A Dictionary of Genetics phần 7 ppt

344 polar body polar body the minute cell produced and dis- the progenitors of the germ cells. See Appendix C, 1866, Metchnikoff.carded during the development of an oocyte. A po- lar body contains one of the nuclei derived from the pole plasm in many vertebrate and invertebrate first or second division of meiosis, but has practically species, a specialized cytoplasmic region of the egg no cytoplasm. See ootid nucleus. or the zygote that contains germ-cell determinants polar fusion nucleus in plants, the product of the and other maternal products required for normal de- fusion of the two polar nuclei. This, after fusing with velopment in the early embryo. The cytoplasm lo- a male nucleus, gives rise to the tripoid endosperm cated at the posterior pole of Drosophila and the nucleus. See double fertilization. vegetal pole of Xenopus embryos are examples of pole plasm. In Drosophila, several maternal effect polar gene conversion a phenomenon in which a genes involved in pole plasm formation have been gradient of conversion frequencies exists from one identified and are known as grandchildless genes end of a gene to the other; sites closer to one end of (q.v.). Females carrying mutations in these genes a gene usually have higher conversion frequencies produce embryos that lack polar granules (q.v.) and than do those farther from that end. show other developmental defects. See cytoplasmic polar granules electron-dense, membrane-less, determinants, cytoplasmic localization, maternal ef- RNA-protein complexes often associated with mito- fect gene, maternal polarity mutants. chondria, found in the pole plasm (q.v.) and subse- polio virus the cause of poliomyelitis (infantile pa- quently incorporated into primordial germ cells ralysis). It is a positive-stranded RNA virus with a (q.v.) in a variety of species. Also called germinal 6.1 kb genome. Enormous polysomes (q.v.) contain- granules or P granules. ing 60 or more ribosomes occur in infected cells. polarity gradient the quantitative effect of a po- The entire genome is translated from a single initia- larity mutation in one gene on the expression of later tion site to form a single polyprotein molecule. This genes in the operon. The effect is a function of the is subsequently cleaved into both structural and non- distance between the nonsense codon and the next structural proteins. The structural proteins assemble chain-initiation signal. to form the icosahedral capsule of the virus. The vi- rus is remarkably stable, and it has been successfully polarity mutant 1. a mutant gene that is able to grown from archaeological specimens centuries old. reduce the rate of synthesis of the proteins that nor- See icosahedron. mally would be produced by wild-type alleles of the genes lying beyond it on the chromosome. Such Polish wheat I Triticum polonicum (N = 14). See genes exert their effect during the translation of a wheat. polycistronic message (q.v.). See regulator gene, translation. 2. a mutant gene that influences polar- pollen grain a microspore in flowering plants that ized patterns of embryonic development. See bicoid, germinates to form the male gametophyte (pollen engrailed, hunchback, maternal polarity mutants, zy- grain plus pollen tube), which contains three hap- gotic segmentation mutants. loid nuclei. One of these fertilizes the ovum, a sec- ond fuses with the two polar nuclei to form the 3N polarization microscope a compound light micro- endosperm, and the third (the vegetative nucleus) scope used for studying the anisotropic properties of degenerates once double fertilization (q.v.) has been objects and for rendering objects visible because of accomplished. their optical anisotropy. polar nuclei See ootid nucleus , pollen gra in, polo- pollen mother cell microsporocyte. cyte. pollen-restoring gene a gene that permits normal polaron a chromosomal segment within which po- microsporogenesis to occur in the presence of a cy- larized genetic recombination takes place by gene toplasmic male sterility factor. conversion. pollen tube the tube formed from a germinating polar tubules microtubules of the spindle appara- pollen grain that carries male gametes to the ovum. tus that originate at the centriolar or polar regions of See Appendix C, 1830, Amici. the cell. See chromosomal tubules. pole cell one of the cells that are precociously seg- pollination the transfer of pollen from anther to stigma. See Appendix C; 1694, Camerarius; pollenregated into the posterior pole of the insect embryo before blastoderm formation. Among these cells are grain, self-pollination. polycystic kidney disease 345 polocyte the small degenerate sister cell of the sec- polyclone See compartmentalization. ondary oocyte. This cell generally divides into two Polycomb (Pc) a Drosophila mutation that pro- polar bodies, which disintegrate. See polar body. duces additional sex combs (q.v.) on the second and polyacrylamide gel a gel prepared by mixing a third pairs of legs in males. Pc is at 3-47.1 on the monomer (acrylamide) with a cross-linking agent genetic map. Proteins encoded by the normal allele (N,N′-methylenebisacrylamide) in the presence of a inhibit Hox genes. Binding sites of Pc proteins have polymerizing agent. An insoluble three-dimensional been visualized by immunochemical staining of gi- network of monomer chains is formed. In water, the ant polytene chromosomes. The Pc protein and the network becomes hydrated. Depending upon the heterochromatin-associated protein 1 (HP1) (q.v.) relative proportions of the ingredients, it is possible share a homologous domain 37 amino acids long to prepare gels with different pore sizes. The gels near their N termini. The Pc proteins bind to histone can then be used to separate biological molecules 3 molecules (q.v.) that have been tagged by the ad- like proteins of a given range of sizes. dition of methyl groups to their tails. See Appendix C, 1989, Zink