin filaments are attached to integrin.
  226. heterochromatin
    Condensed, transcriptionally inactive chromatin.
  227. high-energy bonds
    Chemical bonds that release a large amount of free energy when they are hydrolyzed.
  228. histones
    Proteins that package DNA in eukaryotic chromosomes.
  229. HMG-14
    and HMG-17Nonhistone chromosomal proteins associated with decondensed transcriptionally active chromatin.
  230. Holliday junction
    The central intermediate in recombination, consisting of a crossed-strand structure formed by homologous base pairing between strands of two DNA moleucles.
  231. homeobox
    Conserved DNA sequences of 180 base pairs that encode homeodomains.
  232. homeodomain
    A type of DNA binding domain found in transcription factors that regulate gene expression during embryonic development.
  233. homologous recombination
    Recombination between segments of DNA with homologous nucleotide sequences.
  234. hormones
    Signaling molecules produced by endocrine glands that act on cells at distant body sites.
  235. hydrophilic
    Soluble in water.
  236. hydrophobic
    Not soluble in water.
  237. immunoblotting
    A method that uses antibodies to detect proteins separated by SDS-polyacrylamide gel electrophoresis.
  238. immunoglobulin
    See antibody.
  239. immunoprecipitation
    The use of antibodies to isolate proteins.
  240. importin
    A receptor protein that recognizes nuclear localization signals and directs nuclear import.
  241. in situ hybridization
    The use of radioactive or fluorescent probes to detect RNA or DNA sequences in cell extracts, chromosomes, or intact cells.
  242. in vitro mutagenesis
    The introduction of mutations into cloned DNA in vitro.
  243. in vitro translation
    Protein synthesis in a cell-free extract.
  244. inositol 1,4,5-trisphosphate (IP3)
    A second messenger, formed from the hydrolysis of PIP2, that signals the release of calcium ions from the endoplasmic reticulum.
  245. integral membrane proteins
    Proteins embedded within the lipid bilayer of cell membranes.
  246. integrin
    A transmembrane protein that mediates the adhesion of cells to the extracellular matrix.
  247. intermediate filament
    A cytoskeletal filament about 10 nm in diameter that provides mechanical strength to cells in tissues. See also keratins and neurofilaments.
  248. interphase
    The period of the cell cycle between mitoses that includes G1, S, and G2 phases.
  249. intracellular signal transduction
    A chain of reactions that transmits chemical signals from the cell surface to their intracellular targets.
  250. intron
    A noncoding sequence that interrupts exons in a gene.
  251. ion channel
    A protein that mediates the rapid passage of ions across a membrane by forming open pores through the phospholipid bilayer.
  252. ion pump
    A protein that couples ATP hydrolysis to the transport of ions across a membrane.
  253. Janus kinase (JAK)
    A family of nonreceptor protein-tyrosine kinases associated with cytokine receptors.
  254. keratin
    A type of intermediate filament protein of epithelial cells.
  255. kilobase (kb)
    One thousand nucleotides or nucleotide base pairs.
  256. kinesin
    A motor protein that moves along microtubules towards the plus end.
  257. kinetochore
    A specialized structure consisting of proteins attached to a centromere that mediates the attachment and movement of chromosomes along the mitotic spindle.
  258. kinetochore microtubules
    Microtubules of the mito-tic spindle that attach to condensed chromosomes at their centromeres.
  259. Krebs cycle
    See citric acid cycle.
  260. lagging strand
    The strand of DNA synthesized opposite to the direction of movement of the replication fork by ligation of Okazaki fragments.
  261. lamellipodium
    A broad, actin-based extension of the plasma membrane involved in the movement of fibroblasts.
  262. laminin
    The principal adhesion protein of basal laminae.
  263. lamins
    Intermediate filament proteins that form the nuclear lamina.
  264. leading strand
    The strand of DNA synthesized continuously in the direction of movement of the replication fork.
  265. leptotene
    The initial stage of the extended prophase of meiosis I during which homologous chromosomes pair before condensation.
  266. leucine zipper
    A protein dimerization domain containing repeated leucine residues; found in many transcription factors.
