delay the softening (i.e., spoilage) process by reducing the production of polygalacturonase through genetic engineering of the plant. In 1986, William Hiatt of the American company Calgene discovered the gene for polygalacturonase. That led to the company commercializing a tomato variety that had been genetically engineered to reduce production of polygalacturonase in that variety’s tomatoes (in 1994). See also EPSP SYNTHASE,

G E N E T I C E N G I N E E R I N G , A N T I S E N S E (D N A SEQUENCE), ENZYME, GENE, ACC SYNTHASE.

Polygenic A trait or end product (e.g., in a grain-produced crop) that requires simultaneous expression of more than one gene. For example, the level of protein produced in soybeans is controlled by five genes. See

also POLYHYDROXYLBUTYLATE (PHB), PROTEIN,

SOYBEAN PLANT, GENE, TRAIT, SOYBEAN OIL,

BCE4, ARABIDOPSIS THALIANA, PLASTID.

Polyhydroxyalkanoates See POLYHYDROXYAL-

KANOIC ACID (PHA).

Polyhydroxyalkanoic Acid (PHA) A “family” of chemically related “energy storage” substances (i.e., polyesters) that is naturally produced by certain bacteria (90 strains known). When PHA is removed from the bacteria and purified, this substance has physical properties quite similar to thermoplastics like polystyrene. PHA can quickly

Pbe broken down by soil microorganisms, so PHA is a biodegradable plastic.

During the 1990s, Daniel Solaiman and coworkers at the U.S. Department of Agriculture developed some bacteria strains (e.g.,

Bacillus thermoleovorans) that can produce PHA utilizing vegetable oils (e.g., soybean oil) as a major part of their “diet” (energy source). The precise chemical composition (and physical characteristics) of the PHA thereby produced varies according to the particular vegetable oil that is used as the energy source for those bacteria. For example, PHA thus produced utilizing soybean oil is very amorphous (formable).

In 1994, researchers transferred genes for the production of one PHA into the weed plant Arabidopsis thaliana and the crop plant rapeseed (canola). In 1997, researchers transferred phaB and phaC genes into the crop plant cotton (Gossypium hirsutum),

which caused those transformed plants to express (produce) PHA inside the fibers (seed hair cells) in the amount of 0.34% of the fiber weight. That PHA (inside those cotton fibers) resulted in a fabric (i.e., cottonPHA “blend”) possessing better insulation properties than traditional cotton fabric. See

also POLYHYDROXYLBUTYLATE (PHB), STARCH,

BACTERIA, BIOPOLYMER, ARABIDOPSIS THALIANA,

CANOLA, GENE, TRANSFORMATION, EXPRESS, BIO-

DEGRADABLE, MICROORGANISM, SOYBEAN OIL.

Polyhydroxylbutylate (PHB) O n e o f t h e PHAs, polyhydroxylbutylate is an “energy storage” substance that is naturally produced by certain bacteria, yeasts, and plants. When removed from the bacteria and purified, this substance has physical properties quite similar to thermoplastics like polystyrene. PHB can quickly be broken down by soil microorganisms, so PHB is a biodegradable plastic. Three separate enzymes are utilized by the organism in order to make the PHB molecule.

In 1994, researchers succeeded in transferring genes for PHB production into the weed plant Arabidopsis thaliana and the crop plant rapeseed (canola). Later (1997), researchers transferred phaB and phaC genes into the crop plant cotton (Gossypium hirsutum), which caused those transformed plants to express (produce) PHA inside the fibers (seed hair cells) in the amount of 0.34% of the fiber weight. That PHA (inside those cotton fibers) resulted in a fabric (i.e., cottonPHA “blend”) possessing better insulation properties than traditional cotton fabric. See

also STARCH, BACTERIA, BIOPOLYMER, ENZYME,

POLYGENIC, MICROORGANISM, POLYHYDROXYAL-

KANOIC ACID (PHA), CANOLA, ARABIDOPSIS

THALIANA, GENE, EXPRESS, BIODEGRADABLE.

Polymer A molecule possessing a regular, repeating, covalently bonded arrangement of smaller units called monomers. By analogy, a chain (polymer) that is composed of links (monomer) hooked together. See also OLIGO-

MER, PROTEIN, NUCLEIC ACIDS.

Polymerase An enzyme that catalyzes the assembly of nucleotides into RNA (RNA polymerase) and of deoxynucleotides into DNA (DNA polymerase). See also

MERASE, RNA POLYMERASE, REVERSE TRANSCRIPTASES, DNA, RNA, TAQ.