and Paro; histones, SUMO proteins. polyacrylamide gel electrophoresis See electro- phoresis. polycomplex structures, observed in certain in- sects, within oocyte nuclei, formed by the fusion of polyadenylation enzymatic addition of several ad- components from synaptonemal complexes (q.v.) enine nucleotides to the 3′ end of mRNA molecules that have detached from the diplotene chromo- as part of the processing that primary RNA tran- somes. scripts undergo prior to transport from the nucleus to the cytoplasm. The added segment is referred to as a “poly-A tail.” Histone mRNAs lack poly-A tails. See Appendix C, 1971, Darnell et al.; posttranscrip- tional processing. polyandry the state of having more than one male mate at one time. poly-A tail See polyadenylation. polycentric chromosome polycentromeric chro- mosome. See centromere. polycentromeric chromosome See centromere. polycistronic mRNA a messenger RNA that en- codes two or more proteins. The messenger may polycystic kidney disease one of the most com- later be cleaved into individual messages, each of mon genetic diseases in humans with about 1 in which is translated into a single protein, or a giant 1,000 individuals affected. The major feature of polypeptide chain may be translated that is later PKD is the development of fluid-filled cysts in the cleaved to yield the individual proteins. Polycis- kidneys that damage or destroy them. The disease is tronic mRNAs are common in prokaryotes. For ex- due to dominant mutations in a gene that maps to ample, the lac operon (q.v.)ofE. coli generates a 13.3 on the short arm of chromosome 16. The gene polycistronic mRNA. Contrast with monocistronic (PKD1) spans 52 kilobase pairs and generates a tran- mRNA. See histone genes, polyprotein, retroviruses, script containing 14,148 nucleotides distributed transcription unit, trans-splicing, ubiquitin. among 46 exons. The predicted PKD1 protein, poly- cystin, is a glycoprotein with a carboxyl tail that con-polyclonal an adjective applied to cells or mole- cules arising from more than one clone; e.g., an anti- tains about 225 amino acids and which protrudes into the cytoplasm. This is followed by about 1,500genic preparation (even a highly purified one) elicits the synthesis of various immunoglobulin molecules. amino acids containing transmembrane domains. The N-terminal extracellular portion of the proteinThese antibodies would react specifically with differ- ent components of the complex antigen molecule. contains about 2,500 amino acids, and these are sub- divided into domains that are thought to bind to aThus, the antibody preparation generated by such an antigen would be polyclonal in the sense that it variety of proteins and carbohydrates in the extra- cellular matrix. Polycystin is thought to function inwould contain immunoglobulins synthesized by dif- ferent clones of B lymphocytes. cellular signal transduction (q.v.) mediated through 346 polycythemia vera its cytoplasmic tail. See Appendix C, 1995, Hughes terized by ovarian tumors, polyfusomes often fail to form properly, and the pattern of germ cell divisionset al. and their differentiation are abnormal. See bag of polycythemia vera a disease in humans character- marbles (bam), fusome, hu-li tai shao (hts), otu, pro- ized by the overproduction of red blood cells. Eryth- oocyte. roblasts in the bone marrow are hypersensitive to erythropoietin (q.v.). See Janis kinase 2. polygamy polandry and/or polygyny. Compare with monogamy. polydactyly the occurrence of more than the usual number of fingers or toes. polygene one of a group of genes that together control a quantative character. See Appendix C, polyembryony the formation of multiple embryos 1941, Mather; oligogene, quantitative inheritance. from a zygote by its fission at an early developmental stage. Monozygotic twins constitute the simplest ex- polygenic character a quantitatively variable phe- ample of polyembryony. Monozygotic quadruplets notype dependent on the interaction of numerous are commonly formed by armadillos. In certain para- genes. sitic wasps, as many as 2,000 embryos can be formed polyglucosan a polymer such as glycogen made by polyembryony from a single zygote. up of a chain of glucose units. polyestrous mammal See estrous cycle. polygyny the mating of a male with more than polyethylene glycol a chemical used to promote one female during a single reproductive cycle. Com- the fusion of tissue-cultured cells, as in the produc- pare with monogamy, polyandry. tion of a hybridoma (q.v.). polyhedrin See baculoviruses. polyfusome a gelatinous mass assembled by the polylinker site a stretch of DNA engineered to fusion of the adjacent fusomes (q.v.) formed at con- have multiple sites for cleavage by specific restric- secutive cystocyte divisions in Drosophila. The dia- tion endonucleases (q.v.). polymer a macromolecule composed of a cova- lently bonded collection of repeating subunits or monomers linked together during a repetitive series of similar chemical reactions. Each strand of DNA is a linear polymer of nucleotide monomers. A linear polypeptide chain is a polymer of amino acid mono- mers. See monomer, oligomer. polymerase any enzyme that catalyzes the forma- tion of DNA or RNA molecules from deoxyribonu- cleotides and ribonucleotides, respectively (e.g., DNA polymerase, RNA polymerase). polymerase chain reaction (PCR) a highly sensi- tive technique for quickly amplifying a DNA seg- ment. PCR involves three major steps. First, the re-gram illustrates a polyfusome in a cystocyte clone during the divison of 8 cells into 16. Cell 1 is ob- action mixture containing the target DNA is heated to separate complementary DNA strands. Second,scured by the cells lying above it. In each of the other seven cells, a spindle and a ring canal (q.v.) the mixture is cooled, and synthetic primers with nucleotide sequences complementary to each end ofcan be seen. Pairs of centrioles lie at the spindle poles. The