  267. leukemia
    Cancer arising from the precursors of circulating blood cells.
  268. ligand
    A molecule that binds to a receptor.
  269. light reactions
    The reactions of photosynthesis in which solar energy drives the synthesis of ATP and NADPH.
  270. lignin
    A polymer of phenolic residues that strengthens secondary cell walls.
  271. lipids
    Hydrophobic molecules that function as energy storage molecules, signaling molecules, and the major components of cell membranes.
  272. long terminal repeat (LTR)
    DNA sequences found at the ends of retroviral and retrotransposon DNA that are direct repeats of several hundred nucleotides resulting from reverse transcriptase activity.
  273. lymphocyte
    A blood cell that functions in the immune response. B lymphocytes produce antibodies and T lymphocytes are responsible for cell mediated immunity.
  274. lymphoma
    A cancer of lymphoid cells.
  275. lysogeny
    A viral infection leading to integration of an inactive copy of viral DNA into the cell genome.
  276. lysosome
    A cytoplasmic organelle containing enzymes that break down biological polymers.
  277. M phase
    The mitotic phase of the cell cycle.
  278. macrophage
    A type of white blood cell specialized for phagocytosis.
  279. malignant tumor
    A tumor that invades normal tissue and spreads throughout the body.
  280. MAP kinases
    A family of mitogen-activated protein-serine/threonine kinases that are ubiquitous regulators of cell growth and differentiation.
  281. matrix
    The inner mitochondrial space.
  282. maturation promoting factor (MPF)
    A complex of Cdc2 and cyclin B that promotes entry into the M phase of either mitosis or meiosis.
  283. megabase (Mb)
    One million nucleotides or nucleotide base pairs.
  284. meiosis
    The division of diploid cells to haploid progeny, consisting of two sequential rounds of nuclear and cellular division.
  285. membrane-anchored growth factors
    Growth factors associated with the plasma membrane that function as signaling molecules during cell-cell contact.
  286. mesoderm
    The middle germ layer; gives rise to connective tissues and the hematopoietic system.
  287. messenger RNA (mRNA)
    An RNA molecule that serves as a template for protein synthesis.
  288. metaphase
    The phase of mitosis during which the chromosomes are aligned on a metaphase plate in the center of the cell.
  289. metastasis
    Spread of cancer cells through the blood or lymphatic system to other organ sites.
  290. 5� methylguanosine cap
    A structure consisting of GTP and methylated sugars that is added to the 5� ends of eukaryotic mRNAs.
  291. microfilament
    A cytoskeleton filament composed of actin.
  292. microspike
    See filopodium.
  293. microtubule
    A cytoskeletal component formed by the polymerization of tubulin into rigid, hollow rods about 25 nm in diameter.
  294. microtubule-organizing center
    An anchoring point near the center of the cell from which most microtubules extend outward.
  295. microvillus
    An actin-based protrusion of the plasma membrane, abundant on the surfaces of cells involved in absorption.
  296. mismatch repair
    A repair system that removes mismatched bases from newly synthesized DNA strands.
  297. mitochondria
    Cytoplasmic organelles responsible for synthesis of most of the ATP in eukaryotic cells by oxidative phosphorylation.
  298. mitosis
    Nuclear division.
  299. mitotic spindle
    An array of microtubules extending from the spindle poles that is responsible for separating daughter chromosomes during mitosis. See also kinetochore microtubules, polar microtubules, and astral microtubules.
  300. molecular motor
    A protein that generates force and movement by converting chemical energy to mechanical energy.
  301. monocistronic
    Messenger RNAs that encode a single polypeptide chain.
  302. monoclonal antibody
    An antibody produced by a clonal line of B lymphocytes.
  303. monosaccharides
    Simple sugars with the basic formula of (CH2O)n.
  304. mutagen
    A chemical that induces a high frequency of mutations.
  305. mutation
    A genetic alteration.
  306. myosin
    A protein that interacts with actin as a molecular motor.
