p53 Tumor suppressor gene found mutated in about half of human cancers. It
encodes a gene regulatory protein that is activated by damage to DNA and is
involved in blocking further progression through the cell cycle.
pachytene Third stage of division I of meiosis, in which synapsis is complete.
palindromic sequence Nucleotide sequence that is identical to its
complementary strand when each is read in the same chemical direction for
example, GATC.
paracrine signaling Short-range cell-cell communication via secreted signal
molecules that act on adjacent cells.
parthenogenesis Production of a new individual from an egg cell in the
absence of fertilization by sperm.
passive transport Transport of a solute across a membrane down its
concentration gradient or its electrochemical gradient, using only the energy
stored in the gradient.
patch-clamp recording Electrophysiological technique in which a tiny
electrode tip is sealed onto a patch of cell membrane, thereby making it
possible to record the flow of current through individual ion channels in the
patch.
pathogen (adjective pathogenic) An organism or other agent that causes
disease.
PCR (polymerase chain reaction) Technique for amplifying specific regions
of DNA by the use of sequence-specific primers and multiple cycles of DNA
synthesis, each cycle being followed by a brief heat treatment to separate
complementary strands.
peptide bond Chemical bond between the carbonyl group of one amino acid
and the amino group of a second amino acid a special form of amide linkage.
Peptide bonds link amino acids together in proteins.
peptide map Characteristic two-dimensional pattern (on paper or gel) formed
by the separation of the mixture of peptides produced by the partial digestion
of a protein.
peripheral lymphoid organ (secondary lymphoid organ) Lymphoid organ
in which T cells and B cells interact with foreign antigens. Examples are
spleen, lymph nodes, and mucosal-associated lymphoid tissue.
peripheral membrane protein Protein that is attached to one face of a
membrane by noncovalent interactions with other membrane proteins, and
which can be removed by relatively gentle treatments that leave the lipid
bilayer intact.
peroxisome Small membrane-bounded organelle that uses molecular oxygen
to oxidize organic molecules. Contains some enzymes that produce and others
that degrade hydrogen peroxide (H2O2).
pH Common measure of the acidity of a solution: "p" refers to power of 10,
"H" to hydrogen. Defined as the negative logarithm of the hydrogen ion
concentration in moles per liter (M). Thus on the pH scale, pH 3 (10 3 M H+)
is acidic and pH 9 (10 9 M H+) is alkaline.
PH domain see pleckstrin homology domain
phage display Technique for detecting proteins that interact with each other
by screening a protein against a library of genetically modified phage, each
displaying a potential binding protein on their surface.
phage see bacteriophage
phagocyte General term for a professional phagocytic cell that is, a cell such
as a macrophage or neutrophil that is specialized to take up particles and
microorganisms by phagocytosis.
phagocytosis Process by which particulate material is endocytosed ("eaten")
by a cell. Prominent in carnivorous cells, such as Amoeba proteus, and in
vertebrate macrophages and neutrophils. (From Greek phagein, to eat.)
phagosome Large intracellular membrane-bounded vesicle that is formed as a
result of phagocytosis. Contains ingested extracellular material.
phase-contrast microscope Type of light microscope that exploits the
interference effects that occur when light passes through material of different
refractive indexes. Used to view living cells.
phenotype The observable character of a cell or an organism.
phosphatase Enzyme that removes phosphate groups from a molecule.
phosphatidylinositol 3-kinase (PI 3-kinase) A kinase involved in
intracellular signaling pathways activated by a variety of cell-surface
receptors. It phosphorylates inositol phospholipids at the 3 position of the
inositol ring. (15 58.)
phosphatidylinositol An inositol phospholipid. (15 34.)
phosphodiester linkage Set of covalent chemical bonds formed when two
hydroxyl groups are linked in ester linkage to the same phosphate group. This
linkage joins adjacent nucleotides in RNA or DNA. (2 28.)
phosphoinositide see inositol phospholipid
phospholipase C-b (PLC-b) Enzyme bound to the cytoplasmic face of the
plasma membrane that converts membrane phosphatidylinositol 4,5-
bisphosphate to diacylglycerol (which remains in the plasma membrane) and
inositol 1,4,5-trisphosphate (IP3). It is activated by certain G proteins to trigger
the inositol phospholipid signaling pathway.
phospholipase C-g (PLC-g) Like phospholipase C-b, an enzyme that cleaves
inositol phospholipids to diacylglycerol and IP3 to trigger the inositol
phospholipid signaling pathway. Activated by certain receptor tyrosine kinases.
phospholipid exchange protein Water-soluble carrier protein that transfers a
phospholipid molecule from one membrane to another.
phospholipid The main category of lipid molecules used to construct
biological membranes. Generally composed of two fatty acids linked through
glycerol phosphate to one of a variety of polar groups.
phosphoprotein phosphatase Enzyme that removes a phosphate group from a
protein by hydrolysis.