polyfusome protrudes through each ring the DNA are allowed to anneal to the separated strands. Finally, the temperature is raised again, andcanal and touches one pole of each spindle. As a re- sult of this orientation, one cell of each dividing pair a heat-stable DNA polymerase (q.v.) in the reaction mixture synthesizes new DNA strands by adding nu-will retain all previously formed ring canals, while the other will receive none. These spindle-fusome cleotide bases to the primers. These steps are re- peated for a series of replication cycles, each lastingalignments during the cycle of cystocyte divisions (q.v.) produce a branched chain of interconnected a few minutes, in an automated cycler that controls the required temperature variations. The number ofcells. There are always two central cells, each with four ring canals. In female sterile mutations charac- DNA strands doubles with each successive cycle, re- polyploidy 347 sulting in an exponential increase in the number of polyoma virus a virus that induces tumors in new- born mice, rats, and hamsters and can also transformcopies of the target DNA. Twenty cycles yield a mil- lionfold amplification; 30 cycles yield an amplifica- cultured mouse or rat cells. The genome of the vi- rus is a double-stranded, supercoiled, circular DNAtion factor of 1 billion. The ability of PCR to quickly and accurately generate billions of copies of the min- molecule containing about 5,300 base pairs. See Ap- pendix C, 1983, Rassoulzadegan et al.; oncogenic vi-utest amount of DNA has created a wealth of new practical applications in numerous areas, including rus, transformation. DNA cloning and sequencing, screening for genetic polyp 1. the sedentary form of a coelenterate. 2. disorders, detection of disease-causing organisms, a small stalked neoplasm projecting from a mucous DNA fingerprinting, and examination of species dif- surface (for example, an intestinal polyp). ferences. See Appendix C, 1985, Saiki, Mullis et al.; 1993, Smith and Mullis; ligase chain reaction, reverse polypeptide a polymer made up of less than 50 transcription-polymerase chain reaction, Taq DNA amino acids. See amino acid, peptide bond. polymerase. polyphasic lethal a mutation characterized by polymerization the formation of a polymer from two or more lethal phases separated by develop- a population of monomeric molecules. mental periods in which it produces no deaths. polymerization start site the nucleotide in a polyphenism the occurrence of several pheno- DNA promoter sequence from which the first nucle- types in a population that are not due to genetic dif- otide of an RNA transcript is synthesized. ferences between the individuals in question. polymorphic locus a genetic locus, in a popula- tion, at which the most common allele has a fre- polypheny pleiotropy (q.v.). quency less than 0.95. Compare with monomorphic polyphyletic group a group of species classified locus. together, some members of which are descended polymorphism the existence of two or more ge- from different ancestral populations. Contrast with netically different classes in the same interbreeding monophyletic group. population (Rh-positive and Rh-negative humans, for example). The polymorphism may be transient, polyploid designating a cell or an individual that or the proportions of the different classes may re- has more than two sets of chromosomes. main the same for many generations. In the latter polyploidy the situation where the number of case, the phenomenon is referred to as balanced poly- chromosome sets is greater than two. If N is the morphism. If the classes are located in different re- value for one set of chromosomes, a somatic cell can gions, geographic polymorphism exists. See Appendix be 2N (diploid), 3N (triploid), 4N (tetraploid), 5N C, 1954, Allison; 1966, Lewontin and Hubby. (pentaploid), 6N (hexaploid), and so forth. When polymorphonuclear leukocyte See granulocyte. compared to diploids, polyploid cells are generally larger and metabolically more active. Most genes polyneme hypothesis the concept that a newly continue to be expressed at the same relative levels. formed chromatid contains more than one DNA du- However, a few genes seem to sense increasing gene plex. Contrast with unineme hypothesis. dosage and raise or lower their levels of transcription polynucleotide a linear sequence of 20 or more appropriately. Polyploidy is a dominant factor in joined nucleotides. See oligonucleotide. plant evolution, where rounds of large scale genomic duplication have been followed by selective gene polynucleotide kinase an enzyme that phospho- loss. This conclusion arises from observations of an- rylates the 5′ hydroxyl termini produced by endonu- notated genomes where genes commonly occur in cleases (q.v.). duplicate copies. The frequency of polyploidy varies across plant groups. It is rare in conifers, whereaspolynucleotide phosphorylase the first enzyme shown to catalyze the synthesis of polynucleotides. 95% of fern species and 70% of angiosperms are polyploid. See Appendix A, Plantae; Appendix C,It was isolated from Azotobacter vinelandii in 1955, and it linked ribonucleotides together in a random 1917, Winge; 1937, Blakeslee and Avery; 1999, Gal- itski et al.; allopolyploid, Arabidopsis thaliana, auto-fashion. Subsequently this enzyme was used to pro- duce artificial messenger RNA molecules. See Ap- polyploid, bananas, colchicine, DNA chip, euploid, Gossypium, haploid or haploidy, Nicotiana, -ploid,pendix C, 1955, Grunberg-Manago and Ochoa; 1961, Nirenberg and Matthaei. Raphanobrassica, wheat. 