  307. N-myristoylation
    The addition of myristic acid (a 14-carbon fatty acid) to the N-terminal glycine residue of a polypeptide chain.
  308. Na+-K+ pump
    An ion pump that transports Na+ out of the cell and K+ into the cell.
  309. Nernst equation
    The relationship between ion concentration and membrane potential.
  310. neurofilament
    A type of intermediate filament that supports the axons of nerve cells.
  311. neuron
    A nerve cell specialized to receive and transmit signals throughout the body.
  312. neurotransmitter
    A small, hydrophilic molecule that carries a signal from a stimulated neuron to a target cell at a synapse.
  313. nexin
    A protein that links microtubule doublets to each other in the axoneme.
  314. nicotinamide-adenine dinucleotide (NAD+)
    A coenzyme that functions as an electron carrier in oxidation/reduction reactions.
  315. nitrogen fixation
    The reduction of atmospheric nitrogen (N2) to NH3.
  316. nonreceptor protein-tyrosine kinase
    An intracellular protein-tyrosine kinase.
  317. nuclear envelope
    The barrier separating the nucleus from the cytoplasm, composed of an inner and outer membrane, a nuclear lamina, and nuclear pore complexes.
  318. nuclear export signal
    An amino acid sequence that targets proteins for transport from the nucleus to the cytosol.
  319. nuclear lamina
    A meshwork of lamin filaments providing structural support to the nucleus.
  320. nuclear localization signal
    An amino acid sequence that targets proteins for transportation from the cytoplasm to the nucleus.
  321. nuclear membranes
    Membranes forming the nuclear envelope; the outer nuclear membrane is continuous with the endoplasmic reticulum and the inner nuclear membrane is adjacent to the nuclear lamina.
  322. nuclear pore complex
    A large structure forming a transport channel through the nuclear envelope.
  323. nucleic acid hybridization
    The formation of double stranded DNA and/or RNA molecules by complementary base pairing.
  324. nucleolus
    The nuclear site of rRNA transcription, processing, and ribosome assembly.
  325. nucleoside
    A purine or pyrimidine base linked to a sugar (ribose or deoxyribose).
  326. nucleosome
    The basic structural unit of chromatin consisting of DNA wrapped around a histone core.
  327. nucleosome remodeling factors
    Proteins that disrupt chromatin structure, allowing transcription factors to bind nucleosomal DNA.
  328. nucleotide
    A phosphorylated nucleoside.
  329. nucleotide excision repair
    A mechanism of DNA repair in which oligonucleotides containing damaged bases are removed from a DNA molecule.
  330. nucleus
    The most prominent organelle of eukaryotic cells; contains the genetic material.
  331. Okazaki fragments
    Short DNA fragments that are joined to form the lagging strand of DNA.
  332. oligonucleotide
    A short polymer of only a few nucleotides.
  333. oligosaccharide
    A short polymer of only a few sugars.
  334. oncogene
    A gene capable of inducing one or more characteristics of cancer cells.
  335. operator
    A regulatory sequence of DNA that controls transcription of an operon.
  336. operon
    A group of adjacent genes transcribed as a single mRNA.
  337. origin of replication
    A specific DNA sequence that serves as a binding site for proteins that initiate replication.
  338. origin replication complex (ORC)
    A protein complex that initiates DNA replication at yeast origins.
  339. oxidative phosphorylation
    The synthesis of ATP from ADP coupled to the energetically favorable transfer of electrons to molecular oxygen as the final acceptor in an electron transport chain.
  340. p53
    A transcription factor (encoded by the p53 tumor suppressor gene) that arrests the cell cycle in G1 in response to damaged DNA and is required for apoptosis induced by a variety of stimuli.
  341. pachytene
    The stage of meiosis I during which recombination takes place between homologous chromosomes.
  342. palmitoylation
    The addition of palmitic acid (a 16-carbon fatty acid) to cysteine residues of a polypeptide chain.
  343. paracrine signaling
    Local cell-cell signaling in which a molecule released by one cell acts on a neighboring target cell.
  344. parenchyma cell
    A type of plant cell responsible for most metabolic activities.