phosphorylation Reaction in which a phosphate group becomes covalently
coupled to another molecule.
photochemical reaction center The part of a photosystem that converts light
energy into chemical energy.
photon Elementary particle of light and other electromagnetic radiation.
photoreceptor Cell or molecule that is sensitive to light.
photosynthesis Process by which plants, algae and some bacteria use the
energy of sunlight to drive the synthesis of organic molecules from carbon
dioxide and water.
photosynthetic electron-transfer Light-driven reactions in photosynthesis in
which electrons move along the electron-transport chain in the thylakoid
membrane, generating ATP and NADPH.
photosystem Multiprotein complex involved in photosynthesis that captures
the energy of sunlight and converts it to useful forms of energy.
phragmoplast Structure made of microtubules and actin filaments that forms
in the prospective plane of division of a plant cell and guides formation of the
cell plate.
phylogeny Evolutionary history of an organism or group of organisms, often
presented in chart form as a phylogenetic tree.
pinocytosis Type of endocytosis in which soluble materials are taken up from
the environment and incorporated into vesicles for digestion. Literally, "cell
drinking." (See also fluid-phase endocytosis.)
PKA see cyclic AMP-dependent protein kinase
PKC see protein kinase C
plant growth regulator Signal molecule (also known as a plant hormone) that
helps coordinate growth and development. Examples are ethylene, auxins,
gibberellins, cytokines, abscisic acid, and the brassinosteroids.
plasma membrane Membrane that surrounds a living cell.
plasmid Small circular DNA molecule that replicates independently of the
genome. Modified plasmids are used extensively as plasmid vectors for DNA
cloning.
plasmodesma (plasmodesmata) Communicating cell cell junction in plants
in which a channel of cytoplasm lined by plasma membrane connects two
adjacent cells through a small pore in their cell walls.
plastid Cytoplasmic organelle in plants, bounded by a double membrane, that
carries its own DNA and is often pigmented. Chloroplasts are plastids.
platelet Cell fragment, lacking a nucleus, that breaks off from a
megakaryocyte in the bone marrow and is found in large numbers in the
bloodstream. It helps initiate blood clotting when blood vessels are injured.
PLC-b see phospholipase C-b
PLC-g see phospholipase C-g
pleckstrin homology domain (PH domain) Protein domain found in
intracellular signaling proteins by which they bind to inositol phospholipids
phosphorylated by PI 3-kinase.
ploidy The number of complete sets of chromosomes in a genome. Diploid
organisms have two sets in their somatic cells, polyploid organisms more than
two. Natural polyploidy is the result of previous duplications of the whole
genome or the introduction of complete genomes from another species during
evolution.
plus end The end of a microtubule or actin filament at which addition of
monomers occurs most readily; the "fast-growing" end of a microtubule or
actin filament. The plus end of an actin filament is also known as the barbed
end. (See Panel 16 2, pp. 912 913.)
point mutation Change of a single nucleotide in DNA, especially in a region
of DNA coding for protein.
polar In the electrical sense, describes a structure (for example, a chemical
bond, chemical group, or molecule) with positive charge concentrated toward
one end and negative charge toward the other as a result of an uneven
distribution of electrons. Polar molecules are likely to be soluble in water.
polyisoprenoid see isoprenoid
polymer Large molecule made by covalently linking multiple identical or
similar units (monomers) together.
polymerase chain reaction see PCR
polymorphic Describes a gene with many different alleles, none of which is
predominant in the population.
polypeptide Linear polymer composed of multiple amino acids. Proteins are
large polypeptides, and the two terms can be used interchangeably.
polypeptide backbone The chain of repeating carbon and nitrogen atoms,
linked by peptide bonds, in a polypeptide or protein. The side chains of the
amino acids project from this backbone.
polyploid Describes a cell or an organism that contains more than two sets of
homologous chromosomes.
polyribosome (polysome) Messenger RNA molecule to which are attached a
number of ribosomes engaged in protein synthesis.
polysaccharide Linear or branched polymer of monosaccharides. They
include glycogen, starch, hyaluronic acid, and cellulose.
polytene chromosome Giant chromosome in which the DNA has undergone
repeated replication without separation into new chromosomes.
position effect Differences in gene expression that depend on the position of
the gene on the chromosome and probably reflect differences in the state of the
chromatin along the chromosome.
positional information Information supplied to or possessed by cells
according to their position in a multicellular organism. A cell's internal record
of its positional information is called its positional value.
positive control Type of control of gene expression in which the active DNAbinding
form of the regulatory protein turns the gene on.
posterior Situated toward the tail end of the body.
posttranscriptional control Any control on gene expression that is exerted at
a stage after transcription has begun.
posttranslational Describes any process involving a protein that occurs after
protein synthesis is completed.