348 polyprotein polyprotein a cistronic product that is posttransla- allel and forming a cable-like structure. See somatic pairing.tionally cleaved into several independent proteins. For example, an enkephalin precursor protein con- polythetic group a group of organisms that share tains six copies of met-enkephalin and one copy of a large number of features, no single one of which is leu-enkephalin. See enkephalins, retroviruses. either essential for group membership or is sufficient polyribonucleotide phosphorylase See polynu- to make an organism a member of the group. cleotide phosphorylase. polytopic pertaining to the distribution of subspe- polyribosome polysome (q.v.). cies in two or more geographically discontinuous areas. polysaccharide a carbohydrate formed by the polymerization of many monosaccharide units. polytrophic meroistic ovary See insect ovary Starch, cellulose, and glycogen (q.v.) are examples types. of polysaccharides. polytypic species a species subdivided into a polysomaticism the phenomenon where an indi- number of specialized races. vidual contains diploid and polyploid cells in the pome a fleshy, many-seeded fruit such as the apple same tissue. or pear, in which the enlarged end of the flower stalk polysome a multiple structure containing a group forms much of the flesh. of ribosomes held together by a molecule of messen- Pompe disease a hereditary glycogen storage dis- ger RNA. A contraction of polyribosome. See Appen- ease in humans arising from a deficiency of the lyso- dix C, 1962, Warner et al. somal enzyme α-1,4-glucosidase, due to a recessive polysomy the reduplication of some but not all of gene on chromosome 17. Prevalence of the disease the chromosomes of a set beyond the normal diploid is 1/100,000. number. A metafemale Drosophila is polysomic (tri- Pongidae the family of primates containing all an- somic for the X). thropoid apes. polyspermy the penetration of more than one Pongo pygmaeus the orangutan, a primate with a sperm into one ovum at the time of fertilization. haploid chromosome number of 24. About 30 bio- Polysphondylium pallidum See Acrasiomycota. chemical marker genes have been distributed among 20 linkage groups. See Hominoidea. polytene chromosome a giant cable-like chromo- some consisting of many identical chromatids lying popcorn See corn, quantitative inheritance. in parallel. The chromatin is hypercoiled in localized population a local (geographically defined) group regions, and since the chromatids are in register, a of conspecific organisms sharing a common gene pattern of bands is produced vertical to the long axis pool; also called a deme. of the chromosome. Polytene chromosomes are found within a limited number of organisms. They are population biology the study of the patterns in present in the macronucleus anlage of some ciliates, which organisms are related in space and time. Such in the synergids and antipodal cells of the ovules of disciplines as ecology, taxonomy, ethology, popula- certain angiosperms, and in various tissues of dipter- tion genetics, and others that deal primarily with the ans. The Drosophila salivary gland chromosomes interactions of organisms or groups of organisms (q.v.) have been studied most extensively. See Ap- (demes, species, etc.) are included under this term. pendix C, 1881, Balbiani; 1912, Rambousek; 1934, population cage a special cage in which Drosoph- Bauer; 1952, Beermann; 1959, Pelling; 1969, Am- ila populations can be reared for many generations. mermann; 1980, Gronemeyer and Pongs; Anophe- The cage is designed so that samples of the popu- les, Balbiani ring, Calliphora erythrocephala, Chiro- lation can be conveniently withdrawn and food nomus, Culex pipiens, Glyptotendipes barbipes, supplies can be replenished. See Appendix C, 1934, insulator DNAs, otu mutation, Rhynchosciara, Sciara, L’He ´ ritier and Teissier. Smittia. polytenization the continued replication of each population density 1. in ecology, the number of individuals of a population per unit of living spaceinterphase chromosome to produce giant chromo- somes made up of multiple chromatids lying in par- (e.g., per acre of land, per cubic meter of water, positional cloning 349 etc.). 2. in cell or tissue culture, the number of cells rates. The disease has a very high prevalence in South African populations of Dutch descent, pre-per unit area or volume of a culture vessel. See satu- ration density. sumably because of a founder effect (q.v.). population doubling level in cell or tissue cul- porphyrin any of a class of organic compounds in ture, the total number of population doublings of a which four pyrrole nuclei are connected in a ring cell line or strain since its initiation in vitro. structure usually associated with metals (like iron or magnesium). Porphyrins form parts of the hemoglo- population doubling time See doubling time. bin, cytochrome, and chlorophyll molecules. See heme. population genetics the study of the genetic com- position of populations. Population geneticists try to positional candidate approach in human genet- estimate gene frequencies and detect the selective ics, a strategy for identifying the gene responsible for influences that determine them in natural popu- a disease by mapping the mutant gene to a specific lations. They also build mathematic models to elu- chromosomal region and then looking for an appro- cidate the interaction of factors such as selection, priate candidate among the genes already localized population size, mutation, and migration upon the in that region. Individuals suffering from the disease fixation and loss of linked and unlinked genes. See are then tested for mutations in the candidate gene. Appendix C, 1908, Hardy, Weinberg; 1930–32, For example, a gene encoding a fibroblast growth Wright, Fisher, Haldane. factor receptor (FGFR 3 ) protein was discovered dur- population structure the manner in which a pop- ing a chromosome walk (q.v.) toward the Hunting- ulation is subdivided into local breeding groups or ton disease gene. Next, the gene for achondroplasia demes, the sizes of such demes in terms of the num- (q.v.) was mapped to the same chromosomal region. ber of breeding individuals, and the amount of mi- Finally, the FGFR 3 genes of dwarfs were found to gration or gene flow between demes. contain missense