  345. passive diffusion
    The diffusion of small hydrophobic molecules through a phospholipid bilayer.
  346. passive transport
    The transport of molecules across a membrane in the energetically favorable direction.
  347. pectin
    A gel-forming polysaccharide in plant cell walls.
  348. peptide bond
    The bond joining amino acids in polypeptide chains.
  349. peptidoglycan
    The principal component of bacterial cell walls consisting of linear polysaccharide chains crosslinked by short peptides.
  350. peptidyl prolyl isomerase
    An enzyme that facilitates protein folding by catalyzing the cis-trans isomerization of prolyl peptide bonds.
  351. pericentriolar material
    The material in the centrosome that initiates microtubule assembly.
  352. peripheral membrane proteins
    Proteins indirectly associated with cell membranes by protein-protein interactions.
  353. peroxisome
    A cytoplasmic organelle specialized for carrying out oxidative reactions.
  354. phagocytosis
    The uptake of large particles, such as bacteria, by a cell.
  355. phalloidin
    A drug that binds to actin filaments and prevents their disassembly.
  356. phenotype
    The physical appearance of an organism.
  357. phorbol esters
    A class of tumor promoters that stimulate protein kinase C by acting as analogs of diacylglycerol.
  358. phosphatidylinositide 3-kinase (PI 3-kinase)
    An enzyme that phosphorylates PIP2, yielding the second messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3).
  359. phosphatidylinositol 4,5-bisphosphate (PIP2)
    A minor phospholipid component of the inner leaflet of the plasma membrane. Hormones and growth factors stimulate its hydrolysis by phospholipase C, yielding the second messengers diacylglycerol and inositol trisphosphate.
  360. phosphodiester bond
    A bond between the 5�-phosphate of one nucleotide and the 3�-hydroxyl of another.
  361. phospholipase C
    An enzyme that hydrolyzes PIP2 to form the second messengers diacylglycerol and inositol trisphosphate.
  362. phospholipid bilayer
    The basic structure of biological membranes, in which the hydrophobic tails of phospholipids are buried in the interior of the membrane and their polar head groups are exposed to the aqueous solution on either side.
  363. phospholipid transfer protein
    A protein that transports phospholipid molecules between cell membranes.
  364. phospholipids
    The principal components of cell membranes, consisting of two hydrocarbon chains (usually fatty acids) joined to a polar head group containing phosphate.
  365. phosphorylation
    The addition of a phosphate group to a molecule.
  366. photoreactivation
    A mechanism of DNA repair in which solar energy is used to split pyrimidine dimers.
  367. photosynthesis
    The process by which cells harness energy from sunlight and synthesize glucose from CO2 and water.
  368. pinocytosis
    The uptake of fluids or molecules into a cell by small vesicles.
  369. plant hormones
    A group of small molecules that coordinate the responses of plant tissues to environmental signals.
  370. plasma membrane
    A phospholipid bilayer with associated proteins that surrounds the cell.
  371. plasmalogens
    A family of phospholipids that have an ether bond and an ester bond.
  372. plasmid
    A small, circular DNA molecule capable of independent replication in a host cell.
  373. plasmodesma
    A cytoplasmic connection between adjacent plant cells formed by a continuous region of the plasma membrane.
  374. plastids
    A family of plant organelles including chloroplasts, chromoplasts, leucoplasts, amyloplasts, and elaioplasts.
  375. platelet-derived growth factor (PDGF)
    A growth factor released by platelets during blood clotting to stimulate the proliferation of fibroblasts.
  376. polar microtubules
    Microtubules of the mitotic spindle that overlap in the center of the cell and push the spindle poles apart.
  377. poly-A tail
    A tract of about 200 adenine nucleotides added to the 3� ends of eukaryotic mRNAs.
  378. polyadenylation
    The process of adding a poly-A tail to a pre-mRNA.
  379. polycistronic
    Messenger RNAs that encode multiple polypeptide chains.
  380. polymerase chain reaction (PCR)
    A method for amplifying a region of DNA by repeated cycles of DNA synthesis in vitro.
