مصادر الكيمياء موقع لتعليم الكيمياء بصورة مبسطة

ablation 1 (in surgery) the removal or destruction of tissue by a surgical procedure. 2 (in genetics) a technique for the removal of a tissue or a particular cell type during development. It depends on the tissue-specific expression of a toxin gene such as diphtheria A (dipA) in a transgenic organism.

abl an oncogene from murine Abelson leukemia virus. The human equivalent is ABL (locus at 9q34), which encodes a tyrosine protein kinase. In humans, inappropriate activation of ABL occurs via a reciprocal translocation between chromosomes 9 and 22 in which ABL is joined at the breakpoint cluster region (bcr) of the ph1 gene on chromosome 22(q11), resulting in an altered …

ABH antigens one of the systems of blood group antigens having determinants associated with oligosaccharide structures. It is the basis of the ABO system, which was the first human blood group antigen system to be detected, by Austrian-born US pathologist Karl Landsteiner (1868–1943) in 1901, and it remains the most important in blood transfusion. Individuals having neither A nor B …

abetalipoproteinemia or (Brit.) abetalipoproteinaemia an autosomal recessive disorder in which plasma lipoproteins lack apolipoprotein B. There is defective assembly and secretion both of chylomicrons in intestinal mucosa and of very-low-density lipoproteins in the liver. The cause is a deficiency of the 88 kDa subunit of microsomal triglyceride transfer protein.

ABC transporter a membrane transport protein having the ABC molecular domain, named after ATP-binding cassette, characteristic of all members of a large superfamily of membrane transport proteins that hydrolyse ATP and transfer a diverse array of small molecules across membranes.

ABC model a model for specification of floral organs especially in Arabidopsis thaliana. It views the floral primordium as comprising four whorls whose developmental fate is determined by the concentric and combinatorial activity of three classes of gene, denoted A, B, and C, which encode transcription factors. Class A determines the fate of whorls 1 and 2 (sepals and petals, …

Abbe refractometer a refractometer in which the critical angle for total reflection at the interface of a film of liquid between two similar glass prisms is used in determining the refractive index of the liquid. [After Ernst Abbe (1840–1905), German physicist famous for his researches in optics.]

Aarskog–Scott syndrome or Aarskog syndrome or faciogenital dysplasia an extremely rare genetically heterogeneous developmental disorder in which individuals have widely spaced eyes, anteverted nostrils, a broad upper lip and a ‘saddlebag’ or ‘shawl scrotum’. The X-linked form has been ascribed to mutations in the FGD1 gene. [After Dagfinn Aarskog (1928– ), Norwegian paediatrician, and Charles I. Scott Jr (1934– ), …

abbreviation for ATPase associated with varied activities; any member of a family of conserved ATP-dependent proteases that mediate degradation of nonintegrated membrane proteins in bacteria, mitochondria, and chloroplasts. They form large complexes composed of identical or homologous subunits. Each subunit contains two transmembrane segments, an ATP-binding omain, and a metal-dependent catalytic domain. Mitochondria contain a matrix-facing AAA protease (m-AAA protease) and …

ABCR abbreviation for ATP-binding cassette transporter retina; other name: rim protein. A protein found in the disc membrane of the outer segment of photoreceptor cells of the retina. It consists of 2273 amino acids, and is presumed to function in the transport of retinoids. Mutations in the ABCR gene, at 1p21-p23, are associated with Stargardt and age-related macular dystrophies.