releases more energy, which can be used in certain reactions that require more of a “push” to get them going. See also ATP,

ORTHOPHOSPHATE CLEAVAGE.

Pyrrolizidine Alkaloids A c l a s s o f t o x i c chemical compounds which are produced naturally by certain plants, as a defense mechanism (against predators). One of the

pyrrolizidine alkaloids, monocrotaline is consumed (preferentially) by the larvae (caterpillars) of the moth Utetheisa ornatrix. That moth subsequently utilizes the monocrotaline content of its body as a defense mechanism itself, against spiders that would otherwise eat that moth. See also ALKALOIDS,

TOXIN.

P

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Q

	Q-beta Replicase A viral RNA polymerase	Quantitative Structure-Activity Relation-
	secreted by a bacteriophage that infects	ship (QSAR) A computer modeling tech-
	Escherichia coli bacteria. Q-beta replicase	nique that enables researchers (e.g., drug
	can copy a naturally occurring RNA (mole-	development chemists) to predict the likely
	cule) sequence (e.g., from bacteria, viruses,	activity (e.g., effect on tissue) of a new com-
	fungi, or tumor cells) at a geometric (i.e.,	pound before that compound is actually cre-
	very fast) rate. See also POLYMERASE, BACTE-	ated. QSAR is based on data from decades
	RIOPHAGE, RIBONUCLEIC ACID (RNA), Q-BETA	of research investigating the impact on
	REPLICASE TECHNIQUE.	“activity” of the chemical structures of thou-
	Q-beta Replicase Technique An RNA assay	sands of thoroughly studied molecules. For
	(test) that “amplifies RNA probes” that a	example, the biological activity (i.e., bacte-
	researcher is seeking. For instance, by using	ria-killing effectiveness) of most antibiotics
	the Q-beta replicase technique to assay for	correlates with their tendency to dimerize
	the presence of RNA specific to the AIDS	(i.e., link two molecules into a single molec-
	virus, it is possible to detect an AIDS infec-	ular unit). See also BIOLOGICAL ACTIVITY,
	tion in a patient’s blood sample long before	PHARMACOPHORE, ANTIBIOTIC, PHARMACOKINET-
	that infection has progressed to the point	ICS, PHARMACOLOGY, ANALOGUE, RATIONAL
	where antibodies would appear in the blood.	DRUG DESIGN, IN SILICO SCREENING, POLYMER.
	See also Q-BETA REPLICASE, RNA PROBES, RIBO-	Quantitative Structure-Property Relation-
	NUCLEIC ACID (RNA), POSITIVE AND NEGATIVE	ship (QSPR) A computer modeling tech-
	SELECTION (PNS), ASSAY, IMMUNOASSAY, ANTI-	nique that enables scientists to predict the
	BODY, POLYMERASE CHAIN REACTION (PCR) TECH-	likely properties of a new chemical com-
	NIQUE, COCLONING, WESTERN BLOT TEST.	pound before that chemical compound is
	QCM Acronym for Quartz Crystal Microbal-	actually created. See also QUANTITATIVE
			Q
	ances. See also QUARTZ CRYSTAL MICROBAL-	STRUCTURE-ACTIVITY RELATIONSHIP (QSAR),
	ANCES.	ANALOGUE, RATIONAL DRUG DESIGN.
	QPCR Acronym for Quantitative Polymerase	Quantitative Trait Loci (QTL) I n d i v i d u a l
	Chain Reaction. Uses include gene expres-	specific DNA sequences that are related to
	sion analysis (i.e., quantitatively determine	known traits (e.g., litter size in animals, egg
	the amounts of each protein being expressed	production in birds, yield in crop plants.).
	by a cell), genotyping, DNA quantification,	See also MARKER (DNA SEQUENCE), TRAIT, LINK-
	etc. See also POLYMERASE CHAIN REACTION	AGE, DEOXYRIBONUCLEIC ACID (DNA), LINKAGE
	(PCR), CELL, GENE EXPRESSION PROFILING, PRO-	GROUP, LINKAGE MAP, GENE, SEQUENCE (OF A
	TEIN, GENOTYPE, DEOXYRIBONUCLEIC ACID (DNA).	DNA MOLECULE), MARKER ASSISTED SELECTION,
	QSAR See QUANTITATIVE STRUCTURE-ACTIVITY	CORN, HIGH-OIL CORN, RESTRICTION FRAGMENT
	RELATIONSHIP (QSAR).	LENGTH POLYMORPHISM (RFLP) TECHNIQUE, RAN-
	QSPR See QUANTITATIVE STRUCTURE-PROPERTY	DOM AMPLIFIED POLYMORPHIC DNA (RAPD) TECH-
	RELATIONSHIP (QSPR).	NIQUE, AFLP, SIMPLE SEQUENCE REPEAT (SSR),
	QTL See QUANTITATIVE TRAIT LOCI (QTL).	DNA MARKER TECHNIQUE.

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© 2002 by CRC Press LLC

BIO-

PHY-

Quantum Dot A “molecular structure” that is between 1–100 nanometers in size, so it is midway between molecular and solid states. Quantum dots have been constructed of semiconductor materials, crystallites (grown via molecular beam epitaxy), etc. Quantum dots could conceivably be constructed to act as receptors (e.g., on “biochips”) for specific ligands (e.g., a blood component that is only present in a diseased patient), in a way that would signal the presence of disease when a (blood) sample was passed over the quantum dot. That signal might be electronic, emission of specific-wavelength light, etc.