  381. polynucleotide
    A polymer containing up to millions of nucleotides.
  382. polyp
    A benign tumor projecting from an epithelial surface.
  383. polypeptide
    A polymer of amino acids.
  384. polysaccharide
    A polymer containing hundreds or thousands of sugars.
  385. polysome
    A series of ribosomes translating a messenger RNA.
  386. pre-mRNA
    The primary transcripts that are processed to form messenger RNAs in eukaryotic cells.
  387. pre-rRNA
    The primary transcript, which is cleaved to form individual ribosomal RNAs (the 28S, 18S, and 5.8S rRNAs of eukaryotic cells).
  388. pre-tRNA
    The primary transcript, which is cleaved to form transfer RNAs.
  389. prenylation
    The addition of specific types of lipids (prenyl groups) to C terminal cysteine residues of a polypeptide chain.
  390. primary structure
    The sequence of amino acids in a polypeptide chain.
  391. primase
    An RNA polymerase used to initiate DNA synthesis.
  392. processed psuedogene
    A psuedogene that has arisen by reverse transcription of mRNA.
  393. progesterone
    A steroid hormone produced by the ovaries.
  394. programmed cell death
    A normal physiological form of cell death characterized by apoptosis.
  395. prokaryotic cells
    Cells lacking a nuclear envelope, cytoplasmic organelles, and a cytoskeleton (primarily bacteria).
  396. prometaphase
    A transition period between prophase and metaphase during which the microtubules of the mitotic spindle attach to the kinetochores and the chromosomes shuffle until they align in the center of the cell.
  397. promoter
    A DNA sequence to which RNA polymerase binds to initiate transcription.
  398. pronuclei
    Two haploid nuclei in a newly fertilized egg.
  399. proofreading
    The selective removal of mismatched bases by DNA polymerase.
  400. prophase
    The beginning phase of mitosis, marked by the appearance of condensed chromosomes and the development of the mitotic spindle.
  401. prosthetic groups
    Small molecules bound to proteins.
  402. proteasome
    A large protease complex that degrades proteins tagged by ubiquitin.
  403. protein disulfide isomerase
    An enzyme that catalyzes the formation and breakage of disulfide (S S) linkages.
  404. protein kinase
    An enzyme that phosphorylates proteins by transferring a phosphate group from ATP.
  405. protein kinase A
    A protein kinase regulated by cyclic AMP.
  406. protein kinase C
    A family of protein-serine/threonine kinases that are activated by diacylglycerol and Ca2+ and function in intracellular signal transduction.
  407. protein phosphatase
    An enzyme that reverses the action of protein kinases by removing phosphate groups from phosphorylated amino acid residues.
  408. protein-serine/threonine kinase
    A protein kinase that phosphorylates serine and threonine residues.
  409. protein-tyrosine kinase
    A protein kinase that phosphorylates tyrosine residues.
  410. protein-tyrosine phosphatase
    An enzyme that removes the phosphate groups from phosphotyrosine residues.
  411. proteins
    Polypeptides with a unique amino acid sequence.
  412. proteoglycan
    A protein linked to glycosaminoglycans.
  413. proteolysis
    Degradation of polypeptide chains.
  414. proto-oncogene
    A normal cell gene that can be converted into an oncogene.
  415. pseudogene
    A nonfunctional gene copy.
  416. pseudopodium
    An actin-based extension of the plasma membrane responsible for phagocytosis and amoeboid movement.
  417. PTEN
    A lipid phosphatase that dephosphorylates PIP3 and acts as a tumor suppressor.
  418. pyrimidine dimer
    A common form of DNA damage caused by UV light in which adjacent pyrimidines are joined to form a dimer.
  419. quaternary structure
    The interactions between polypeptide chains in proteins consisting of more than one polypeptide.
  420. Raf
    A protein-serine/threonine kinase (encoded by the raf oncogene) that is activated by Ras and leads to activation of MAP kinase.
  421. Ran
    A small GTP-binding protein involved in nuclear import and export.
