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U 1. Uracil. 2. Uridine. 3. Enzyme Unit. 4. Uranium. 5. Uniformly Labeled. Ul

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U U 1. Uracil. 2. Uridine. 3. Enzyme unit. 4. UEP Unit evolutionary period. Uranium. 5. Uniformly labeled. UFA Unesterified fatty acids. Ul One of a group of small nuclear UIBC Unsaturated iron-binding capacity. ribonucleoproteins (snRNP) that function in ultimate carcinogen The form of a carcinogen the splicing of nuclear pre-mRNA. Ul con- in which it ultimately reacts with cellular mac- sists of several polypeptides and an RNA romolecules; the reactive intermediate formed molecule (165 nucleotides) that contains a from a carcinogen that is inacitve per se. The segment that is complementary to the accep- ultimate carcinogen is believed to be a form tor and donor junctions of introns. Other that contains an electrophilic center (such as a members of the group that have been iden- carbonium ion, free radical, epoxide, or metal tified include U2, U4, U5, and U6. cation) that attacks electron-rich centers in Ubbelohde viscometer A capillary-type visco- proteins and nucleic acids. meter that permits the measurements of vis- ultimate precision See method of ultimate pre- cosity as a function of concentration, since the cision. solution can be diluted directly in the visco- ultra- 1. Prefix meaning excessive. 2. Prefix meter. meaning beyond the range of. ubiquinol Reduced coenzyme Q; CoQH2. ultracentrifugation Centrifugation, performed ubiquinol: cytochrome C oxidoreductase Com- in an ultracentrifuge, in which high centrifugal plex III. forces are used. ubiquinone Coenzyme Q. ultracentrifuge A high-speed centrifuge cap- ubiquitin A small, heat-stable, nonenzymatic able of generating speeds of approximately polypeptide of 76 amino acids that is widely 60,000 rpm and centrifugal forces of approx- distributed (as its name implies) in eukaryotic imately 500,000 x g. cells. ATP hydrolysis drives a reaction where- ultracryotomy The preparation of thin sections by ubiquitin becomes covalently linked, via its from frozen tissue specimens. carboxyl group, to the amino group of lysine ultrafiltration The filtration of a solution in proteins that are substrates for proteolysis. through a filter or a membrane that will retain It is believed that the attachment of ubiquitin macromolecules. marks a given protein for rapid intracellular ultramicrotome A microtome for cutting very degradation; it may serve to tag damaged or thin sections of tissues, about 0.05 to 0.10 jim abnormal proteins. Ubiquitin is also involved in thickness, for electron microscopic ex- in histone modification and appears to be part amination. of certain receptor sites on lymphocyte sur- ultrasonic Of, or pertaining to, ultrasound. faces. ultrasonication The exposure of material to ubiquitination The covalent linking of ubi- ultrasound; used for the rupture of cells and quitin to a protein. the denaturation of proteins. ubiquitous RNA A class of RNA molecules ultrasound Sound waves that have frequencies that form part of the small nuclear RNA greater than those of audible sound; refers complement and that have wide occurrence. particularly to sound waves that have frequen- Ribosomal 5S RNA and transfer RNAs are cies of 500,000 cps or higher. examples. ultrastructure The fine structure of tissues, UDP 1. Uridine diphosphate. 2. Uridine-5'- cells, and subcellular particles beyond that re- diphosphate. vealed by the light microscope. UDPG Uridine diphosphate glucose; see UDP- ultraviolet dichroism The dichroism produced glucose. when ultraviolet light is absorbed by oriented UDP-GaI Uridine diphosphate galactose, samples such as nucleic acid preparations. UDP-GIc Uridine diphosphate glucose; see ultraviolet microscope A microscope that util- UDP-glucose. izes ultraviolet light for illumination of the UDP-glucose Uridine diphosphate glucose; a specimen and for image formation. It has nucleoside diphosphate sugar that serves as twice the resolving power of an ordinary light the donor of a glucose residue in the biosyn- microscope and may be used for estimating thesis of glycogen. Abbr UDPG; UDP-GIc. the amount of nucleic acid in a sample. ultraviolet optics ABSORPTION OPTICAL SYSTEM. uncompensated acidosis An acidosis in which ultraviolet spectrum That part of the electro- the blood pH falls due to insufficient com- magnetic spectrum that covers the wavelength pensatory mechanisms. range of about 1.3 x 10~6 to 4 x 10~5 cm and uncompensated alkalosis An alkalosis in which that includes photons which are emitted or the blood pH rises due to insufficient com- absorbed during electronic transitions; radia- pensatory mechanisms. tion of the shorter wavelengths is known as uncompetitive inhibition The inhibition of the far ultraviolet, and that of the longer wave- activity of an enzyme that is characterized by lengths is known as near ultraviolet. Abbr a decrease in the maximum velocity compared UV spectrum. to that of the uninhibited reaction and by a umber codon The codon UGA; one of the reciprocal plot (I/velocity versus I/substrate three termination codons. Aka opal codon. concentration) which is parallel to that of the umber mutation A mutation in which a codon uninhibited reaction. See also degree of in- is mutated to the umber codon, thereby caus- hibition. ing the premature termination of the synthesis uncompetitive inhibitor An inhibitor that binds of a polypeptide chain. to the enzyme-substrate complex but does not UMP 1. Uridine monophosphate (uridylic bind to the free enzyme. acid). 