pig See Sus scrofa. pigeon domestic breeds include the homing pi- geon, carneaux, dragoon, white Maltese, white king, fantail, pouter, tumbler, roller, Jacobin, barb, carrier pigeon, and the ptarmigan. The species is Columbia livia domestica. pigmentation See agouti, albinism, melanin, plum- age pigmentation genes, SLC 24A5. pilin See pilus. pillotinas large symbiotic spirochaetes living in the hind gut of termites. These spirochaetes are of interest because they contain microtubules, and the presence of microtubules in spirochaetes supports the symbiotic theory of the origin of undulipodia. See motility symbiosis. pilus (plural pili) a filamentous, hollow appendage extending from the surface of a conjugating bacte- rium. In E.
coli pili serve as tubes through which DNA from a “male” (F+ and Hfr) cell is transferred to a recipient, “female” (F−) cell. Pili are composed of a glucophosphoprotein called pilin. Pilin mole- cules are arranged helically around a central canal about 2.5 nanometers in diameter. See androphages, F factor, fimbria. pin the type of flower characterized by long styles and low anthers found among distylic species such as seen in the genus Primula. See thrum. pinocytosis the engulfment of liquid droplets by a cell through the production of pinosomes (q.v.). pinosome a membrane-bounded cytoplasmic ves- icle produced by the budding off of localized in- vaginations of the plasma membrane. Recently pino- cytosed fluids are segregated within the cell in pinosomes. pioneer the first plant or animal species to become established in a previously uninhabited area. PIR databases Protein Information Resource data- bases. See Appendix E.
pistil the female reproductive organ of the flower (q.v.), consisting of ovary, style, and stigma. See carpel. pistillate designating a flower having one or more pistils and no stamens. Pisum sativum the garden pea, Mendel’s experi- mental organism. It has 7 pairs of chromosomes and a genome size of 4.1 gbp. See Appendix A, Plantae, Angiospermae, Dicotyledoneae, Leguminales, Appen- dix C, 1822-24, Knight, Gross, Seton; 1856, 1865, Mendel; 1990, Bhattcharyya et al.; Appendix E. pitch the number of base pairs in one complete revolution of a DNA double helix. See twisting number. pith parenchymatous tissue in the center of roots and stems. Pithecanthropus erectus currently called Homo erectus erectus (q.v.).
pituitary dwarfism in humans, a form of ateliosis inherited as an autosomal recessive. There are two types: (1) primordial dwarfism, in which only growth hormone is deficient, and (2) panhypopituitarism, in which all hormones of the anterior pitiutary are de- ficient. See ateliosis, hereditary growth hormone de- ficiencies, human growth hormone, Laron dwarfism, midget. pituitary gland the master endocrine gland, which lies beneath the floor of the brain, within the skull, of vertebrates. See adenohypophysis, human growth hormone, hypothalamus, neurohypophysis. pK dissociation constant (q.v.). PKU phenylketonuria (q.v.). placebo an inactive substance given to certain pa- tients randomly chosen (without their knowledge) from a group. A new medicinal compound is admin- istered to the other patients. The effectiveness of the compound is determined by comparing the progress of the treated patients with those receiving the pla- cebo.
placenta an organ consisting of embryonic and maternal tissues in close union through which the embryo of a viviparous animal is nourished. placoderms cartilagenous fishes representing the first vertebrates with jaws. Placoderm fossils appear in the upper Silurian and survive through the lower Permian. plague a disease caused by Yersinia pestis (q.v.). Bubonic plague is transmitted to humans by the bites of fleas living on infected rats. Pneumonic plague is spread by inhaling respiratory droplets ex- pelled by infected persons. Plantae the plant kingdom. Its members are all eu- karyotes made up of cells containing green plastids. See Appendix A; Eukaryotes. plantain See Musaceae.
