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.