mutations, proving that mutations of the FGFR 3 gene were responsible for the retarded Populus a genus containing trees such as the growth characterizing the disease. trembling aspen (P. tremuloides), the black cotton- wood (P. trichocarpa), and the white poplar (P. positional cloning a strategy for identifying and alba.). Most species are dioecious (q.v.). The prog- cloning a gene based on a knowledge of its position eny of crosses between certain related species are in the genome (q.v.), with little or no information typically grown on plantations. These hybrids are about the function or product of the gene at the out- fertile and have exceptional growth and vigor. Popu- set. This strategy has been applied in a variety of lus trichocarpa is the first tree species selected for plant and animal species. In humans, the chromo- DNA sequencing because of its small genome size. somal position of the gene of interest is usually de- It is ϳ550 mbp, which is only 4× larger than Arabi- termined by linkage analyses of families affected by dopsis, but 40× smaller than pine. See Appendix A, a particular disease. A search is then made for genet- Plantae, Angiospermae, Dicotyledonae, Salicales. ically linked molecular markers, and the closest ones flanking the gene are used to start chromosome porcine referring to members of the pig family, es- walking (q.v.) in order to identify additional markers pecially the domestic pig Sus scrofa. with the closest possible linkage to the gene. The DNA defined by these molecular markers is thenporphyrias diseases caused by toxic accumulations of porphyrins (q.v.) and related compounds in tis- cloned and the gene residing between them identi- fied by a variety of means, such as by searching data-sues. Inherited porphyrias are due to mutations in genes that encode enzymes which catalyze steps in bases for genes within the identified genomic region, by sequencing the gene-containing region and look-the biosynthesis of heme (q.v.). There are at least eight enzymes that control steps in the heme biosyn- ing for an open reading frame(s), by comparing the suspected gene’s sequence and expression patternsthetic pathway. One is protoporphyrinogen oxidase (PPOX) which is encoded by a gene that maps to in mutant and wild-type individuals, and where pos- sible, by the ability of the putative gene to rescue1q22. This gene has 13 exons and spans about 8 kb. Mutations in the PPOX gene cause variegate por- (q.v.) a mutant phenotype (q.v.). Once identified, the gene is cloned for further analysis. Human dis-phyria (VP). The symptoms include photosensitiv- ity, abdominal pain, and mental disturbances includ- ease genes identified by positional cloning include cystic fibrosis, Duchenne muscular dystrophy, fragileing hallucinations, depression, and paranoia. Attacks of VP are often triggered by drugs such as barbitu- X syndrome, and Huntington disease (all of which 350 Positional Information Hypothesis See). Also called map-based cloning. Compare with CAP-cAMP complexes to promoters of bacterial genes involved in catabolism of sugars other thanfunctional cloning. See marker, open reading frame, positional candidate approach. glucose facilitates binding of RNA polymerase to these operons when glucose is absent. See glucose- Positional Information Hypothesis a model de- sensitive operons. Compare with negative gene con- veloped by Louis Wolpert to explain pattern speci- trol. fication (q.v.) during development. His idea is that each population of cells in an embryonic structure positive interference the interaction between lies in a field that contains a gradient of a chemical crossovers such that the occurrence of one exchange morphogen (q.v.). The position in the field deter- between homologous chromosomes reduces the mines the concentration of morphogen, and the cells likelihood of another in its vicinity. Compare with are programmed to enter one of a number of devel- negative interference. See Appendix C, 1916, Muller. opmental pathways depending on the concentration positive sense ssDNA or RNA See plus (+) and mi- level of the morphogen to which they are exposed. nus (−) viral strands. See Appendix C, 1969, Wolpert. position effects the change in the expression of a positive supercoiling See supercoiling. gene accompanying a change in the position of the positron a particle of the atomic nucleus equal in gene with respect to neighboring genes. The change mass to the electron and having an equal but oppo- in position may result from crossing over or from a site (positive) charge. chromosomal aberration. Position effects are of two types: the stable (S) type and the variegated (V) postcoitum after mating. type. S-type position effects are also called cis-trans postmating isolation mechanism See postzygotic position effects. S-type position effects involve cis- isolation mechanism. trons that possess at least two mutated sites separa- ble by intragenic recombination. In the cis configu- postmeiotic fusion a method for restoring dip- ration (m 1 m 2 /++) a normal phenotype is observed, loidy in eggs produced by parthenogenesis, involving whereas in the trans configuration (m 1 +/+m 2 ) a mu- union of two identical haploid nuclei formed by a tant phenotype is produced. A reasonable explana- mitotic division of the egg nucleus. tion for such an observation would be that the mRNA transcribed from a (++) chromatid would postmeiotic segregation in ascomycete fungi function normally, whereas the mRNAs transcribed such as Neurospora, the formation of heteroduplex from (m 1 m 2 ), (m 1 +), or (+m 2 ) chromatids would not. regions (by meiotic crossing over) that results in ab- V-type position effects generally involve the sup- errant 4 : 4 pattern of asci in which adjacent pairs of pression of activity of a wild-type gene when it is ascospores produced by mitotic division after meio- placed in contact with heterochromatin because of sis have different genetic