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sis (q.v.). P elements have been cloned in E. coli plas- mids. When DNA molecules carrying P elements are microinjected into Drosophila embryos, some P ele- ments integrate into the germ-line chromosomes and are transmitted to the progeny of the injected flies. Active autonomous P elements are 2.9 kb in length and are flanked by inverted repeats that are 31 bp long. They have four exons (designated ORF 0, 1, 2, and 3). All four exons encode an 87 kDa transpos- ase, and the first three exons specify a 66 kDa re- pressor. P elements were discovered in D. melano- gaster, but they are absent from other sister species. However, P elements are common in D. willistoni and species related to it. D. melanogaster is believed to have received its P elements in the 1950s from D. willistoni.

Horizontal transfer may have been ac- complished by ectoparasitic mites. See Appendix C, 1982, Bingham et al., Spradling and Rubin; 1991, Houck et al.; 1994, Clark et al.; horizontal mobile elements (HMEs), promiscuous DNA, transposable el- ements, transposon tagging. P element transformation the transfer of specific DNA segments into germ-line cells of Drosophila us- ing the transposable P element to carry exogenous DNA fragments. See Appendix C, 1982, Spradling and Rubin. Pelger-Huet anomaly See Pelger nuclear anomaly. Pelger nuclear anomaly a hereditary abnormality in humans involving the nuclear morphology of the polymorphonuclear leukocytes. A similar syndrome occurs in the rabbit. Homozygotes show chondro- dystrophy in the rabbit, but not in humans.

pellagra a disease caused by deficiency of niacin. Pelomyxa a genus of amoebas. P. carolinensis is a favorite species for nuclear transplantation studies. P. palustris is of interest, since it lacks mitochondria and instead harbors anaerobic bacteria in a perma- nent symbiotic relationship. See Appendix A, Protoc- tista, Caryoblastea; serial symbiosis theory. penetrance the proportion of individuals of a specified genotype that show the expected pheno- type under a defined set of environmental condi- tions. For example, if all individuals carrying a domi- nant mutant gene show the mutant phenotype, the gene is said to show complete penetrance. See mani- festing heterozygote. penicillin any of a family of antibiotics derived from the mold Penicillium notatum and related spe- cies. It was once thought that the disruption of cell wall synthesis caused the cell wall to weaken and thus led to cell lysis. Evidence now exists that peni- cillin and almost all antibiotics indirectly activate a class of enzymes (autolysins) that are responsible for


cell lysis. Different penicillins differ only in the side chain  symbolized  by  R  in  the  above  formula. In the parent molecule, R = C6H5CH2CO−. Ampi- cillin is a penicillin derivative that is effective against a larger variety of Gram-negative bacteria than are most  other  penicillins.  Here,  R = C6H5CH(NH2) CO−. Penicillin-resistant bacteria synthesize peni- cillinases. These enzymes attack the beta-lactam ring to produce penicilloic acid, which has no bacterio- cidal activity. See Appendix C, 1929, Fleming; 1940, Florey et al.; 1949, Hodgkin et al.; ampR, transforma- tion.

penicillinases See penicillin.
penicillin enrichment technique  See penicillin se-lection technique.

penicillin selection technique a method for iso- lating an auxotrophic mutant from a wild-type cul- ture of bacteria by adding penicillin to minimal medium. Penicillin interferes with cell wall develop- ment, causing growing wild-type cells to rupture. However, nongrowing auxotrophic mutant cells are not killed. After one hour, about 99% of the wild- type cells have lysed, releasing their pool of metabo- lites into the medium. The culture must be filtered to remove these metabolites, because the auxotro- phic mutants would use them for growth and be subject to penicillin-induced lysis; the filtering also removes the penicillin. Alternatively, the enzyme penicillinase can be used to destroy the penicillin. Surviving auxotrophs are then supplied with en- riched medium. These cells produce the colonies that are harvested. See Appendix C, 1948, Lederberg and Zinder, Davis.

Penicillium notatum  the ascomycote fungus that synthesizes penicillin (q.v.).

penicilloic acid  See penicillin.
Pennsylvanian  See Carboniferous.
pentabarbital  Nembutal (q.v.).

pentosephosphate pathway  a pathway of hexose oxidation that is an alternative to the glycolysis-citric acid cycle (q.v.). The pathway involves an intercon version of phosphates of 7-, 6-, 5-, 4-, and 3-carbon sugars, and of a sugar lactone and sugar acids in a cycle that effects the complete oxidation of glucose to CO2 and H2O with the formation of 36 molecules of ATP. peplomers spikes that protrude from the enve- lopes of certain viruses. For example, the rabies virus (q.v.) has about 400 peplomers dispersed evenly over its surface, except for its planar end. Each spike consists of a trimer of virus-encoded glycoproteins. See enveloped viruses, virus.

