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O ANTIGEN

The somatic antigen of Gram-negative bacteria, eg genera Escherichia, Shigella, Salmonella etc. Species specific cell wall antigen with an outer side chain of repeating units of oligosaccharides which confers strain specificity. The polysaccharide is internally linked to lipid A forming lipopolysaccharide.

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O-RING

A toroidal ring, usually of circular cross-section, made of neoprene or similar materials, used eg as an oil or air seal.

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O-SHELL

The electron shell in an atom corresponding to a principal quantum number of five. In naturally occuring elements the shell is never completely filled but has most electrons, 21, for the element uranium. See panel on Atomic structure

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OAKUM

Tarred, untwisted rope or hemp used for caulking joints.

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OBCONIC, OBCONICAL

Cone-shaped but attached by the point.

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OBDIPLOSTEMONOUS

Having stamens in two whorls, those in the outer opposite to the petals, eg Geranium. Cf diplostemonous.

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OBDUCTION

The process during collisions between tectonic plates whereby a piece of the subducted plate is broken off and pushed on to the overriding plate,

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OBELISK

A slender stone shaft, generally monolithic, square in section, and tapering towards the top, which is surmounted by a small pyramid.

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OBERON

A natural satellite of Uranus, discovered in 1787. Distance from the planet 583 000 km; diameter of 1520 km.

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OBESITY

An excessive proportion of fat on the body; cultural norms vary. It is produced by a large range of factors including faulty metabolism and abnormal behaviour.

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