Polymerase Chain Reaction (PCR) A reaction that uses the enzyme DNA polymerase to catalyze the formation of more DNA strands from an original one by the execution of repeated cycles of DNA synthesis. Functionally, this is accomplished by heating and melting double-stranded (hydrogen bonded) DNA into single-stranded (nonhydrogen bonded) DNA and producing an oligonucleotide primer complementary to each DNA strand. The primers bind to the DNA and mark it in such a way that the addition of DNA polymerase and deoxynucleoside triphosphates cause a new strand of DNA to form which is complementary to the target section of DNA. The process described previously is repeated (trait, product, etc.) again and again to produce millions of copies (amplicons) of the desired strand of DNA. PCR and its registered trademarks are the property of F. Hoffmann-La Roche & Co. AG, Basel, Switzerland. See also

MERASE CHAIN REACTION (PCR) TECHNIQUE,

NESTED PCR, DEOXYRIBONUCLEIC ACID (DNA),

DNA PROBE, PROBE, Q-BETA REPLICASE TECHNIQUE, COCLONING (OF MOLECULES), POSITIVE

AND NEGATIVE SELECTION (PNS), AMPLICON,

NESTED PCR, PRIMER (DNA).

Polymerase Chain Reaction (PCR) Technique Developed in 1984 and 1985 by Kary B. Mullis, Randall K. Saiki, Stephen J. Scharf, Fred A. Faloona, Glenn Horn, Henry A. Erlich, and Norman Arnheim, the PCR technique is an in vitro method that greatly amplifies (makes millions of copies of) DNA sequences that otherwise could not be detected or studied. It can be utilized to amplify a given DNA sequence that constitutes less than one part per million of initial sample (e.g., a 100- base-pair target DNA sequence within the genome of one of the higher organisms, which can contain up to 500 million base pairs). The procedure alleviates the necessity of in vivo replication of a target DNA sequence, or of replication of one-of-a-kind tiny DNA samples (e.g., from a crime scene). See also

IN VITRO , IN VIVO , POLYMERASE CHAIN REACTION

(PCR), AMPLICON, NESTED PCR, DEOXYRIBONUCLEIC

ACID (DNA), BASE PAIR (bp), GENOME, SEQUENCE

(OF A DNA MOLECULE), TAQ, DNA POLYMERASE,

PRIMER (DNA).

POLYMORPHISM (GENETIC), VIRUS, BACTERIA,

GLYCOPROTEIN, INTERLEUKIN- 8 (IL- 8), CELL,

NUCLEUS, PLASMA MEMBRANE.

Polypeptide (protein) A molecular chain of amino acids linked by peptide bonds. Synonymous with protein. Via the synthesis (of this “chain”) performed by ribosomes, each polypeptide (protein) in nature is the ultimate expression product of a gene. All of the amino acids commonly found in proteins have an asymmetric carbon atom, except the amino acid glycine. Thus, the polypeptide is potentially chiral in nature. See also PROTEIN,

AMINO ACID, GENE, PEPTIDE, STEREOISOMERS,

CHIRAL COMPOUND, EXPRESS, RIBOSOMES,

POLYRIBOSOME (POLYSOME), MESSENGER RNA

(mRNA).

Polyphenols Phytochemicals (e.g., naturally found in coffee, certain types of grapes, certain red wines, green tea, cocoa, etc.) that act as antioxidants when consumed by humans. For example, polyphenols are naturally produced within the beans of the cocoa (cacao) tree (Theobroma cacao), and thus are present in chocolate made from those beans. Polyphenols naturally produced in apples have been shown to inhibit certain bacteria in the human mouth from producing the particular glucans that lead to a buildup of plaque on teeth; prevention of such plaque

Pbuild-up may help prevent cavities from

forming in teeth. See also PHYTOCHEMICALS,

FLAVONOIDS, ATHEROSCLEROSIS, ANTIOXIDANTS,

OXIDATIVE STRESS, PHENOLIC HORMONES,

NUTRACEUTICALS, BACTERIA, GLUCANS.

Polyribosome (polysome) A complex of a messenger RNA (mRNA) molecule on which ribosomes (ribosomal RNA; rRNA) are anchored. A number of ribosomes bound to only a single mRNA molecule. One mRNA molecule hence functions as a template for a number of polypeptide chains at one time. See also RIBOSOMES, rRNA (ribosomal

RNA), MESSENGER RNA (mRNA).

Polysaccharides Linear and/or branched (structure) macromolecules (large molecules) composed of many monosaccharide units (monomers such as glucose) linked by glycosidic bonds. See also GLYCOSIDE,

MONOSACCHARIDES, AMYLOSE, AMYLOPECTIN.

