| absolute configuration | <chemistry> The three-dimensional arrangement of the atoms around the chiral centre of a molecule. (09 Oct 1997) |
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| cis configuration | <molecular biology> The configuration of an organic molecule containing a double bond between two carbon atoms, where the largest, most complex R groups are on the same side: H H \ / C=C / \ H3C CH3 -OR-The configuration of an organic molecule containing a ring, where the largest, most complex R groups are on the same side: H H | ___ \ / | // \\ [ring viewed edge-on] | \___/ / \ | /---\ H3C CH3 | H3C CH3. A genetics term meaning an event or a gene whose action occurs on the same chromosome. Two mutations in different genes coding for the same phenotype which are on the same chromosome (as opposed to the trans configuration where each homologue has one of the mutations). Compare: trans configuration. (11 Jan 1998) |
| configuration | <chemistry> The three-dimensional arrangement of an organic molecule that is caused by: double bonds that prevent free rotation, or chiral centres that haveside groups arranged around them in a particular order. A configurational isomer cannot be turned into another configuration without breaking covalent bonds. (09 Oct 1997) |
| stereochemical configuration | <chemistry> The arrangement of atoms in a molecule in three-dimensional space, especially with regard to the differences between enantiomers. The arrangements are specified in chemical formulas with the letters R, S, L, and/or D. In the R, S system, each asymmetric carbon atom is classified as R (for rectus, Latin for right) if the chemical groups attached to it have decreasing priorities in a clockwise direction, or S (for sinister, Latin for left) if the chemical groups attached to it have decreasing priorities in a counterclockwise direction, when the carbon atom in question is viewed along the bond of the chemical group with the lowest priority. Priorities are assigned to chemical groups according to the Cahn-Ingold Prelog priority rules (generally, the larger the group the higher the priority). L and D designations are given if the enantiomers have optical activity, that is, if they will rotate polarized light. The member of the pair which rotates polarized light clockwise is dextrorotatory, or D, and the member of the pair which rotates polarized light counterclockwise is levorotatory, or L (there is always one of each in every pair). This is also often referred to as handedness, where D is right-handed and L is lefthanded. (13 Nov 1997) |
| degenerate configuration | <radiobiology> Magnetic field configuration in which the magnetic lines of force close exactly on themselves after passing around the configuration a finite number of times. (09 Oct 1997) |
| trans configuration | 1. <chemistry> The configuration of an organic molecule containing a double bond between two carbon atoms, where the largest, most complex R groups attached to each carbon are on opposite sides. Alternatively: The configuration of an organic molecule containing a ring, where the largest, most complex R groups attached to each carbon are on opposite sides. These configurations are the opposite of cis configuration. 2. <genetics> A genetics term meaning that the changes in gene expression are due to something acting on the gene from a distance, or from an unrelated location, rather than on the same strand of DNA. Two mutations in different genes coding for the same phenotype which are on opposite homologous chromosomes (as opposed to the cis configuration where both of the mutations are on the same homologue). (17 Mar 1998) |
| field-reversed configuration | <radiobiology> A compact torus produced in a theta pinch and having (in principle) no toroidal field. The potential advantages for a fusion reactor include a simple (linear) machine geometry, an average plasma pressure close to the confining field pressure, and physical separation of formation and burn chambers. The are predicted to be violently unstable to tilting, but this is rarely observed. (09 Oct 1997) |
| GI-tract target lesion | <radiology> Metastatic melanoma, primary neoplasm, spindle cell tumour (benign or malignant), lymphoma, carcinoid, carcinoma, metastasis, breast carcinoma, lung carcinoma, renal carcinoma, Kaposi's sarcoma, eosinophilic granuloma, ectopic pancreas (12 Dec 1998) |
| molybdenum target tube | An X-ray tube with an anode surface made of molybdenum instead of tungsten, used in mammography. (05 Mar 2000) |
| target | 1. An object fixed as goal or point of examination. 2. In the ophthalmometer, the mire. Synonym: target organ. 4. Anode of an X-ray tube. See: X-ray. Origin: It. Targhetta, a small shield (05 Mar 2000) |
| target behaviour | Operant in behaviour modification therapy, the prescribed behaviour. (05 Mar 2000) |
| target cell | An erythrocyte in target cell anaemia, with a dark centre surrounded by a light band that again is encircled by a darker ring; it thus resembles a shooting target; such cell's also appear after splenectomy. A cell lysed by cytotoxic T lymphocytes, as in graft rejection. Synonym: Mexican hat cell. (05 Mar 2000) |
| target cell anaemia | Any anaemia with a conspicuous number of target cells in the peripheral blood; characteristic of the thalassaemias and also found in several haemoglobinopathies. (05 Mar 2000) |
| target gland | The effector that functions when stimulated by the internal secretion of another gland or by some other stimulus. (05 Mar 2000) |
| target lesions of liver/spleen | <radiology> Lymphosarcoma (including Kaposi sarcoma), melanoma (12 Dec 1998) |