Essential Cell Biology 5th edition

Glossary G:9
homologous
Describes genes, chromosomes, or any structures that are
similar because of their common evolutionary origin. Can
also refer to similarities between protein sequences or
nucleic acid sequences.
homologous gene—see homologous.
homologous recombination
Mechanism by which double-strand breaks in a DNA
molecule can be repaired flawlessly; uses an undamaged,
duplicated, or homologous chromosome to guide the repair.
During meiosis, the mechanism results in an exchange of
genetic information between the maternal and paternal
homologs.
homozygous
Possessing identical alleles for a given gene.
horizontal gene transfer
Process by which DNA is passed from the genome of one
organism to that of another, even to an individual from
another species. This contrasts with “vertical” gene transfer,
which refers to the transfer of genetic information from
parent to progeny.
hormone
Extracellular signal molecule that is secreted and
transported via the bloodstream (in animals) or the sap (in
plants) to target tissues on which it exerts a specific effect.
hybridization
Experimental technique in which two complementary
nucleic acid strands come together and form hydrogen
bonds to produce a double helix; used to detect specific
nucleotide sequences in either DNA or RNA.
hydrogen bond
A weak noncovalent interaction H
between a positively charged O H O
hydrogen atom in one molecule
H
and a negatively charged atom,
such as nitrogen or oxygen, in another; hydrogen bonds are
key to the structure and properties of water.
hydrolysis
Chemical reaction that involves cleavage of a covalent
bond with the accompanying consumption of water (its –H
being added to one product of the cleavage and its –OH to
the other); the reverse of a condensation reaction.
hydronium ion
The form taken by a proton (H + ) in aqueous solution.
hydrophilic
Molecule or part of a molecule that readily forms hydrogen
bonds with water, allowing it to readily dissolve; literally,
“water loving.”
hydrophobic
Nonpolar, uncharged molecule or part of a molecule
that forms no hydrogen bonds with water molecules and
therefore does not dissolve; literally, “water fearing.”
hydrophobic force
A noncovalent interaction that forces together the
hydrophobic portions of dissolved molecules to minimize
their disruption of the hydrogen-bonded network of water;
causes membrane phospholipids to self-assemble into a
bilayer and helps to fold proteins into a compact, globular
shape.
in situ hybridization
Technique in which a single-stranded RNA or DNA probe
is used to locate a complementary nucleotide sequence
in a chromosome, cell, or tissue; used to diagnose genetic
disorders or to track gene expression.
induced pluripotent stem (iPS) cell
Somatic cell that has been reprogrammed to resemble
and behave like a pluripotent embryonic stem (ES) cell
through the artificial introduction of a set of genes encoding
particular transcription regulators.
initiator tRNA
Special tRNA that initiates the translation of an mRNA in a
ribosome. It always carries the amino acid methionine.
inorganic
Not composed of carbon atoms.
inositol 1,4,5-trisphosphate (IP 3 )
Small intracellular signaling molecule that triggers the
release of Ca 2+ from the endoplasmic reticulum into the
cytosol; produced when a signal molecule activates a
membrane-bound protein called phospholipase C.
inositol phospholipid
Minor lipid component of plasma membranes that plays a
part in signal transduction in eukaryotic cells; cleavage yields
two small messenger molecules, IP 3 and diacylglycerol.
integrin
One of a family of transmembrane proteins present on cell
surfaces that enable cells to make and break attachments
to the extracellular matrix, allowing them to crawl through
a tissue.
intermediate filament
Fibrous cytoskeletal element, about 10 nm in diameter, that
forms ropelike networks in animal cells; helps cells resist
tension applied from outside.
interphase
Long period of the cell cycle between one mitosis and the
next. Includes G 1 phase, S phase, and G 2 phase.
intracellular condensate
A large aggregate of phase-separated macromolecules that
creates a region with a special biochemistry without the use
of an encapsulating membrane.
intracellular signaling pathway
A set of proteins and small-molecule second messengers
that interact with each other to relay a signal from the
cell membrane to its final destination in the cytoplasm or
nucleus.
intrinsically disordered sequence
Region in a polypeptide chain that lacks a definite structure.
intron
Noncoding sequence within a eukaryotic gene that is
transcribed into an RNA molecule but is then excised by
RNA splicing to produce an mRNA.
ion
An atom carrying an electrical charge, either positive or
negative.
ion channel
Transmembrane protein that forms a pore across the lipid
bilayer through which specific inorganic ions can diffuse
down their electrochemical gradients.
ion-channel-coupled receptor
Transmembrane receptor protein or
protein complex that opens in response to
the binding of a ligand to its external face,
allowing the passage of a specific inorganic
ion.
ionic bond
Interaction formed when one atom donates electrons to
another; this transfer of electrons causes both atoms to
become electrically charged.