2. Uridine-5'-monophosphate (5'- uncoupler Uncoupling agent. uridylic acid). uncoupling 1. The separation of the two pro- unbalanced growth The growth of cells cesses that constitute oxidative phosphoryla- such that not all of the cellular components tion such that ATP synthesis is dissociated increase by the same factor and, hence, the fromt he electron transport system at one or overall composition of the cells does not re- more of the phosphorylation sites. As a result main constant. A given increase in the con- of uncoupling, ATP synthesis is inhibited but centration of protein, DNA, RNA, lipid, or electron transport and respiration are allowed some other macromolecule is not coincident to proceed and may even be stimulated. 2. with the same increase in the mass and num- The decreased sensitivity of target cells to sti- ber of cells. mulation by a ligand that is due to the fact uncertainty principle The principle, enunciated that specific receptors, while still present, are by Heisenberg, that it is impossible to know unable to activate a particular enzyme. The simultaneously, with absolute precision, both two processes of ligand binding to a receptor the position and the speed of a small particle, and enzyme activation by the ligand-receptor such as an electron. Specifically, the product complex have been separated. See also desen- of the uncertainty in position Ax and the un- sitization (3). certainty in momentum Ap can be no smaller uncoupling agent An inhibitor that brings than Planck's constant h. Thus, AJC-Ap ^ h. about the uncoupling of ATP synthesis from Depending on the precise definition of Ax and the electron transport system at one or more Ap, the right side of this expression can be /t, of the phosphorylation sites. /i/2ir, or /i/4ir. Aka Heisenberg uncertainty undecaprenol A long-chain lipid, containing 11 principle. isoprene units, that serves as a carrier (in the uncharged polar amino acid A polar amino form of undecaprenol phosphate) of carbohy- acid that carries no charge in the intracellular drates in the biosynthesis of peptidoglycan in pH range of about 6 to 7. bacteria. See also glycosylation. uncharged tRNA A transfer RNA molecule undecaprenol phosphate See undecaprenol. that does not have an amino acid covalently undernutrition Malnutrition resulting from the linked to it. consumption of inadequate amounts of food uncoating The removal of the protein coat so that one or more of the essential nutrients from a virus that occurs extracellularly in the are lacking in the diet. Aka undernourish- case of bacterial viruses and, apparently, in- ment. tracellularly in the case of animal viruses. underwinding Negative supercoiling; see super- uncoating enzyme An enzyme that functions in helix. the removal of the protein coat from a virus. uneconomic species An undesirable species; the uncoded amino acid An amino acid, such as species that is eliminated by another, desir- hydroxyproline or hydroxylysine, for which no able, species through the use of chemicals. codon exists. Such an amino acid is derived by uni- 1. Referring to one kinetically important enzymatic modification of the parent amino substrate and/or product of a reaction; thus a acid (e.g., proline or lysine) after the parent uni bi reaction is a reaction with one substrate amino acid has become incorporated into a and two products. 2. Prefix meaning one. polypeptide chain in response to its codon. unidirectional replication DNA replication that proceeds in one direction; one replicating fork unit membrane hypothesis The hypothesis that moving in a given direction, as in the rolling the structure of biological membranes can circle model. be described in terms of a unit membrane, uniformitarianism See doctrine of uniformitar- about 90 A thick. The unit membrane is con- ianism. sidered to be a bilayer of polar lipids, such uniformly labeled Designating a compound as phospholipids, surrounded on both sides that is labeled in all of the positions in the by protein. The lipids are arranged with their molecule in a statistically uniform or nearly nonpolar portions inward and their polar por- uniform manner; generally refers to 14C- tions outward where they are coated with labeled compounds. Sym U. protein molecules that are in their beta con- unifunctional feedback A feedback mechanism figuration. Aka Danielli-Davson-Robertson that affords control in only one direction so model; Robertson model. See also Danielli- that the input of a system is affected by either Davson model; fluid mosaic model. an increase or a decrease of the output; a unit mitochondrion hypothesis A controversial system in which the pH will be adjusted if it idea, proposed by some biologists, according rises above the normal value is an example.
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