| a | 1. First letter of the Greek alphabet, alpha, used as a classifier in the nomenclature of many sciences. 2. Symbol for atto-, Bunsen's solubility coefficient, specific optical rotation, absorbance, specific absorption coefficient, degree of dissociation, adenosine or adenylic acid in polynucleotides, alanine in polypeptides, first substrate in a multisubstrate enzyme=catalyzed reaction 3. In chemistry, denotes the first in a series, a position immediately adjacent to a carboxyl group, the first of a series of closely related compounds, an aromatic substituent on an aliphatic chain, or the direction of a chemical bond away from the viewer. 4. <abbreviation> Alpha particle, ampere, adenine, alanine, Helmholtz energy, total acidity, ante, area, asymmetric, auris, artery, arteria, absorptivity, alveolar gas, systemic arterial blood. (05 Mar 2000) |
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| A | Symbol for angstrom. (05 Mar 2000) |
| A bands | The dark-staining anisotropic cross striations in the myofibrils of muscle fibres, comprising regions of overlapping thick (myosin) and thin (actin) filaments. Synonym: A disks, anisotropic disks, Q bands, Q disks. (05 Mar 2000) |
| a cappella | In church or chapel style; said of compositions sung in the old church style, without instrumental accompaniment; as, a mass a capella, i. E, a mass purely vocal. A time indication, equivalent to alla breve. Origin: It. See Chapel. Source: Websters Dictionary (01 Mar 1998) |
| A chain | A polypeptide component of insulin containing 21 amino acyl residues, beginning with a glycyl residue (NH2-terminus); insulin is formed by the linkage of an A chain to a B chain by two disulfide bonds; the amino-acid composition of the A chain is a function of species. Synonym: glycyl chain. In general, one of the polypeptides in a multiprotein complex. (05 Mar 2000) |
| A disks | The dark-staining anisotropic cross striations in the myofibrils of muscle fibres, comprising regions of overlapping thick (myosin) and thin (actin) filaments. Synonym: A disks, anisotropic disks, Q bands, Q disks. (05 Mar 2000) |
| a fetoprotein | A protein normally produced during the 12th to 15th week of gestation, decreasing thereafter, but appearing in the blood in certain tumours, such as embryonal carcinomas of the testis and ovary, hepatoma, and less often in patients with carcinomas of the pancreas, stomach, colon, or lung. When present, a useful marker in following the course of a tumour. (05 Mar 2000) |
| A fibres | Myelinated nerve fibre's in somatic nerves, measuring 1 to 22 um in diameter, conducting nerve impulses at a rate of 6 to 120 m/sec. (05 Mar 2000) |
| a granules | Large, rodlike, or filamentous granules found in several types of cells, especially platelets where they are the most numerous type of granule; contain secretory proteins, including fibrinogen, fibronectin, fibrospondin, von Willebrand factor (collectively known as adhesive proteins) and other proteins (platelet factor 4, platelet-derived growth factor, coagulation factor V, etc.). (05 Mar 2000) |
| a haemolysin | See: a' haemolysis. (05 Mar 2000) |
| a helix | The helical (commonly right-handed) form present in many proteins, deduced by Pauling and Corey from X-ray diffraction studies of proteins such as alpha-keratin; the helix is stabilised by hydrogen bonds between, e.g., ==C==O and HN== groups (symbolised by the centre dot in ==CO-HN==) of different eupeptide bonds. In a true a helix, there are 3.6 amino acid residues per turn of the helix. Synonym: 3.613 helix, Pauling-Corey helix. Collagen helix, an extended left-handed helix resulting from the high levels of glycine, l-proline, and l-hydroxyproline present in the collagens. There are 3.3 amino acids per turn of the helix. Three of those left-handed helices form a triple superhelix that is right-handed. (05 Mar 2000) |
| A layer | <molecular biology> Right handed double helical DNA with approximately 11 residues per turn. Planes of base pairs in the helix are tilted 20 deg away from perpendicular to the axis of the helix. Formed from B DNA by dehydration. (18 Nov 1997) |
| a posteriori | 1. <logic> Characterising that kind of reasoning which derives propositions from the observation of facts, or by generalisations from facts arrives at principles and definitions, or infers causes from effects. This is the reverse of a priori reasoning. 2. <philosophy> Applied to knowledge which is based upon or derived from facts through induction or experiment; inductive or empirical. Origin: L. A (ab) + posterior latter. Source: Websters Dictionary (01 Mar 1998) |
| a priori | 1. <logic> Characterising that kind of reasoning which deduces consequences from definitions formed, or principles assumed, or which infers effects from causes previously known; deductive or deductively. The reverse of a posteriori. 3. <philosophy> Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible. "A priori, that is, form these necessities of the mind or forms of thinking, which, though first revealed to us by experience, must yet have preexisted in order to make experience possible." (Coleridge) Origin: L. A (ab) + prior former. Source: Websters Dictionary (01 Mar 1998) |
| a thalassaemia intermedia | See: haemoglobin H. (05 Mar 2000) |