See also NANOMETERS (nm), NANOTECHNOLOGY,

RECEPTORS, MEMS (NANOTECHNOLOGY), BIOCHIP,

BIOELECTRONICS, MICROARRAY (TESTING),

LIGAND (IN BIOCHEMISTRY).

Quantum Wire A strip or “wire” of (electric- ity-) conducting material that is ten nanometers (nm) or less in its thickness or width. Indications from some research show that some forms of DNA molecules might be used as “quantum wires.” See also NANOMETERS

(NM), NANOTECHNOLOGY, DEOXYRIBONUCLEIC

A C I D (D N A ), M E M S (N A N O T E C H N O L O G Y ),

BIOELECTRONICS.

Quartz Crystal Microbalances Abbreviated QCM. Refers to biosensors consisting of small quartz crystals (to which are attached a source

of appropriate electric current), with sensitive measurement devices utilized to detect when the “attachment” of specific molecules (e.g., viruses, DNA sequences, antigens) to the quartz (or to layers of certain materials previously deposited on the quartz surface) causes the specific oscillation frequency of that quartz crystal to change in a way that enables (electronic) identification of the specific molecule(s) that attached themselves to the QCM.

See also BIOSENSORS (ELECTRONIC), VIRUS,

SEQUENCE (OF A DNA MOLECULE), ANTIGEN.

Quaternary Structure The three-dimensional structure of an oligomeric protein; particularly the manner in which the subunit chains fit together. See also PROTEIN, OLIGOMER, CON-

FIGURATION, NATIVE CONFORMATION.

Quencher Dye See MOLECULAR BEACON.

Quercetin A phytochemical naturally produced in apples, onions, and some other plants. Research indicates that human consumption of quercitin helps prevent prostate and some other cancers. See also

TOCHEMICALS, NUTRACEUTICALS, CANCER,

LOGICAL ACTIVITY.

Quick-Stop The term used to describe how DNA mutants of Escherichia coli cease replication immediately when the temperature is increased to 42°C (108°F). See also

ESCHERICHIA COLIFORM (E. COLI).

Q

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R

R Genes Refers to genes within some plants that confer resistance (to certain plant diseases) through common signaling pathways involved in (“surveillance” and activation of) natural plant defense responses (e.g., SAR). For example, the gene that codes for (causes the “manufacture” of) harpin protein is only present in a few bacteria (e.g., Erwinia amylovora), but R genes (i.e., those responsible for “surveillance” and activation of plant defense responses) which respond to the presence of harpin are present within the genomes of numerous species of plants. Thus, the spraying of man-made harpin protein onto any of those numerous species of (crop) plants causes those particular plants to initiate a protective/defensive response (cascade) against pathogenic bacteria, viruses, fungi, and even some insects. See also GENE, SIGNALING, PATH-

WAY, PROTEIN, HARPIN, SPECIES, SYSTEMIC

ACQUIRED RESISTANCE (SAR), PATHOGENIC,

PATHOGENESIS RELATED PROTEINS, STRESS PRO-

TEINS, CASCADE, BACTERIA, VIRUS, FUNGUS.

RAC See RECOMBINANT DNA ADVISORY COMMITTEE

(RAC).

Racemate An equimolar (i.e., equal number of molecules) mixture of the D and L stereoisomers of an optically active compound. A solution of dextrorotary (D) isomer (enantiomer) will rotate the plane in which the light was polarized a specific number of degrees to the right (dextro) while a solution containing the same number of levorotary (L) isomer molecules will rotate the plane in which the light was polarized the same number of degrees to the left (levo). The difference between D and L enantiomers is that the rotations of the plane of plane-polarized light are equal in magnitude, but opposite in sign. Hence, a 50:50 mixture of both enantiomers

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(known as a racemic mixture) shows no optical activity. That is, a solution containing a 50:50 mixture of enantiomers will not rotate the plane of plane polarized light when it is passed through the solution. See also ENANTI-

OMERS, STEREOISOMERS, LEVOROTARY (L) ISOMER,

DEXTROROTARY (D) ISOMER.

Racemic (mixture) See RACEMATE. Radioactive Isotope An isotope with an

unstable (atomic) nucleus that spontaneously emits radiation. The radiation emitted includes alpha particles, nucleons, electrons, and gamma rays. See also ISOTOPE.

Radioimmunoassay A very sensitive method of quantitating a specific antigen using a specific radiolabeled antibody. Functionally, the antibody is made radioactive by the covalent incorporation of radioactive iodine. The radioimmuno probe thus prepared is exposed to its antigen (which may be a protein, or a receptor, etc.) in excess (the exact amount will have to be determined). The radiolabeled probe then binds to the antigen and the unbound, free probe is washed away. The radioactivity is then determined (counted) and by comparison to a standard plot which has been constructed

previously, the amount of antigen (binding) is R determined. See also ANTIBODY, ASSAY, HOR-

MONE, RADIOIMMUNOTECHNIQUE.

Radioimmunotechnique A method of using a radiolabeled antibody to quantitate a known

antigen. See also RADIOIMMUNOASSAY, ANTIGEN,

ANTIBODY.

Radiolabeled From the Latin radiare, to emit beams. See also LABEL (RADIOACTIVE).

Random Amplified Polymorphic DNA (RAPD) Technique A genetic mapping methodology that utilizes as its basis the fact that specific DNA sequences (polymorphic DNA) are “repeated” (i.e., appear in

© 2002 by CRC Press LLC