Plant growth regulators
plant growth regulators small molecules synthe- sized by a variety of plant tissues that can act locally or at a distance to regulate plant development. The regulators include abscisic acid, auxins, cytokinins, ethylene, and gibberellins (all of which see). plaque a clear, round area on an otherwise opaque layer of bacteria or tissue-cultured cells where the cells have been lysed by a virulent virus. In the ex- ample shown below, the petri dish contains a gel in which a nutrient broth is suspended. Covering the surface of the medium is a lawn of bacteria. These arose from a layer of 1 × 108 bacteria that was depos- ited on the agar surface.
Subsequently, a small num- ber of viruses was spread over these cells. The holes represent points at which a phage particle was pres- ent. The initial phage particle was adsorbed to a bac- terium and after a short period the bacterium lysed, releasing new phage. These new phage in turn at- tacked neighboring bacteria and produced more phage. The process continued until holes visible to the naked eye appeared. Different phages can some- times be recognized by the morphology of the plaques they produce. The numerous small plaques in the figure are from T6 bacteriophage, while the four large plaques are from phage T7. Animal vi- ruses will also attack monolayer cultures of animal cells on petri plates, so it is possible to assay virus titer in the same way by plaque counts. See Appen- dix C, 1932, Ellis and Delbruck; 1952, Dulbecco.
plaque assay a technique for counting the num- ber of complete, infective phage in a culture by an appropriate dilution to ensure that no more than one phage can infect a given host cell, followed by counting the number of plaques that develop on a bacterial lawn. plaque-forming cells in immunology, antibody- secreting cells that can cause a hemolytic plaque with the aid of complement on a lawn of erythro- cytes. The term is also applied to cells in certain assay systems where the cell killing that creates the plaque is cell mediated, rather than antibody depen- dent. plasma See blood plasma.
plasmablasts highly proliferative cells that are de- velopmental intermediates between small B lympho- cytes and immunoglobulin-secreting mature plasma cells. plasma cell a terminally differentiated immuno- globulin-secreting cell of the B lymphocyte lineage. plasmacytoma synonymous with myeloma (q.v.). plasmagene a self-replicating cytoplasmic gene within an organelle or a symbiont of a eukaryotic cell or in a plasmid of a bacterial cell. plasmalemma plasma membrane (q.v.). plasma lipoproteins a multicomponent complex of proteins and lipids circulating in the plasma.
Plasma lipoproteins are grouped by their densities into four classes: high-density lipoproteins (HDL), low-density lipoproteins (LDL), intermediate-den- sity lipoproteins (IDL), and very-low-density lipo- proteins (VLDL). LDLs are about 25% protein and 75% lipid, and about half of the lipid is cholesterol. LDLs serve as the major cholesterol transport sys- tem in human plasma. See Appendix C, 1975, Gold- stein and Brown; familial hypercholesterolemia. plasma membrane the membrane that surrounds the cell. It is made up of a bilayer of phospholipids that contain interspersed proteins.
Cells have ways of confining membrane proteins to specific regions. For example, the cells that line the digestive tract have the proteins that function in transport localized at their apical surfaces. See cystic fibrosis, fluid mosaic concept, lipid bilayer model, unit membrane. plasma membrane receptors See cell-surface re- ceptors. plasma protein any of the dissolved proteins of vertebrate blood plasma that are responsible for holding fluid in blood vessels by osmosis. See Appen- dix A, 1955, Smithies. plasma thromboplastin component a protein (also called factor 9) that participates in the cascade of reactions that results in blood clotting (q.v.). A deficiency of factor 9 causes hemophilia B. The fac- tor 9 gene, HEMA (q.v.), is X-linked. It encodes a protein of 461 amino acids, of which 46 are in a leader sequence peptide (q.v.), and the remaining 415 make up the mature clotting growth factor. This
contains two EGF repeats. A catalytic domain gives it a serine protease activity. See epidermal growth factor, hemophilia, serine proteases. plasma transferrins beta globins that bind and transport iron to the bone marrow and tissue-storage areas. Numerous heritable transferrin variants are known. plasmid an extrachromosomal genetic element found in a variety of bacterial species that generally confers some evolutionary advantage to the host cell (i.e., resistance to antibiotics, production of colicins, etc.). Plasmids are double-stranded, closed DNA molecules ranging in size from 1 to 200 kilobases.