posttranslational modification The enzyme-catalyzed change to a protein
made after it is synthesized. Examples are acetylation, cleavage, glycosylation,
methylation, phosphorylation, and prenylation.
pre-B cell Iimmediate precursor of a B cell.
prenylation Covalent attachment of an isoprenoid lipid group to a protein.
preprophase band Circumferential band of microtubules and actin filaments
that forms around a plant cell under the plasma membrane prior to mitosis and
cell division.
primary immune response Adaptive immune response to an antigen that is
made on first encounter with that antigen.
primary structure Sequence of monomer units in a linear polymer, such as
the amino acid sequence of a protein.
primordial germ cell Cell set aside early in embryonic development that is a
precursor to germ cells that give rise to gametes.
primosome A complex of DNA primase and DNA helicase that is formed on
the lagging strand during DNA replication, improving the efficiency of
replication.
prion An infectious abnormal form of a normal protein that is replicated in the
host by forcing the normal proteins of the same type to adopt the aberrant
structure.
prion disease Transmissible spongiform encephalopathies such as
Kreutzfeld Jacob disease in humans, scrapie in sheep and bovine spongiform
encephalopathy (BSE) in cattle, that are apparently caused and transmitted by
abnormal forms of a protein (prions).
probe Defined fragment of RNA or DNA, radioactively or chemically labeled,
used to locate specific nucleic acid sequences by hybridization.
procaryote (prokaryote) Single-celled microorganism whose cells lack a welldefined,
membrane-enclosed nucleus. The procaryotes comprise two of the
major domains of living organisms the Bacteria and the Archaea.
programmed cell death see apoptosis
prometaphase Phase of mitosis preceding metaphase in which the nuclear
envelope breaks down and chromosomes first attach to the spindle.
promoter Nucleotide sequence in DNA to which RNA polymerase binds to
begin transcription.
prophase First stage of mitosis, during which the chromosomes are condensed
but not yet attached to a mitotic spindle.
protease (proteinase, proteolytic enzyme) Enzyme such as trypsin that
degrades proteins by hydrolyzing some of their peptide bonds.
proteasome Large protein complex in the cytosol with proteolytic activity that
is responsible for degrading proteins that have been marked for destruction by
ubiquitylation or by some other means.
protein The major macromolecular constituent of cells. A linear polymer of
amino acids linked together by peptide bonds in a specific sequence.
protein domain Portion of a protein that has a tertiary structure of its own.
Larger proteins are generally composed of several domains, each connected to
the next by short flexible regions of polypeptide chain.
protein glycosylation Posttranslational addition of oligosaccharide side chains
to a protein.
protein kinase Enzyme that transfers the terminal phosphate group of ATP to
a specific amino acid of a target protein.
protein kinase C (PKC) Ca2+-dependent protein kinase that, when activated
by diacylglycerol and an increase in the concentration of Ca2+, phosphorylates
target proteins on specific serine and threonine residues.
protein module see module
protein phosphatase see phosphoprotein phosphatase
protein phosphorylation The covalent addition of a phosphate group to a side
chain of a protein catalyzed by a protein kinase.
protein translocator Membrane-bound protein that mediates the transport of
another protein across an organelle membrane.
proteoglycan Molecule consisting of one or more glycosaminoglycan (GAG)
chains attached to a core protein.
proteolysis Degradation of a protein by hydrolysis at one or more of its
peptide bonds.
proteolytic enzyme see protease
protofilament A linear chain of protein subunits joined end to end, which
associates laterally with other protofilaments to form cytoskeletal components
such as microtubules and intermediate filaments.
proton Positively charged subatomic particle that forms part of an atomic
nucleus. Hydrogen has a nucleus composed of a single proton (H+).
proton-motive force Driving force that moves protons across a membrane as a
result of an electrochemical proton gradient.
proto-oncogene Normal gene, usually concerned with the regulation of cell
proliferation, that can be converted into a cancer-promoting oncogene by
mutation.
protozoa Free-living or parasitic, nonphotosynthetic, single-celled, motile
eucaryotic organisms, such as Paramecium and Amoeba. Free-living protozoa
feed on bacteria or other microorganisms.
pseudogene Gene that has accumulated multiple mutations that has rendered it
inactive and nonfunctional.
pseudopodium (pseudopodia) Large cell-surface protrusion formed by
amoeboid cells as they crawl. More generally, any dynamic actin-rich
extension of the surface of an animal cell.
pulse-chase Technique for following the movement of a substance through a
biochemical or cellular pathway, by briefly adding the radioactively labeled
substance (the pulse) followed by the unlabeled substance (the chase).
pump Transmembrane protein that drives the active transport of ions or small
molecules across the lipid bilayer.
purine One of the two categories of nitrogen-containing ring compounds
found in DNA and RNA. Examples are adenine and guanine.
pyrimidine One of the two categories of nitrogen-containing ring compounds
found in DNA and RNA. Cytosine, thymine and uracil are pyrimidines.