compositions. See tetrad a chromosome aberration. Under some conditions, segregation types. the gene may escape suppression, and consequently postreductional disjunction referring to the sepa- the final phenotype may be variegated, with patches ration of alleles at particular heterozygous loci dur- of normal and mutant tissues. See Appendix C, 1925, ing the first meiotic division. If the loci are repre- Sturtevant; 1936, Schultz; 1945, Lewis; heterochro- sented by A and A′, in the case of postreductional matization, transvection. disjunction the two chromatics that enter one sister positive assortative mating See assortative mat- nucleus have one A and one A′ allele, whereas in the ing. case of prereductional disjunction both have A al- positive control control by a regulatory protein leles or both A′ alleles. that must bind to an operator before translation can postreplication repair repair to a DNA region take place. after a replication fork has passed that region or in positive eugenics See eugenics. nonreplicating DNA. positive feedback the enhancement or amplifica- posttranscriptional processing those modifica- tion of an effect by its own influence on the process tions made to pre-mRNA molecules before they that gives rise to it. leave the nucleus; also called nuclear processing. A gene containing three exons (E 1 ,E 2 , and E 3 ) and twopositive gene control enhancement of DNA tran- scription through binding of specific expressor mole- introns (I 1 and I 2 ) is diagrammed (page 351). RNA polymerase II transcribes the 3′-5′ strand of the genecules to promoter sites. For example, the binding of POU genes 351 Posttransciptional processing to form a 5′-3′ pre-mRNA molecule. Next, a meth- potato Solanum tuberosum, a tetraploid with a ge- nome size of 1.8 gigabases. Together with corn,ylated cap (MC) is added to the 5′ end of the pri- mary transcript; a poly-A tail is added to the 3′ end. wheat, and rice it is one of the four most valuable of the world’s crops. Although it is called the IrishFinally, the introns are removed and the exons are spliced together during reactions that occur within a potato, S. tuberosum originated in South America. So- lanum exists as two principal cultivated races desig-spliceosome, and the mature mRNA leaves the nu- cleus. See alternative splicing, Cajal body, cis-splicing, nated as subspecies (ssp. andigena and ssp. tubero- sum). Both subspecies arose in South America, butdystrophin, exon, hemoglobin genes, heterogeneous nuclear RNA (hnRNA), intron, methylated cap, polya- ssp. andigena was the first species introduced into Europe. This cultivated race was wiped out by thedenylation, RNA editing, RNA splicing, small nuclear RNAs, snurposomes, spliceosome, transcriptosomes. potato blights of the 1840s and was replaced by the American cultivar, which belonged to ssp. tubero- posttranslational processing alterations to poly- sum. See Appendix A, Plantae, Angiospermae Dicto- peptide chains after they have been synthesized: tyledonae, Solanales; Phytophthora infestans. e.g., removal of the formyl group from methionine in bacteria, acetylation, hydroxylation, phosphoryla- potato virus Y a virus that causes diseases of com- merically important crop plants, such as peppers,tion, attachment of sugars or prosthetic groups, oxi- dation of cysteines to form disulfide bonds, cleavage potatoes, and tomatoes. The virion is about 730 × 110 nm, and it contains a helically disposed ssRNAof specific regions that convert proenzymes to en- zymes, etc. See cystine, N-formylmethionine. surrounded by protein subunits. potency in developmental biology, the capacity ofposttranslational sorting See protein sorting. a cell or its descendants to give rise to differentiated postzygotic isolation mechanism any factor that structures (e.g., specific cell lineage(s), tissue(s), or- tends to reduce or prevent interbreeding between ge- gan(s), or a whole organism), given a specific envi- netically divergent populations or species, but func- ronment. See pluripotent, totipotent, unipotent. tioning after fertilization has occurred; includes hybrid inviability, hybrid sterility, and hybrid breakdown. Potorous tridactylus the rat kangaroo, a marsupial favorable for chromosomal studies because of the potassium an element universally found in small small number and individuality of its chromosomes. amounts in tissues. Atomic number 19; atomic See Appendix A, Mammalia, Metatheria. weight 39.102; valence 1 + ; most abundant isotope 39 K; radioisotope 42 K, half-life 12.4 hours, radiations POU genes genes that encode related, DNA-bind- ing proteins. The family is large, and it includesemitted—beta particles and gamma rays. 352 poultry breeds many genes expressed in the central nervous system. pp60c-src the 60-kilodalton protein kinase en- coded by the c-src gene (q.v.) in normal cells. SeeThe DNA-binding domains consist of an upstream homeobox (q.v.) and a downstream POU-specific pp60v-src. domain about 80 amino acids long. The name POU pp60v-src the protein encoded by the oncogene of comes from the initials of the first genes included in the Rous sarcoma virus. It is a 60-kilodalton phos- the family (Pit-1, Oct-1, and Unc-86.) The Pit-1 phoprotein, hence the pp60 in the name; the v-src gene is expressed in the pituitary gland of mouse indicates that it is encoded by viral gene src. The embryos and Oct-1 is expressed in many tissues of molecule is a protein kinase (q.v.) that phosphoryl- both developing and adult mice. The unc-86 gene ates tyrosine subunits in cellular proteins, particu- activates the differentiation of specific embryonic larly those that form the adhesion portions of the cells into neurons in Caenorhabditis elegans. See Ap- plasmalemma. See Appendix C, 1978, Collett and Er- pendix C, 1988, Herr et al.; selector genes. ickson; pp60c-src. poultry breeds Plymouth Rock, New Hampshire, Prader-Willi syndrome (PWS) a syndrome due to White Leghorn, Blue