pepsin a proteolytic enzyme from the gastric mu- cosa that functions at low pH. peptidase an enzyme catalyzing the hydrolytic cleavage of peptide bonds (q.v.). peptide a compound formed of two or more amino acids. peptide bond a covalent bond between two amino acids formed when the amino group of one is bond- ed to the carboxyl group of the other and water is eliminated. See Appendix C, 1902, Hofmeister and Fischer; amino acid, translation.

peptidoglycan a heteropolymer present in the cell walls of most bacteria. It consists of linear polysac- charide chains cross linked by short peptides (con- taining 8 amino acids). The structure that results forms a hollow net surrounding the bacterium, and it functions to strengthen the cell and to protect it from osmotic lysis. The cells of Archaebacteria (q.v.) do not contain peptidoglycans. See bacterial cell walls. peptidyl transferase See translation. peptidyl-RNA binding site See translation. PER the protein encoded by the gene called period (q.v.). perdurance in a concealed, yet durable state. In genetics literature, it refers to a situation in which


the phenotypic expression of a gene remains un- changed, after it has been deleted or inactivated, be- cause of the long-lived nature of its product. perennation the survival of plants from growing season to growing season with a period of reduced activity in between. perennial a plant that continues to grow from year to year. perfect flower See flower. perfusion the introduction of fluids into organs by injection into their arteries. pericarp the wall of the ovary after it has matured into a fruit; it may be dry and hard (as in the case of a nut) or fleshy (as in the case of a berry). See kernel. pericentric inversion an inversion that includes the centromere. periclinal refermng to a layer of cells running par- allel to the surface of a plant part. See anticlinal. periclinal chimera a plant made up of two geneti- cally different tissues, one surrounding the other. perikaryon that portion of the cell body of a neu- ron surrounding the nucleus as distinguished from the axon and dendrites. perinuclear cisterna the fluid-filled reservoir en- closed by the inner and outer membranes of the nu- clear envelope. period the first gene shown to control a circadian clock (q.v.).

Mutations at the per locus alter the rhythmicity of eclosion and of adult locomotor ac- tivity. This sex-linked gene has been cloned and se- quenced in D. melanogaster, pseudoobscura, and viri- lis. Null mutants eclosion or locomotor activity. Missense mutations shorten (perS) or lengthen (perL) both rhythms. The per+ gene is expressed in the normal adult nervous system, and it encodes a PER protein containing about 1,200 amino acids. The protein is present at extremely low levels, and it does not contain struc- tural motifs of the sort that characterize DNA-bind- ing domains, signal factors, or membrane-spanning domains.

The gene also modulates the interpulse in- tervals during courtship “songs.” These are generated by the extension and vibration of the males’ wings, and the songs enhance the females’ mating behavior. The songs are species-specific, and per has been shown to contain species-specific song instructions by introducing a cloned copy of per from D. simulans into the genome of a D. melanogaster per0 mutant. See Appendix C, 1971, Konopka and Benzer; 1984, Bargiello and Young; 1991, Wheeler et al.; fre- quency. periodic acid Schiff procedure a staining proce- dure for demonstrating polysaccharides. Abbrevi- ated PAS.

See Schiff’s reagent. periodicity in molecular genetics, the number of base pairs per turn of the DNA double helix. periodic table an arrangement of the chemical ele- ments (q.v.) in order of increasing atomic number (see page 330). Elements with similar properties are placed one under the other, yielding groups and families of elements. All living organisms consist of organic compounds derived from hydrogen (H), car- bon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S).

The alkali metals, sodium (Na) and potassium (K), the alkaline earth metals magnesium (Mg) and calcium (Ca), and the halogens chlorine (Cl) and iodine (I) are also biologically important. Other elements are vital but occur in trace amounts. The dark boxes in the table show the relative abun- dance of the biologically important elements found in the human body. The values give the number of atoms in a total of 100,000. See Appendix C, 1869, Mendeleev; chemical elements.

peripatric speciation a model proposing that spe- ciation occurs in small populations isolated on the periphery of the distribution of the parental popula- tion, as opposed to parapatric speciation (q.v.). The isolated populations may undergo shifts in their gene frequencies under the influence of genetic drift. This is most likely to occur if new populations arise from a few founder individuals and no gene flow occurs between the isolates and the main population. Con- sequently the most rapid evolutionary changes do not occur in widespread populous species, but in small founder populations. See Chronology C, 1954, Mayr; founder effect. Peripatus See living fossil.

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