Hewas a member of several scientific societies and
was elected a fellow of the Royal Society in 1918, serv-
ing as secretary for the period 1935–45, and foreign
secretary in 1946. Hill died on June 3, 1977.
hill topping A behavior exhibited by butterflies
where males and females congregate at a high point in
the landscape, increasing each individual butterfly’s
chance of finding a mate.
hilum The area where blood vessels, nerves, and
ducts enter an organ.
HiPIP Formerly used abbreviation for high-potential
IRON–SULFUR PROTEIN, now classed as a FERREDOXIN.
An ELECTRON TRANSFER PROTEINfrom photosynthetic
and other bacteria, containing a [4FE-4S]CLUSTERthat
undergoes oxidation-reduction between the [4Fe-4S]2+
and [4Fe-4S]3+states.
See alsoPHOTOSYNTHESIS.
hirudin Anonenzymatic chemical secreted from the
leech that prevents blood clotting. Today, the genetical-
ly engineered lepirudin and desirudin and the synthetic
bivalirudin are used as anticoagulants.
histamine A hormone and chemical transmitter
found in plant and animal tissues. In humans it is
involved in local immune response that will cause blood
vessels to dilate during an inflammatory response; also
regulates stomach acid production, dilates capillaries,
and decreases blood pressure. It increases permeability
of the walls of blood vessels by vasodilation when
released from mast cells and causes the common symp-
toms of allergies such as running nose and watering
eyes. It will also shut the airways in order to prevent
allergens from entering, making it difficult to breath.
Antihistamines are used to counteract this reaction.
histology The study of the microscopic structure of
plant and animal tissue.
histone Abasic unit of chromatin structure; several
types of protein characteristically associated with the
DNA in chromosomes in the cell nucleus of eukaryotes.
They function to coil DNA into nucleosomes, which
are a combination of eight histones (a pair each of
H2A, H2B, H3, and H4) wrapped by two turns of a
DNA molecule. A high number of positively charged
amino acids bind to the negatively charged DNA.HIV SeeAIDS.holoblastic cleavage A complete and equal division
of the egg in an early embryo that has little yolk. Char-
acteristic of amphibians, mammals, nonvertebrate
chordates, echinoderms, most mollusks, annelids, flat-
worms, and nematodes.holocene The present epoch of geological time start-
ing approximately 10,000 years ago to the present.
See alsoGEOLOGICAL TIME.holoenzyme An ENZYMEcontaining its characteristic
PROSTHETIC GROUP(s) and/or metal(s).holotype The exact specimen of a new animal or
plant representing what is meant by the new name and
designated so by publication. The holotype specimen
does not have to be the first ever collected, but it is the
official one with which all others are compared.homeobox(HOX genes) A shortstretch of similar or
identical 180-base-pair (nucleotide) sequences of DNA
within a homeotic gene in most eukaryotic organisms
that plays a major role in controlling body develop-
ment by regulating patterns of differentiation. Homeot-
ic genes create segments in an embryo that become
specific organs or tissues. Homeoboxes determine posi-
tional cell differentiation and development. Mutations
in these genes will cause one body part to convert into
a totally different one.
See alsoHOMEOTIC GENE.164 hill topping