Andalusian, Rhode Island Red, a genetic deletion of human chromosome 15 (q11– Rhode Island White, Australorp, and Orpington. See 13). The condition was first described in 1956 by Gallus domesticus. Andrea Prader and Heinrich Willi. PWS patients are pox viruses viruses that belong to the family Pox- generally mildly retarded and have insatiable appe- viridae. These are the largest and most complex vi- tites. They are obese from overeating. This condition ruses known, with genomes made up of linear dou- is often discussed in conjunction with the Angelman ble-stranded DNAs. These molecules are 130–300 syndrome (AS), which is also characterized by dele- kbp and contain 200–300 genes. Pox viruses pro- tions in the same region of chromosome 15. Individ- duce both specific and cross-reacting antibodies. For uals with PWS have a very different phenotype than this reason it is possible to vaccinate against a deadly those with AS. In the case of PWS, the deleted chro- disease caused by one pox virus with a related spe- mosome 15 is usually of paternal origin, whereas in cies that causes a much milder disease. The classic AS the deleted chromosome is maternally derived. example is vaccinating against smallpox (caused by Human chromosome 15 contains the genes SNRPN, the Variola virus) with the Vaccinia virus. Both Va- IPW, and UBE3A lying in 1, 2, 3 order. Some defi- riola and Vaccinia viruses are believed to have ciencies lack all three genes. Genes 1 and 2 are pa- evolved from the same rodent pox virus about ternally imprinted, and gene 3 is maternally im- 10,000 BC. See enveloped viruses, smallpox, small- printed. In the diagram, the male and female signs pox vaccine, vaccine, virus. mark the source of the chromosomes in the zygotes. The inactive genes are methylated, as shown by m’s pp inorganic pyrophosphate. alongside the circles. The active genes produce prod- P particle See kappa. ucts P 1 ,P 2 , and P 3 . The patient with Prader-Willi syndrome has lost the ability to produce the prod- P1 phage a temperate bacteriophage that is wide- ucts of the genes that normally undergo paternal ly used in transduction experiments with E. coli. Its imprinting. The same deficiency (df) causes the genome consists of a linear double-stranded DNA Angelman syndrome (AS) because the maternally molecule of about 90 kilobases. The molecule is ter- transmitted deficiency lacks UBE3A, which is mater- minally redundant and cyclically permuted. See cy- nally imprinted under normal circumstances. The clically permuted sequences, P1 artificial chromo- UBE3A gene is active in localized regions of the fe- somes (PACs). male brain, where it specifies a ubiquitin-protein li- gase. The PW syndrome may result from the loss of P22 phage a temperate bacteriophage that infects SNRPN and IPW or of other paternally expressed Salmonella. The prophage inserts at a specific site on genes farther to the left. It is known that an imprint- the host chromosome (between pro A and pro C). ing center lies immediately to the left of SNRPN. Insertion is catalyzed by an integrase (q.v.) specified This center contains CpG islands that are methyl- by the phage. Transduction (q.v.) was discovered in ated on the maternal chromosome and unmethyl- Salmonella typhimurium that carried the P22 pro- ated on the paternal chromosome. Patients who lack phage. See Appendix C, 1952, Zinder and Lederberg. the 15q11-13 deficiency but have the PW or A syn- PPLO pleuropneumonia-like organism (q.v.). dromes often have mutations in the imprinting cen- ter. See DNA methylation, parental imprinting. ppm parts per million. ppt precipitate. PRD domain, PRD repeat See Paired. prepupal period 353 12 3 12 3 12 3 12 3 Normal PWS AS P 1 P 3 P 3 P 2 P 1 P 2 mm mm mm df df Prader-Willi syndrome preadaptation See exaptation. to undergo rapid condensation to metaphase dimen- sions. This is done by fusing an interphase cell with pre-adoptive parents See germinal choice. a cell in mitosis. The interphase cell is induced to enter mitosis, and its chromosomes contract accord- Precambrian the eon between the Phanerozoic ingly. See Appendix C, 1970, Johnson and Rao. and the Hadean eons. The protists arose and evolved during this 3.2-billion-year interval. See geologic premessenger RNA the giant RNA molecule tran- time divisions. scribed from a structural gene. It will undergo post- transcriptional processing (q.v.) before it leaves the precursor ribosomal RNA See preribosomal RNA. nucleus. pre-embryo See embryo. prenatal genetic testing the sampling of cells preferential association an immunological theory from a fetus to determine whether or not it has a that specific viral antigens interact more strongly genetic disorder. Such testing is offered to mothers with certain allelic products of the major immu- who will be older than 35 at the time of delivery, or nogene complex than with others. This preferential to those who have had a previous child or pregnancy association may make the virus more immunogenic, with a birth defect, or in situations where the family and hosts with the strongly interacting allelic prod- history indicates that the baby may be at risk of in- uct would tend to be more immune to viral infec- heriting a genetic abnormality. See amniocentesis, tion than would those with weakly interacting allelic chorionic villi sampling, Down syndrome, genetic products. counseling, informed consent, maternal contamina- tion, pedigree. preformation See epigenesis. prenylation the covalent binding of a protein to an prehensile adapted for grasping. isoprenoid lipid (q.v.), generally by its C-terminal cysteine. Prenylation increases the hydrophobicity of preimplantation genotyping the determination proteins and facilitates their interactions with mem- of the genotype of an in vitro-fertilized, human em- brane lipids. bryo prior to its implantation. The technique sam- ples one blastomere from an eight-cell embryo, and prepatent period the interval