haploid Having only one set of chromosomes, as in a sperm cell or a
bacterium, as distinct from diploid (having two sets of chromosomes).
heat shock protein (stress-response protein) Protein synthesized in increased
amounts in response to an elevated temperature or other stressful treatment,
and which usually helps the cell to survive the stress. Prominent examples are
hsp60 and hsp70.
heavy chain (H chain) The larger of the two types of polypeptide in an
immunoglobulin molecule.
HeLa cell Line of human epithelial cells that grows vigorously in culture.
Derived from a human cervical carcinoma.
helix-loop-helix (HLH) DNA-binding structural motif present in many gene
regulatory proteins. Should not be confused with the helix-turn-helix.
helper T cell Type of T cell that helps stimulate B cells to make antibodies
and activates macrophages to kill ingested microorganisms.
heme Cyclic organic molecule containing an iron atom that carries oxygen in
hemoglobin and carries an electron in cytochromes. (14 22.)
hemidesmosome Specialized anchoring cell junction between an epithelial
cell and the underlying basal lamina.
hemoglobin The major protein in red blood cells that associates with O2 in the
lungs by means of a bound heme group.
hemopoiesis Generation of blood cells, mainly in the bone marrow.
hepatocyte Liver cell.
heterocaryon Cell with two or more genetically different nuclei; produced by
the fusion of two or more different cells.
heterochromatin Region of a chromosome that remains unusually condensed
chromatin; transcriptionally inactive during interphase.
heterodimer Protein complex composed of two different polypeptide chains.
heterozygote Diploid cell or individual having two different alleles of one or
more specified genes.
high-energy bond Covalent bond whose hydrolysis releases an unusually
large amount of free energy under the conditions existing in a cell. A group
linked to a molecule by such a bond is readily transferred from one molecule
to another. Examples include the phosphodiester bonds in ATP and the
thioester linkage in acetyl CoA.
high-mannose oligosaccharide Chain of sugars attached to a glycoprotein
which contains many mannose residues. It is generated by a trimming of the
original mannose-rich oligosaccharide that leaves most of the mannose
residues with no subsequent addition of further sugars. (13 26.)
high-performance liquid chromatography (HPLC) Type of
chromatography that uses columns packed with tiny beads of matrix; the
solution to be separated is pushed through under high pressure.
histidine-kinase-associated receptor Type of transmembrane receptor found
in the plasma membrane of bacteria, yeast and plant cells, and involved, for
example, in sensing stimuli that cause bacterial chemotaxis. Associated with a
histidine protein kinase on its cytoplasmic side.
histone One of a group of small abundant proteins, rich in arginine and lysine,
four of which form the nucleosome on the DNA in eucaryotic chromosomes.
HIV Human immunodeficiency virus, the retrovirus that is the cause of AIDS.
HLH see helix-loop-helix
hnRNP protein (heterogeneous nuclear ribonuclear protein) Any of a
group of proteins that assemble on newly synthesized RNA, organizing it into
a more compact form.
Holliday junction X-shaped structure observed in DNA undergoing
recombination, in which the two DNA molecules are held together at the site
of crossing-over, also called a cross-strand exchange.
homeobox Short (180 base pairs long) conserved DNA sequence that encodes
a DNA-binding protein motif (homeodomain) famous for its presence in genes
that are involved in orchestrating development in a wide range of organisms.
homeodomain DNA-binding domain that defines a class of gene regulatory
proteins important in animal development.
homeotic mutation Mutation that causes cells in one region of the body to
behave as though they were located in another, causing a bizarre disturbance of
the body plan.
homolog (1) One of two or more genes that are similar in sequence as a result
of derivation from the same ancestral gene. The term covers both orthologs
and paralogs. (2) See homologous chromosome.
homologous Describes organs or molecules that are similar because of their
common evolutionary origin. Specifically it describes similarities in protein or
nucleic acid sequence.
homologous chromosome (homolog) One of the two copies of a particular
chromosome in a diploid cell, each copy being derived from a different parent.
homozygote Diploid cell or organism having two identical alleles of a
specified gene or set of genes.
hormone Signal molecule secreted by an endocrine cell into the bloodstream,
which can then carry it to distant target cells.
housekeeping gene Gene serving a function required in all the cell types of an
organism, regardless of their specialized role.
Hox complex Two tightly linked clusters of genes in Drosophila (the bithorax
and Antennapedia complexes) that control the differences between the
different segments of the body. Homologous Hox complexes are found in
other animals, where they also determine pattern along the anteroposterior axis.
HPLC see high-performance liquid chromatography
hybridization In molecular biology, the process whereby two complementary
nucleic acid strands form a double helix. Forms the basis of a powerful
technique for detecting specific nucleotide sequences.
hybridoma Cell line used in the production of monoclonal antibodies.
Obtained by fusing antibody-secreting B cells with cells of a lymphocyte
tumor.
hydrocarbon Compound that has only carbon and hydrogen atoms.
hydrogen bond Noncovalent bond in which an electropositive hydrogen atom
is partially shared by two electronegative atoms.
hydrolysis (adjective hydrolytic) Cleavage of a covalent bond with
accompanying addition of water, H being added to one product of the
cleavage and OH to the other.
hydronium ion (H3O) Water molecule associated with an additional proton.
hydrophilic Describes a polar molecule or part of a molecule that forms
enough energetically favorable interactions with water molecules to dissolve
readily in water. (Literally, "water loving.")
hydrophobic (lipophilic) Describes a nonpolar molecule or part of a molecule
that cannot form energetically favorable interactions with water molecules and
therefore does not dissolve in water. (Literally, "water hating.")
hydrophobic force Force exerted by the hydrogen-bonded network of water
molecules that brings two nonpolar surfaces together by excluding water
between them.
hydroxyl ( OH) Chemical group consisting of a hydrogen atom linked to an
oxygen, as in an alcohol.
hypertonic Describes any medium with a sufficiently high concentration of
solutes to cause water to move out of a cell due to osmosis.
hypervariable region Any of three small regions within the variable region of
an immunoglobulin light or heavy chain that show the highest variability from
molecule to molecule. These regions determine the specificity of the antigenbinding
site.
hypotonic Describes any medium with a sufficiently low concentration of
solutes to cause water to move into a cell due to osmosis.