  422. Ras
    A family of small GTP binding proteins (encoded by the ras oncogenes) that couple growth factor receptors to intracellular targets, including the Raf protein-serine/threonine kinase and the MAP kinase pathway.
  423. Rb
    A transcriptional regulatory protein encoded by a tumor suppressor gene that was identified by the genetic analysis of retinoblastoma.
  424. receptor protein-tyrosine kinase
    Membrane-spanning protein-tyrosine kinases that are receptors for extracellular ligands.
  425. receptor-mediated endocytosis
    The selective uptake of macromolecules that bind to cell surface receptors that concentrate in clathrin-coated pits.
  426. recessive
    An allele that is masked by a dominant allele.
  427. recombinant DNA library
    A collection of genomic or cDNA clones.
  428. recombinant molecule
    A DNA insert joined to a vector.
  429. recombination
    The exchange of genetic material.
  430. replication fork
    The region of DNA synthesis where the parental strands separate and two new daughter strands elongate.
  431. repressor
    A regulatory molecule that blocks transcription.
  432. restriction endonuclease
    An enzyme that cleaves DNA at a specific sequence.
  433. restriction map
    The locations of restriction endonuclease cleavage sites on a DNA molecule.
  434. restriction point
    A regulatory point in animal cell cycles that occurs late in G1. After this point, a cell is committed to entering S and undergoing one cell division cycle.
  435. retroposon
    A transposable element that moves via reverse transcription of an RNA intermediate.
  436. retrovirus
    A virus that replicates by making a DNA copy of its RNA genome by reverse transcription.
  437. reverse transcriptase
    A DNA polymerase that uses an RNA template.
  438. rhodopsin
    A G protein-coupled photoreceptor in retinal rod cells that activates transducin in response to light absorption.
  439. ribonucleic acid (RNA)
    A polymer of ribonucleotides.
  440. ribosomal RNA (rRNA)
    The RNA component of ribosomes.
  441. ribosomes
    Particles composed of RNA and proteins that are the sites of protein synthesis.
  442. ribozyme
    An RNA enzyme.
  443. RNA editing
    RNA processing events other than splicing that alter the protein coding sequences of mRNAs.
  444. RNA polymerase
    An enzyme that catalyzes the synthesis of RNA.
  445. RNA splicing
    The joining of exons in a precursor RNA molecule.
  446. RNase H
    An enzyme that degrades the RNA strand of RNA-DNA hybrid molecules.
  447. RNase P
    A ribozyme that cleaves the 5� end of pre-tRNAs.
  448. rough endoplasmic reticulum
    The region of the endoplasmic reticulum covered with ribosomes and involved in protein metabolism.
  449. Rous sarcoma virus (RSV)
    An acutely transforming retrovirus, in which the first oncogene was identified.
  450. ryanodine receptors
    Calcium channels in muscle and nerve cells that open in response to changes in membrane potential.
  451. S phase
    The phase of the cell cycle during which DNA replication occurs.
  452. sarcoma
    A cancer of cells of connective tissue.
  453. sarcomere
    The contractile unit of muscle cells composed of interacting myosin and actin filaments.
  454. sarcoplasmic reticulum
    A specialized network of membranes in muscle cells that stores a high concentration of Ca2+.
  455. scanning electron microscopy
    See electron microscopy.
  456. sclerenchyma cells
    Plant cells characterized by thick cell walls that provide structural support to the plant.
  457. SDS-polyacrylamide gel electrophoresis (SDS-PAGE)
    A commonly used method to separate proteins by gel electrophoresis on the basis of size.
  458. second messenger
    A compound whose metabolism is modified as a result of a ligand-receptor interaction; it functions as a signal transducer by regulating other intracellular processes.
  459. secondary structure
    The regular arrangement of amino acids within localized regions of a polypeptide chain. See a helix and b sheet.
  460. secretory vesicles
    Membrane-enclosed sacs that transport proteins from the Golgi apparatus to the cell surface.
  461. selectins
    Cell adhesion molecules that recognize oligosaccharides exposed on the cell surface.
  462. self-splicing
    The ability of some RNAs to catalyze the removal of their own introns.
