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parts of the world show differences in the propor- tions that can taste PTC, ranging from 30%-90%. The ability to taste PTC is controlled by a taste re- ceptor gene (q.v.). See Appendix C, 1931, Fox.

phenylthiourea a synonym for phenylthiocarbam- ide (q.v.). pheoplasts a brown plastid of brown algae, dia- toms, and dinoflagellates. Also spelled phaeoplast. pheromone a chemical signal released by one or- ganism that, at low concentrations, can cause non- harmful responses in a second organism, often of the same species. Examples of pheromones are sex at- tractants, alarm substances, aggregation-promotion substances, territorial markers, and trail substances of insects. Pheromones functioning as sex attractants are also known in fungi and algae. See courtship rit- ual, mate choice, odorant, odorant receptor gene.

Philadelphia (Ph1) chromosome an aberrant chromosome first observed by researchers at the University of Pennsylvania and named after the city where the discovery was made. The chromosome is often found in patients suffering from chronic my- eloid leukemia, a disease in which several bone mar- row-derived cell lineages proliferate uncontrollably. Ph1 is generally a reciprocal translocation between human chromosomes 9 and 22, involving break points at 9q34 and 22q11. The translocation gener- ates a fusion gene made up of regulatory elements from a gene on chromosome 22 and the majority of the ORF of a proto-oncogene (q.v.) from chromo- some 9. See Appendix C, 1970, Nowell and Hunger- ford; 1971, O’Riordan et al.; 1972, Rowley; Abelson murine leukemia virus, AML1 gene, Burkitt lym- phoma, myeloproliferative disease. philopatric descriptive of organisms that tend to remain in the location where they were born and raised.

phi X174 ( X174) virus a coliphage whose ge- nome consists of a circular, positive sense, single stranded DNA molecule containing 5,286 nucleo- tides. Overlapping genes (q.v.) were discovered in this virus (q.v.). See Appendix C, 1959, Sinsheimer; 1967, Goulian et al.; 1977, Sanger et al.; Appen- dix F. phloem the vascular tissue that conducts nutrient fluids in vascular plants. phocomelia absence of the proximal portion of a limb or limbs, the hands or feet being attached to the trunk by a single bone. A genetic form occurs in humans that is inherited as an autosomal recessive. Phocomelia may also be caused by exposure of the developing embryo to the drug thalidomide (q.v.). phosphate bond energy the energy liberated as one mole of a phosphorylated compound undergoes hydrolysis to form free phosphoric acid. See ATP. phosphatidylinositol enzyme that functions at DNA damage checkpoints (q.v.). See RAD. phosphodiester any molecule containing the linkage

where R and R′ are carbon-containing groups, O is oxygen, and P is phosphorus. This type of covalent chemical bond involves the 5′ carbon of one pentose sugar (ribose or deoxyribose) and the 3′ carbon of an adjacent pentose sugar in RNA or DNA chains. See deoxyribonucleic acid. phosphodiesterase I a 5′ exonuclease that re- moves, by hydrolysis, 5′ nucleotides from the 3′ hy- droxy-terminus of oligonucleotides. phospholipid a lipid containing phosphate esters of glycerol or sphingosine. phosphorescence See luminescence. phosphorus an element universally found in small amounts in tissues, a component of nucleic acids. Atomic number 15; atomic weight 30.4735; valence 5+; most abundant isotope 3lP; radioisotope 32P (q.v.). phosphorylation the attachment of phosphate groups to appropriate target molecules. Biochemical phosphorylation reactions are used to trap energy and to form compounds that serve as intermediates during metabolic cycles. Phosphorylation is a major mechanism used by cells for the transduction of sig- nals from protein to protein. See adenosine phos-

phate, cellular signal transduction, citric acid cycle, glycolysis, oxidative phosphorylation, phosphate bond energy, protein kinase. phosvitin a phosphoprotein of relative molecular mass 35,000. Two molecules of phosvitin and one of lipovitellin comprise the basic subunit of the am- phibian yolk platelet. See vitellogenin. photoactivated crosslinking a technique for crosslinking a nucleic acid (e.g., a tRNA chain) to a polypeptide chain with which it is functionally asso- ciated (e.g., its cognate synthetase) by irradiating the synthetase-tRNA complex with ultraviolet light. The technique is used to locate the points of inti- mate contact between the two molecules. See RNase protection. photoautotroph an organism that can produce all of its nutritonal and energy requirements using only inorganic compounds and light. photoelectric effect a process in which a photon ejects an electron from an atom so that all the en- ergy of the photon is absorbed in separating the elec- tron and in imparting kinetic energy to it. Contrast with Compton effect. photograph 51 Rosalind Franklin’s x-ray diffrac- tion photograph of the B form of deoxyribonucleic acid (q.v.). The photo provided Watson and Crick with certain data essential for constructing their three-dimensional model of DNA. It revealed the molecule to be a helix that undergoes a complete revolution every 3.4 nm with ten nucleotide pairs per turn. The photograph was taken May 1, 1952. See Appendix C, Bibliography, Secret of Photo 51.

photographic rotation technique a technique used to establish the symmetry of a structure (such as a virus) observed in an electron micrograph. The mi- crograph is printed n times, the enlarging paper be- ing rotated 360°/n between successive exposures. Structures with n-fold radial symmetry show rein- forcement of detail, whereas the micrograph will show no reinforcement when tested for n − 1 or n + 1 symmetry. photolyase See photoreactivating enzyme. photolysis decomposition of compounds by radi- ant energy, especially light. photomicrography the technique of making pho- tographs through a light microscope. photon a quantum of electromagnetic energy. photoperiodism the response of organisms to varying periods of light. In plants, for example, the photoperiod controls flowering. See phytochrome. photophosphorylation the addition of phosphate to AMP and ADP through the energy provided by light during photosynthesis. photoreactivating enzyme an exonuclease cata- lyzing a photochemical reaction that removes UV- induced thymine dimers from DNA.

