| trans-activation (genetics) | Increased rate of gene expression directed by either viral or cellular proteins. These regulatory factors (diffusible gene products) act in trans -- that is, act on homologous or heterologous molecules of DNA. (cis-acting factors act only on homologous molecules.) (12 Dec 1998) |
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| energy of activation | Energy that must be added to that already possessed by a molecule or molecules in order to initiate a reaction; usually expressed in the Arrhenius equation relating a rate constant to absolute temperature. (05 Mar 2000) |
| enzyme activation | Conversion of an inactive form of an enzyme to one possessing metabolic activity. It includes 1) activation by ions (activators); 2) activation by cofactors (coenzymes); and 3) conversion of an enzyme precursor (proenzyme or zymogen) to an active enzyme. (12 Dec 1998) |
| juxtacrine activation | Activation of target cells by membrane anchored growth factors, also used for activation of leucocytes by PAF bound to endothelial cell surface. (18 Nov 1997) |
| feedback activation | The activation of an enzyme by an end product of a biochemical pathway in which that enzyme plays a part. For example, the activation of factors VIII and V by thrombin during blood clotting. (05 Mar 2000) |
| feed-forward activation | The activation of an enzyme by a precursor of the substrate of that enzyme. (05 Mar 2000) |
| upstream activation site | A DNA sequence that regulates transcription like an enhancer but does notwork if its located downstream from a promoter. (09 Oct 1997) |
| low-activation materials | <radiobiology> In fission reactors, one is forced to deal with the radioactive byproducts of the fission process, but in fusion reactors one generally has a choice of what materials to expose to neutrons produced by the fusion process. A major problem for fusion reactors is developing materials (such as for the reactor vacuum vessel structure) which can be exposed to high levels of neutron bombardment without becoming permanently radioactive. Candidate structural materials which have relatively low induced radiactivation (generally relative to stainless steel) are known as low-activation materials, these include titanium, vanadium, and silicon-carbide. (09 Oct 1997) |
| lymphocyte activation | <haematology> The change in morphology and behaviour of lymphocytes exposed to a mitogen or to an antigen to which they have been primed. The result is the production of lymphoblasts, cells that are actively engaged in protein synthesis and that divide to form effector populations. Should not be confused with transformation of the type associated with oncogenic viruses and activation is therefore perhaps a better term. (18 Nov 1997) |
| ABC process | Purification of water or deodorization of sewage by a mixture of alum, blood, and charcoal. (05 Mar 2000) |
| accessory process | A small apophysis at the posterior part of the base of the transverse process of each of the lumbar vertebrae. Synonym: processus accessorius, accessory tubercle. (05 Mar 2000) |
| acromial process | <anatomy> The lateral triangular projection of the spine of the scapula that forms the point of the shoulder and articulates with the clavicle. (27 Sep 1997) |
| activated sludge process | <procedure> A method of treating sewage and wastewater through microbial oxidation. Sewage previously treated in settling tanks is aerated to encourage the growth of nonpathogenic aerobic microorganisms (bacteria, yeasts, moulds and protozoans) which break the organic matter down into carbon dioxide, water and simple salts. After this activated sludge is produced, the wastewater undergoes further processing through anaerobic digestion, filtering and chlorination. (29 Dec 1997) |
| adiabatic process | <chemistry> A process in which the system does not exchange heat with the surroundings. (15 Jan 1998) |
| agene process | Bleaching of flour with nitrogen trichloride (prohibited in the United States). (05 Mar 2000) |