  463. SH2 domain
    A protein domain of approximately 100 amino acids that binds phosphotyrosine-containing peptides.
  464. Shine-Delgarno sequence
    The sequence prior to the initiation site that correctly aligns bacterial mRNAs on ribosomes.
  465. signal patch
    A recognition determinant formed by the three-dimensional folding of a polypeptide chain.
  466. signal peptidase
    An enzyme that removes the signal sequence of a polypeptide chain by proteolysis.
  467. signal recognition particle (SRP)
    A particle composed of proteins and 7SL RNA that binds to signal sequences and targets polypeptide chains to the endoplasmic reticulum.
  468. signal sequence
    A hydrophobic sequence at the amino terminus of a polypeptide chain that targets it for secretion in bacteria or incorporation into the endoplasmic reticulum in eukaryotic cells.
  469. site-specific recombination
    Recombination mediated by proteins that recognize specific DNA sequences.
  470. smooth endoplasmic reticulum
    The major site of lipid synthesis in eukaryotic cells.
  471. Southern blotting
    A method in which radioactive probes are used to detect specific DNA fragments that have been separated by gel electrophoresis.
  472. spectrin
    A major actin-binding protein of the cell cortex.
  473. sphingomyelin
    A phospholipid consisting of two hydrocarbon chains bound to a polar head group containing serine.
  474. spliceosomes
    Large complexes of snRNAs and proteins that catalyze the splicing of pre-mRNAs.
  475. Src
    A nonreceptor protein-tyrosine kinase encoded by the oncogene (src ) of Rous sarcoma virus.
  476. starch
    A polymer of glucose residues that is the principal storage form of carbohydrates in plants.
  477. START
    A regulatory point in the yeast cell cycle that occurs late in G1. After this point a cell is committed to entering S and undergoing one cell division cycle.
  478. STAT proteins
    Trancription factors that have an SH2 domain and are activated by tyrosine phosphorylation, which promotes their translocation from the cytoplasm to the nucleus.
  479. stem cell
    A cell that divides to produce daughter cells that can either differentiate or remain as stem cells.
  480. stereocilium
    A specialized microvillus of auditory hair cells.
  481. steroid hormones
    A group of hydrophobic hormones that are derivatives of cholesterol.
  482. steroid receptor superfamily
    A family of transcription factors that regulate gene expression in response to steroids and related hormones.
  483. substrate
    A molecule acted upon by an enzyme.
  484. symport
    The transport of two molecules in the same direction across a membrane.
  485. synapse
    The junction between a neuron and another cell, across which information is carried by neurotransmitters.
  486. synapsis
    The association of homologous chromosomes during meiosis.
  487. synaptic vesicle
    A secretory vesicle that releases neurotransmitters at a synapse.
  488. synaptonemal complex
    A zipperlike protein structure that forms along the length of paired homologous chromosomes during meiosis.
  489. T cell receptor
    A T lymphocyte surface protein that recognizes antigens expressed on the surface of other cells.
  490. TATA box
    A regulatory DNA sequence found in the promoters of many eukaryotic genes transcribed by RNA polymerase II.
  491. TATA-binding protein (TBP)
    A basal transcription factor that binds directly to the TATA box.
  492. taxol
    A drug that binds to and stabilizes microtubules.
  493. telomerase
    A reverse transcriptase that synthesizes telomeric repeat sequences at the ends of chromosomes from its own RNA template.
  494. telomeres
    Repeats of simple-sequence DNA that maintain the ends of linear chromosomes.
  495. telophase
    The final phase of mitosis, during which the nuclei re-form and chromosomes decondense.
  496. temperature-sensitive mutant
    A cell expressing a protein that is functional at one temperature but not at another, whereas the normal protein is functional at both temperatures.
  497. tertiary structure
    The three-dimensional folding of a polypeptide chain that gives the protein its functional form.
  498. testosterone
    A steroid hormone produced by the testis.
  499. thylakoid membrane
    The innermost membrane of chloroplasts that is the site of electron transport and ATP synthesis.
  500. thymine
    A pyrimidine found in DNA that base-pairs with adenine.
