Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

Glossary G:15
Hedgehog protein Secreted extracellular signal molecule
that has many different roles controlling cell differentiation
and gene expression in animal embryos and adult tissues.
Excessive Hedgehog signaling can lead to cancer.
helper T cell (T H cell) Type of T cell that helps activate B cells
to make antibodies, cytotoxic T cells to become effector cells,
and macrophages to kill ingested pathogens. They can also
help activate dendritic cells.
heterochromatin Chromatin that is highly condensed even
in interphase; generally transcriptionally inactive. (Compare with
euchromatin.)
heterochronic Describes genes involved in developmental
timing; mutation results in cells of a specific fate behaving as
cells at a different stage of development.
high-mannose oligosaccharides Broad class of N-linked
oligosaccharides, attached to mammalian glycoproteins in the
endoplasmic reticulum, containing two N-acetylglucosamine
residues and many mannose residues.
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.
histone One of a group of small abundant proteins, rich in
arginine and lysine, that combine to form the nucleosome cores
around which DNA is wrapped in eukaryotic chromosomes.
(Figure 4–24)
histone chaperone (chromatin assembly factor) Protein
that binds free histones, releasing them once they have been
incorporated into newly replicated chromatin. (Figure 4–27)
histone H1 “Linker” (as opposed to “core”) histone protein
that binds to DNA where it exits from a nucleosome and helps
package nucleosomes into the 30-nm chromatin fiber.
(Figure 4–30)
Holliday junction (cross-strand exchange) 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.
(Figure 5–55)
homeotic selector gene In Drosophila development, a
gene that defines and preserves the differences between body
segments.
homolog 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.
(Figure 1–21) See homologous chromosomes.
homologous Genes, proteins, or body structures that are
similar as a result of a shared evolutionary origin.
homologous chromosomes (homologs) The maternal and
paternal copies of a particular chromosome in a diploid cell.
homologous recombination (general recombination)
Genetic exchange between a pair of identical or very similar
DNA sequences, typically those located on two copies of the
same chromosome. Also a DNA repair mechanism for doublestrand
breaks. (Figures 5–48, 5–50, and 5–54)
homophilic Binding between molecules of the same kind,
especially those involved in cell–cell adhesion. (Figure 19–5)
horizontal gene transfer Gene transfer between bacteria via
natural transformation by released naked DNA, transduction by
bacteriophages, or sexual exchange by conjugation.
hormone Signal molecule secreted by an endocrine cell into
the bloodstream, which can then carry the signal to distant
target cells.
Hox complex A gene complex consisting of a series of Hox
genes
Hox genes Genes coding for transcription regulators,
each gene containing a homeodomain, and specifying bodyregion
differences. Hox mutations typically cause homeotic
transformations.
Hox proteins Transcription regulator proteins encoded by
Hox genes; possess a highly conserved, 60-amino-acid-long
DNA-binding homeodomain.
HPV Human papillomavirus; infects the cervical epithelium
and is important as a cause of carcinoma of the uterine cervix.
hyaluronan (hyaluronic acid) Type of nonsulfated
glycosaminoglycan with a regular repeating sequence of up to
25,000 identical disaccharide units, not linked to a core protein.
Found in the fluid lubricating joints and in many other tissues.
(Figures 19–33 and 19–34)
hybridization In molecular biology, the process whereby two
complementary nucleic acid strands form a base-paired duplex
DNA-DNA, DNA-RNA, or RNA-RNA molecule. Forms the
basis of a powerful technique for detecting specific nucleotide
sequences. (Figures 5–47 and 8–33)
hybridoma Hybrid cell line generated by fusion of a tumor cell
and another cell type. Monoclonal antibodies are produced by
hybridoma lines obtained by fusing antibody-secreting B cells
with cells of a B lymphocyte tumor. (Figure 8–4)
hydrogen bond Noncovalent bond in which an
electropositive hydrogen atom is partially shared by two
electronegative atoms. (Panel 2–3, pp. 94–95)
hydronium ion (H 3 O + ) Water molecule associated with
an additional proton. The form generally taken by protons in
aqueous solution.
hydrophilic Dissolving readily in water. Literally, “water loving.”
hydrophobic (lipophilic) Not dissolving readily in water.
Literally, “water-fearing.”
hydrophobic force Force exerted by the hydrogen-bonded
network of water molecules that brings two nonpolar
surfaces together by excluding water between them.
(Panel 2–3, pp. 94–95)
hypervariable region In immunology: any of the three small
parts of the variable region of an immunoglobulin or T cell
receptor chain that show the highest variability from molecule
to molecule and contribute to the antigen-binding site.
(Figure 24–26)
hypoxia-inducible factor 1α (HIF1α). Transcription
regulator, the intracellular levels of which increase in response
to a shortage of oxygen, that stimulates transcription of the
VEGF gene to promote angiogenesis.
IκB Inhibitory proteins that bind tightly to NFκB dimers
and hold them in an inactive state within the cytoplasm of
unstimulated cells.
Ig superfamily Large and diverse family of proteins that
contain immunoglobulin or immunoglobulin-like domains. Most
are involved in cell–cell interactions or antigen recognition.
(Figure 24–48)
IgA Immunoglobulin A; the principal class of antibody in
secretions, including saliva, tears, milk, and respiratory and
intestinal secretions.