This enzyme, called photolyase, is widely distributed in nature, from bacteria to marsupials, but is not found in pla- cental mammals. Photolyase is very unusual in that it is the only enzyme (other than one involved in photosynthesis) that uses energy from light to drive its activities. photoreactivation reversal of injury to cells caused by ultraviolet light, accomplished by postir- radiation exposure to visible light waves. See Appen- dix C, 1949, Kelner; thymine dimer. photoreceptor a biological light receptor. See cone, ommatidium, rod. photosynthesis the enzymatic conversion of light energy into chemical energy in green plants, algae, and cyanobacteria that results in the formation of carbohydrates and oxygen from carbon dioxide and water. The net chemical reaction is CO2 + H2O → CH2O + O2. Here CH2O represents carbohydrate (q.v.) and chlorophyll (q.v.) functions as the light- harvesting molecule. The fixing of gaseous CO2 into an organic product is catalyzed by ribulose-1, 5-bis- phosphate carboxylase oxygenase (q.v.) See chloro- plast. phototrophs organisms that use light as the source of energy for metabolism and growth. Examples are cyanobacteria, algae and plants, all of which undergo photosynthesis (q.v.). Contrast with chemotrophs.

phototropism a growth movement induced by a light stimulus. Plant shoots grow toward a light source (are positively phototropic); roots grow away (show negative phototropism). phragmoplast a differentiated region of the plant cell that forms during late anaphase or early telo- phase between the separating groups of chromo- somes. The phragmoplast contains numerous micro- tubules that function to transport material used by the developing cell plate. Once the cell plate forms it divides the phragmoplast into two parts, and sub- sequently the cell plate is transformed into the mid- dle lamella of the mature cell wall.

Phycomyces a genus belonging to the same family as the common bread molds Mucor and Rhizopus. Two species, P. nitens and P. blakesleeanus, are fa- vorites for genetic studies. See Appendix A, Fungi, Zygomycota. phyletic evolution the gradual transformation of one species into another without branching; anagen- esis; vertical evolution. phyletic speciation See phyletic evolution. phylogenetic tree a diagram that portrays the hy- pothesized genealogical ties and sequence of histori- cal ancestor/descendant relationships linking individ- ual organisms, populations, or taxa. When species are considered they are represented as line segments, and points of branching correspond to subsequent speciation events.

When possible, the lineage is pre- sented in relation to a geological time scale. See Ap- pendix C, 1936, Sturtevant and Dobzhansky; 1963, Margoliash; cladogram, PhyloCode. phylogeny the relationships of groups of organ- isms as reflected by their evolutionary history. phylum See classification. Physarum polycephalum See Myxomycota. physical map a map that shows the linear order of identifiable landmarks on chromosomal DNA, such as restriction enzyme cutting sites, sequence tagged sites, ORFs, and others. The distances between land- marks are always determined by methods other than genetic recombination (e.g., nucleotide sequencing, overlapping deletions in polytene chromosomes, electron micrographs of heteroduplex DNAs, etc.).

The lowest resolution physical map would show the pattern of horizontal bands on each stained chromo- some; the highest resolution map would give the complete nucleotide sequences for all chromosomes in the genome. Cloning vectors that can carry large DNA inserts are very useful in constructing physical maps of chromosomes or whole genomes. Compare with genetic map, linkage map. See bacterial artificial chromosomes (BACs), cosmid, P1 artificial chromo- somes (PACs), yeast artificial chromosomes (YACs).

physiological saline an isotonic, aqueous solution of salts used for temporarily maintaining living cells. physiology the study of the dynamic processes of living organisms. phytochrome the molecule responsible for the photoperiodic control of flowering. During the day, a form of phytochrome that absorbs light at the far red end of the spectrum accumulates in plants. This form of the pigment inhibits flowering in short-day plants (q.v.) and stimulates flowering in long-day plants (q.v.). During darkness this compound reverts to a red-absorbing form that is stimulatory to the flowering of short-day plants and inhibitory to long- day plants. Whether a plant belongs to the short-or long-day class is genetically controlled. phytohemagglutinin a lectin (q.v.) extracted from the red kidney bean, Phaseolus vulgaris, that aggluti- nates human erythrocytes and stimulates lympho- cytes to undergo mitosis.

See Appendix C, 1960, No- well. phytohormone a plant hormone. See auxin, cytoki- nin, gibberellin. Phytophthora infestans the oomycete responsible for the highly destructive downy mildew or “late blight” of potatoes. This disease was the cause of Irish potato famine, 1843-1847, which was a princi- pal reason for the waves of Irish immigration to the United States. P. infestans is incorrectly called a fun- gus in the earlier literature. See Appendix A, Proto- ctista, Oomycota. phytotron a group of rooms used for growing plants under controlled, reproducible, environmental con- ditions. pI isoelectric point (q.v.). picogram 10−12 gram. picornavirus a group of extremely small RNA vi- ruses. The name is derived from the prefix pico (meaning small) + RNA + virus. The polio virus be- longs to this group. piebald designating an animal, especially a horse, having patches of black and white or of different col- ors. A pinto horse is piebald. Pieris a genus of small butterflies extensively stud- ied by ecological geneticists.

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