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Biology Glossary - J


J chain
A cystine-rich polypeptide (MWt ca. 15000) involved in the formation of dimeric and polymeric forms of both IgA and IgM (q.v.). In an IgA dimer the J chain forms a link between cystine residues situated near the C-terminal ends of the heavy chains in the two monomers.

Jaccard coefficient
See entry SJ.

jack bean lectin
 Concanavalin A.

jack-in-the-box ascus
Syn. Bitunicate Ascus.

Jacquard coefficient
See entry SJ.

JAK
 See cytokines.

janiemycin
A polypeptide antibiotic which inhibits transglycosylation in peptidoglycan synthesis.

Janus green
 a blue basic dye which has both azine and azo chromophores; it is used e.g. in vital staining.

Jarisch–Herxheimer reaction (Herxheimer reaction)
A potentially fatal reaction which may follow the first effective chemotherapy against e.g. brucellosis or syphilis – or, in general, diseases caused by certain bacteria (particularly spirochaetes) and protozoa. Symptoms (e.g. an initial rise in temperature) are associated with a cascade of CYTOKINES which seem to be responsible for at least some of the pathophysiology. The reaction has been prevented by treatment with antibodies against tumour necrosis factor (TNF)

jarrah dieback
 A disease of the jarrah (Eucalyptus marginata) – an important timber tree e.g. in Western Australia; the causal agent is generally believed to be Phytophthora cinnamomi, but waterlogging of the soil may be an important contributory factor.

joint molecule (mol. biol.)
A term sometimes used to refer to a structure composed of two dsDNA molecules held together only by hydrogen bonding – as e.g. in early intermediates formed
during homologous recombination.

jumping gene
Syn. transposable element.

junctional pore complex (JPC)
A carbohydrate-export apparatus, reported to occur in certain motile cyanobacteria, which (in at least some cases) includes a tubular structure (minimum diameter 13 nm) that is capable of spanning the cell envelope. It has been suggested that JPCs form the mechanistic basis of gliding motility in cyanobacteria.

joule (J)
A unit of energy: 1 J = 0.239 cal; 1 cal = 4.184 J.

juvenile hormone (JH)
A hormone in arthropods, secreted by the corpora allata glands, that promotes the retention of larval characteristics.

juxtaglomerular apparatus (JGA)
Specialized tissue located near the afferent arteriole that supplies blood to the kidney glomerulus; the JGA raises blood pressure by producing renin, which activates angiotensin.

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