Polycrates Xpo-'lik-ro-.tezV (fl. 6th century bc) Tyrant of the Aegean island of Samos c. 535-522 bc. He seized sole control during a festival of Hera, eliminating his two brothers, who shared his power. He quickly became notorious for piracy, as he sought to dominate nearby islands and Ionia. Initially a supporter of Egypt, he joined the Persians against Egypt in 525. Attempts by his opponents to remove him were unsuccessful until the Persian governor of Sardis lured him to the mainland and had him crucified. Polycrates brought wealth and prominence to Samos and was a patron of writers, including Anacreon.
polycystic ovary syndrome or Stein-Leventhal syndrome
Endocrine disorder in women, characterized by high androgen levels and infrequent or absent ovulation (see reproductive system). It causes a high proportion of female infertility cases. Symptoms vary but often include hirsutism, acne, and obesity. Menstruation may be irregular, absent, or excessive. The ovaries are usually enlarged and contain cysts. The disease may remain undiagnosed until a woman tries to conceive. The underly¬ ing cause is not fully understood. Treatment attempts to reduce androgen production. Infertility may be treated with clomiphene citrate or gona¬ dotropins to induce ovulation or with laparoscopy.
polyester Organic compound, any of a class of polymers formed by ester linkages between monomers. They are usually prepared from equivalent amounts of glycols and dibasic carboxylic acids, which undergo conden¬ sation polymerization to produce the polyester and water. Polyesters are strong, colourfast, and resistant to corrosion and chemical attack but tend
to build up a static electric charge. In addition to the familiar fibres and films (e.g., Dacron, Mylar), polyesters are used to make reinforced plas¬ tics, automotive parts, boat hulls, foams, laminates, tapes, piping, bottles, disposable filters, encapsulations, and coatings.
polyethylene (PE) Any of the polymers of ethylene, the largest class of plastics. Its simple basic structure, of ethylene monomers, can be linear (high-density and ultrahigh-molecular-weight polyethylene; HDPE and UHMWPE, respectively) or branched to a greater or lesser degree (low- density and linear low-density polyethylene; LDPE and LLDPE, respec¬ tively). The branched polyethylenes have similar structural characteristics (e.g., low crystalline content), properties (high flexibility), and uses (pack¬ aging film, plastic bags, mulch, insulation, squeeze bottles, toys, and housewares). HDPE has a dense, highly crystalline structure of high strength and moderate stiffness; uses include beverage bottles, liquid detergent jugs, crates, barrels, and luggage. UHMWPE is made with molecular weights 6-12 times that of HDPE; it can be spun and stretched into stiff, highly crystalline fibres with a tensile strength many times that of steel; uses include bulletproof vests.
polygamy \p9-'li-g3-me\ Marriage to more than one spouse at a time. Although the term may also refer to polyandry (marriage to more than one man) it is often used as a synonym for polygyny (marriage to more than one woman), which appears to have once been common in most of the world and is still found widely in some cultures. Polygyny seems to offer the husband increased prestige, economic stability, and sexual com¬ panionship in cultures where pregnancy and lactation dictate abstinence, while offering the wives a shared labour burden and an institutionalized role where a surplus of unmarried women might otherwise exist. The polygynous family is often fraught with bickering and sexual jealousy; to preserve harmony, one wife may be accorded seniority, and each wife and her children may have separate living quarters. Polyandry is relatively rare; in Tibet and Nepal, where brothers may marry a single woman, the practice serves to limit the number of descendants and keep limited land within the household.
Polygnotus or Polygnotos N.pa-lig-'no-tosX (b. c. 500, Thasos, Thrace— d. c. 440 bc, Athens, Greece) Greek painter. None of his works are extant, but accounts exist of monumental wall paintings in a severely Classical style at the hall of the Cnidians at Delphi: Sack of Troy and Ulysses Visiting Hades. His compositions were noted for the expression of emotion on faces and for the distribution of figures throughout the composition rather than on a single base line, as was the convention of the day.
polygon In geometry, any closed curve consisting of a set of line seg¬ ments (sides) connected such that no two segments cross. The simplest polygons are triangles (three sides), quadrilaterals (four sides), and pen¬ tagons (five sides). If none of the sides, when extended, intersects the polygon, it is a convex polygon; otherwise it is concave. A polygon with all sides equal is equilateral. One with all interior angles equal is equi¬ angular. Any polygon that is both equilateral and equiangular is a regular polygon (e.g., equilateral triangle, square).
polygraph See lie detector
polyhedron \,pa-le-'he-dr3n\ In Euclidean geometry, a three- dimensional object composed of a finite number of polygonal surfaces (faces). Technically, a polyhedron is the boundary between the interior and exterior of a solid. In general, polyhedrons are named according to number of faces. A tetrahedron has four faces, a pentahedron five, and so on; a cube is a six-sided regular polyhedron (hexahedron) whose faces are squares. The faces meet at line segments called edges, which meet at points called vertices. See also Platonic solid; Euler's formula.
Polykleitos See Polyclitus
polymer Vpa-lo-morX Any of a class of natural or synthetic substances composed of macromolecules that are multiples of monomers. The mono¬ mers need not all be the same or have the same structure. Polymers may consist of long chains of unbranched or branched monomers or may be cross-linked networks of monomers in two or three dimensions. Their backbones may be flexible or rigid. Some natural inorganic materials (e.g., the minerals diamond, graphite, and feldspar) and certain man-made inor¬ ganic materials (e.g., glass) have polymer-like structures. Many impor¬ tant natural materials are organic polymers, including cellulose (from sugar monomers; see polysaccharide), lignin, rubber, proteins (from amino acids), and nucleic acids (from nucleotides). Synthetic organic polymers
© 2006 Encyclopaedia Britannica, Inc.
1524 I polymerase chain reaction ► polyvinylchloride
include many plastics, including polyethylene, the nylons, polyurethanes, polyesters, vinyls (e.g., PVC), and synthetic rubbers. The silicone polymers, with an inorganic backbone of silicon and oxygen atoms and organic side groups, are among the most important mixed organic-inorganic com¬ pounds.
polymerase \pa-'b-m3- I ras\ chain reaction (PCR) Laboratory technique used to make numerous copies of specific DNA segments quickly and accurately. These are needed for various experiments and procedures in molecular biology, forensic analysis (DNA fingerprinting), evolutionary biology (to amplify DNA fragments found in ancient speci¬ mens), and medicine (to diagnose genetic disease or detect low viral counts). Invented by Kary Mulus, PCR requires a DNA template (as little as one molecule) to copy, nucleotides to build the copies, and the enzyme DNA polymerase to catalyze the formation of bonds between the nucle¬ otide monomers. Each three-step cycle (separating the two strands of the DNA double helix, marking the ends of the segment to be copied, and catalyzing the formation of bonds), which takes only minutes to complete, doubles the number of DNA strands present in the reaction medium. Rep¬ etition of this cycle many times results in an exponential increase in the amount of DNA.