Polysome See POLYRIBOSOME.

Polyunsaturated Fatty Acids (PUFA)

Unsaturated fatty acids, possessing more than one molecular double bond in their molecular “backbone” (i.e., they contain at least two less than the maximum possible number of hydrogen atoms). Enzymes (e.g., ∆ 12 desaturase) present in some oilseed plants (soybean, canola, corn/maize, etc.) convert some monounsaturated fatty acids (e.g., oleic acid) to some polyunsaturated fatty acids (e.g., linoleic acid), within their developing seeds. For example, soybean oil contains (historical average) 60% polyunsaturated fatty acids. Extensive research shows that polyunsaturated fatty acids (PUFA) impart a variety of health benefits to humans that consume them. In general, those health benefits include anti-inflammatory, antihypertensive (i.e., prevention of high blood pressure), reduction in cancer risk, reduction in the blood cholesterol levels, reduction in the risk of coronary heart disease (CHD), plus aiding in the development of retina and brain tissues.

For example, the n-3 (“omega-3”) PUFAs possess antithrombotic effects and also reduce blood concentrations of triglycerides. High dietary levels (in human diet) of the n-6 (“omega-6”) PUFAs have been related to a decreased risk of coronary heart disease (CHD). Research indicates that some of the beneficial effects of PUFAs occur via PUFA interactions with several types of nuclear receptors (present in cells of some human tissues), which results in (PUFA) modulation of certain gene(s) expression in those

cells. See also UNSATURATED FATTY ACID,

ESSENTIAL FATTY ACIDS, THROMBOSIS, TRIGLYC-

ERIDES, CORONARY HEART DISEASE (CHD), CAN-

CER, N- 3 FATTY ACIDS, SOYBEAN OIL, N- 6 FATTY

ACIDS, ENZYME, DOCOSAHEXANOIC ACID (DHA),

HIGHLY UNSATURATED FATTY ACIDS (HUFA),

EICOSAPENTANOIC ACID (EPA), CONJUGATED

LINOLEIC ACID (CLA), CELL, GENE, RECEPTORS,

NUCLEAR RECEPTORS, DEOXYRIBONUCLEIC ACID

(DNA), EXPRESS, GENE EXPRESSION, TRANSCRIP-

TION, TRANSCRIPTION FACTORS, SOYBEAN PLANT,

OLEIC ACID, LINOLEIC ACID, LINOLENIC ACID.

Porcine Somatotropin (PST) A h o r m o n e , produced in the pituitary gland of pigs, that increases a swine’s muscle tissue production

efficiency. Injecting this hormone causes a faster growing, leaner pig.

Porphyrins Complex nitrogenous compounds containing four substituted pyrroles covalently joined into a ring structure. When complexed with a central metal atom it is called a metalloporphyrin.

Position Effect A change in the expression of a gene brought about by its translocation to a new site in the genome. For example, a previously active gene may become inactive if placed on a new site in the genome. See

also GENOME, TRANSLATION, GENETIC MAP, MAP

DISTANCE, PROMOTER.

Positional Cloning A technique used by researchers to zero in on the gene (s) responsible for a given trait or disease. A genetic map of the organism’s genome is used to make an educated guess as to the precise location of the gene of interest (e.g., near marker “x” or “y”, etc.). Then those guessed genes are cloned, inserted into living organisms or cells, and tested to see if the guessed gene causes expression of the protein of interest (e.g., a protein that causes the disease that the researcher is attempting to

cure). See also CLONE (A MOLECULE), GENE,

GENE AMPLIFICATION, GENE DELIVERY, DNA PROBE, GENE MACHINE, GENETIC ENGINEERING,

GENETIC MAP, GENETIC MARKER, GENOME, MAP DISTANCE, FUNCTIONAL GENOMICS, POSITION

EFFECT, EXPRESS.

Positive and Negative Selection (PNS) A separation technique; a technique to speed up the task of selecting, from thousands of laboratory specimens, the few cells with precisely the desired genetic changes induced (via genetic engineering). The thousands of genetically altered cells are brought about (produced) by genetic engineering experiments. Many genetic alterations are accomplished by injecting or flooding (specimen) cells with fragments of new genetic material (genes). A few cells are produced that have precisely the desired genetic changes among a large number of cells that do not have the desired changes. Sort of like a “needle in a haystack.” By analogy, the few cells possessing the desired trait represent the needles while the multitude of cells not possessing the trait represent the hay.