between infection the selected templates are amplified by the polymer- with a pathogen or a parasite and the time when the ase chain reaction (q.v.). Tests are then run to see if causative agent of the ensuing disease can be de- the templates contain mutant copies of the gene un- tected by conventional diagnostic techniques. See la- der study. Embryos free of the defect are used to tent period, patent period. start the pregnancy. prepattern a morphogenetic pattern superimposed premating isolation mechanism See prezygotic upon a population of cells arranged in a two-dimen- isolation mechanism. sional array. Specific types of differentiation are stimulated in certain cells located in defined areas. premature initiation a second initiation of replica- An example of a gene that influences a prepattern is tion occurring before the first is completed; a phe- engrailed (q.v.). See compartmentalization. nomenon observed in bacteria grown in a complex nutrient broth or in some phage species that make preprimosome See primosome. replicas very rapidly. prepupal period the period between puparium formation and the eversion of the imaginal discs ofprematurely condensed chromosomes inter- phase chromosomes that are experimentally forced the insect. [...]... divisions prothallus (prothalium) the independent gametophyte of a horsetail or fern See Appendix A, Plantae, Tracheophyta prothetely an experimentally induced abnormality in which an organ appears in advance of the normal time because of a partially inhibited metamorphosis; for example, the formation of pupal antennae on a caterpillar prothoracic gland a gland located in the prothorax of insects that secretes... types of radiation, such as alpha particles, beta particles, and gamma photons radioautograph autoradiograph (q.v.) radioautographic efficiency ficiency (q.v.) radioautography autoradiographic ef- autoradiography (q.v.) radiobiology a branch of biology that deals with the effects of radiation on biological systems It includes radiation genetics (q.v.) radiogenic element an element derived from another... and water habitats, as well as the roots of crop plants It has unusual abilities in breaking down aromatic and other toxic compounds, and it can tolerate heavy metals See Appendix A, Bacteria, Proteobacteria; bacteriocins, bioremediation pseudotumor an aggregation of blackened cells in Drosophila larvae, pupae, and adults of certain genotypes Such “tumors” result from encapsulation during the larval... type of gonadal tissue present Pseudomonas a genus of Gram-negative, motile bacteria that grow as free living organisms in soil, river water, marshes, and coastal marine, habitats and as pathogens of plants and animals Geneticists often study strains of P aeruginosa which are resistant to antibiotics and disinfectants and are responsible for many infections in humans This species is the predominant cause... breakage caused by ionizing radiation In the figure on page 370 are shown the origin and mitotic behavior of a variety of radiation-induced aberrations Original break positions are indicated by short diagonal lines radiation sickness a syndrome characterized by nausea, vomiting, diarrhea, psychic depression, and death following exposure to lethal doses of ionizing radiation The median lethal radiation... line of the gnathostome ancestor, and this transposon was the source of RAG-1 and RAG-2 See Appendix A, Chordata, Craniata, Agantha, Gnathostoma; Appendix C, 1990, Oettinger et al.; 1996, Spanopoulou et al.; immunoglobulin genes, recombination activating genes (RAGs), somatic recombination, T cell receptor genes, Tc1/mariner element ramets buds that can detach from a plant or animal and result in the asexual... transduces signals from the environment that detect damage to DNA The normal allele of a gene in humans that is responsible (when mutated) for the hereditary disease ataxia-telangiectasia encodes a homologous Pl 3-kinase See ATM kinase radial cleavage a pattern of cell divisions seen in the developing embryos of deuterostomes, such as echinoderms and amphibians The first two cleavages are vertical and... rapidly reannealing DNA, rapidly reassociating DNA repetitious DNA (q.v.) rapid-lysing (r ) mutants mutants of T-even phage that enhance the rate at which E coli host cells are lysed; on a bacterial lawn, r-plaques are larger than wild-type plaques (r+) See plaque rare bases purines (other than adenine and guanine) and pyrimidines (other than cytosine and uracil) found in transfer RNA (q.v.) Formulas are... element by atomic disintegration radiograph a shadow image made on photographic emulsion by the action of ionizing radiation The image is the result of the differential attenuation of the radiation during its passage through the object being radiographed A chest x-ray negative is a radiograph r and K selection theory radioimmunoassay a highly sensitive technique for the quantitative determination of antigenically... the adult mouth, and the anus forms anew at the end of the gastrula sac Compare with Deuterostomia See Appendix A prototroph 1 an organism that is able to subsist on a carbon source and inorganic compounds For most bacteria, the carbon source could be a sugar; green plants use carbon dioxide 2 a microbial strain that is capable of growing on a defined minimal medium; wild-type strains are usually regarded . water, marshes, and coastal marine, habitats ma thromboplastin component (both of which see). and as pathogens of plants and animals. Geneticists often study strains of P. aeruginosa which are. analyses of families affected by dopsis, but 40× smaller than pine. See Appendix A, a particular disease. A search is then made for genet- Plantae, Angiospermae, Dicotyledonae, Salicales. ically. to anneal to the separated strands. Finally, the temperature is raised again, andcanal and touches one pole of each spindle. As a re- sult of this orientation, one cell of each dividing pair a