Plasmids whose replication is coupled to that of the host so that only a few would be present per bacte- rium are said to be under stringent control. Under re- laxed control, the number of plasmids per host cell may be from 10 to 100. See Appendix C, 1952, J. Lederberg; Bacillus, bacteriocins, nif (nitrogen-fixing) genes, pBR322, R (resistance) plasmid, sym-plasmid, Ti plasmid, Vibrio cholerae, virulence plasmid.
plasmid cloning vector a plasmid used in recom- binant DNA experiments as an acceptor of foreign DNA. Plasmid cloning vectors are generally small and replicate in a relaxed fashion. They are marked with antibiotic resistance genes and contain recogni- tion sites for restriction endonucleases in regions of the plasmid that are not essential for its replication. One widely used plasmid cloning vector is pBR322 (q.v.). See Appendix C, 1973, Cohen et al.; bacterial artificial chromosomes (BACs), P1 artificial chromo- somes (PACs), Ti plasmid. plasmid conduction the process whereby a conju- gative plasmid can help a nonmobilizable plasmid to be transferred from a donor to a recipient cell. Non- mobilizable plasmids cannot prepare their DNA for transfer, but can become mobilized by recombina- tion with a conjugative plasmid to form a single transferable DNA molecule. See plasmid donation, relaxation complex.
plasmid donation the process by which a noncon- jugative plasmid is transferred from a donor to a re- cipient cell via the effective contact function pro- vided by a conjugative plasmid. In E. coli, for example, the ColE 1 plasmid does not have genes for the establishment of effective contact, but an F plasmid in the same cell is conjugative and can pro- vide this function. plasmid engineering See recombinant DNA re- search, Ti plasmid. plasmid fusion See replicon fusion. plasmid rescue See transformation rescue. plasmin an enzyme, cleaved from plasminogen, that hydrolyzes fibrin. See blood clotting. plasminogen a blood proenzyme that is activated by cleavage of a single arg-val peptide bond to form the functional enzyme plasmin (q.v.).
plasmodesmata cytoplasmic threads that form delicate protoplasmic connections between adjacent plant cells. plasmodium a multinucleate mass of cytoplasm lacking internal cell boundaries. The term is used to refer to the amoeboid stage in the life cycle of Spor- ozoa (q.v.) and to the vegetative stage of Myxomy- cota (q.v.). Plasmodium the genus of protozoa that causes malaria in reptiles, birds, and mammals. The genus is estimated to have arisen 100-180 million years ago. Five species of medical importance are de- scribed below. Three of these species cause malaria characterized by a rhythmicity in the periodic fevers and chills experienced by the human host. These symptoms are associated with the release of merozo- (q.v.) from the synchronously rupturing red blood cells. In tertian and quartan malaria the cycles are 48 and 72 hours, respectively. In subtertian ma- laria the 24-hour period is frequently modified.
Plas- modium vivax and Plasmodium malariae are species that cause benign tertian malaria and quartan ma- laria, respectively, both less serious diseases of hu- mans. P. vivax is known to have 600-1,000 copies of genes that function to suppress the immune de- fenses of the host. These are clustered near the telo- meres. Plasmodium cynomolgi and P. knowlesi are species causing malaria in monkeys. The liver stage in the malaria cycle was first observed in P. cynomolgi and the role of the circumsporozoite protein as an immunogenic decoy has been studied in P. knowlesi. See Appendix A, Protoctista, Protozoa, Apicomplexa, Sporozoa; Appendix C, 1948, Shortt and Garnham; Plasmodium life cycle. Plasmodium falciparum the parasite that causes subtertian malaria, the most deadly form of the dis- ease in humans. It is a sporozoan with a genome size of 22.8 mbp and about 5,300 genes spread among 14 chromosomes. The base composition is unusual in that 81% of the bases are As or Ts. The more common situation is seen in the yeast Saccharomyces where As and Ts make up only 62% of the total. About 60% of the proteins predicted to be encoded by the Plasmodium genome have no similarity to the