  501. thyroid hormone
    A hormone synthesized from tyrosine in the thyroid gland.
  502. tight junction
    A continuous network of protein strands around the circumference of epithelial cells, sealing the space between cells and forming a barrier between the apical and basolateral domains.
  503. topoisomerase
    An enzyme that catalyzes the reversible breakage and rejoining of DNA strands.
  504. transcription factor
    A protein that regulates the activity of RNA polymerase.
  505. transcription
    The synthesis of an RNA molecule from a DNA template.
  506. transcriptional attenuation
    A regulatory mechanism that controls elongation of an mRNA molecule past a certain site on the DNA template.
  507. transcytosis
    The sorting and transport of proteins to different domains of the plasma membrane following endocytosis.
  508. transducin
    A G protein that stimulates cGMP phosphodiesterase when it is activated by rhodopsin.
  509. transfection
    The introduction of a foreign gene into eukaryotic cells.
  510. transfer RNA (tRNA)
    RNA molecules that function as adaptors between amino acids and mRNA during protein synthesis.
  511. transforming growth factor � (TGF-)
    A polypeptide growth factor that generally inhibits animal cell proliferation.
  512. transgenic mouse
    A mouse that carries foreign genes incorporated into the germ line.
  513. translation
    The synthesis of a polypeptide chain from an mRNA template.
  514. transmembrane proteins
    Integral membrane proteins that span the lipid bilayer and have portions exposed on both sides of the membrane.
  515. transmission electron microscopy
    See electron microscopy.
  516. transposition
    The movement of DNA sequences throughout the genome.
  517. transposon
    A DNA sequence that can move to different positions in the genome.
  518. triacylglycerol
    Three fatty acids linked to a glycerol molecule.
  519. tropomyosin
    A fibrous protein that binds actin filaments and regulates contraction by blocking the interaction of actin and myosin.
  520. tubulin
    A cytoskeletal protein that polymerizes to form microtubules.
  521. tumor
    Any abnormal proliferation of cells.
  522. tumor necrosis factor (TNF)
    A polypeptide growth factor that induces programmed cell death.
  523. tumor promoter
    A compound that leads to tumor development by stimulating cell proliferation.
  524. tumor suppressor gene
    A gene whose inactivation leads to tumor development.
  525. tumor virus
    A virus capable of causing cancer in animals or humans.
  526. turgor pressure
    The internal hydrostatic pressure within plant cells.
  527. ubiquinone
    See coenzyme Q.
  528. ubiquitin
    A highly conserved protein that acts as a marker to target other cellular proteins for rapid degradation.
  529. uniport
    The transport of a single molecule across a membrane.
  530. uracil
    A pyrimidine found in RNA that base-pairs with adenine.
  531. vacuole
    A large membrane-enclosed sac in the cytoplasm of eukaryotic cells. In plant cells, vacuoles function to store nutrients and waste products, to degrade macromolecules, and to maintain turgor pressure.
  532. vector
    A DNA molecule used to direct the replication of a cloned DNA fragment in a host cell.
  533. vinblastine
    A drug that inhibits microtubule polymerization.
  534. vincristine
    A drug that inhibits microtubule polymerization.
  535. Western blotting
    See immunoblotting.
  536. X-ray crystallography
    A method in which the diffraction pattern of X rays is used to determine the arrangement of individual atoms within a molecule.
  537. Xenopus laevis
    An African clawed frog used as a model system for developmental biology.
  538. yeast artificial chromosome (YAC)
    A vector that can replicate as a chromosome in yeast cells and can accommodate very large DNA inserts (hundreds of kb).
  539. yeasts
    The simplest unicellular eukaryotes. Yeasts are important models for studies of eukaryotic cells.
  540. zebrafish
    A species of small fish used for genetic studies of vertebrate development.
  541. zinc finger domain
    A type of DNA binding domain consisting of loops containing cysteine and histidine residues that bind zinc ions.
  542. zygote
    A fertilized egg.
  543. zygotene
    The stage of meiosis I during which homologous chromosomes become closely associated.

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