polar body the minute cell produced and dis- carded during the development of an oocyte. A po- lar body contains one of the nuclei derived from the first or second division of meiosis, but has practically no cytoplasm. See ootid nucleus. polar fusion nucleus in plants, the product of the fusion of the two polar nuclei. This, after fusing with a male nucleus, gives rise to the tripoid endosperm nucleus. See double fertilization. polar gene conversion a phenomenon in which a gradient of conversion frequencies exists from one end of a gene to the other; sites closer to one end of a gene usually have higher conversion frequencies than do those farther from that end. polar granules electron-dense, membrane-less, RNA-protein complexes often associated with mito- chondria, found in the pole plasm (q.v.) and subse- quently incorporated into primordial germ cells (q.v.) in a variety of species.
Also called germinal granules or P granules. polarity gradient the quantitative effect of a po- larity mutation in one gene on the expression of later genes in the operon. The effect is a function of the distance between the nonsense codon and the next chain-initiation signal. polarity mutant 1. a mutant gene that is able to reduce the rate of synthesis of the proteins that nor- mally would be produced by wild-type alleles of the genes lying beyond it on the chromosome. Such genes exert their effect during the translation of a polycistronic message (q.v.). See regulator gene, translation. 2. a mutant gene that influences polar- ized patterns of embryonic development. See bicoid, engrailed, hunchback, maternal polarity mutants, zy- gotic segmentation mutants. polarization microscope a compound light micro- scope used for studying the anisotropic properties of objects and for rendering objects visible because of their optical anisotropy. polar nuclei See ootid nucleus, pollen grain, polo- cyte. polaron a chromosomal segment within which po- larized genetic recombination takes place by gene conversion. polar tubules microtubules of the spindle appara- tus that originate at the centriolar or polar regions of the cell. See chromosomal tubules. pole cell one of the cells that are precociously seg- regated into the posterior pole of the insect embryo before blastoderm formation.
Among these cells are the progenitors of the germ cells. See Appendix C, 1866, Metchnikoff. pole plasm in many vertebrate and invertebrate species, a specialized cytoplasmic region of the egg or the zygote that contains germ-cell determinants and other maternal products required for normal de- velopment in the early embryo. The cytoplasm lo- cated at the posterior pole of Drosophila and the vegetal pole of Xenopus embryos are examples of pole plasm. In Drosophila, several maternal effect genes involved in pole plasm formation have been identified and are known as grandchildless genes (q.v.).
Females carrying mutations in these genes produce embryos that lack polar granules (q.v.) and show other developmental defects. See cytoplasmic determinants, cytoplasmic localization, maternal ef- fect gene, maternal polarity mutants. polio virus the cause of poliomyelitis (infantile pa- ralysis). It is a positive-stranded RNA virus with a 6.1 kb genome. Enormous polysomes (q.v.) contain- ing 60 or more ribosomes occur in infected cells. The entire genome is translated from a single initia- tion site to form a single polyprotein molecule. This is subsequently cleaved into both structural and non- structural proteins. The structural proteins assemble to form the icosahedral capsule of the virus.
The vi- rus is remarkably stable, and it has been successfully grown from archaeological specimens centuries old. See icosahedron. Polish wheat I Triticum polonicum (N = 14). See wheat. pollen grain a microspore in flowering plants that germinates to form the male gametophyte (pollen grain plus pollen tube), which contains three hap- loid nuclei. One of these fertilizes the ovum, a sec- ond fuses with the two polar nuclei to form the 3N endosperm, and the third (the vegetative nucleus) degenerates once double fertilization (q.v.) has been accomplished. pollen mother cell microsporocyte. pollen-restoring gene a gene that permits normal microsporogenesis to occur in the presence of a cy- toplasmic male sterility factor. pollen tube the tube formed from a germinating pollen grain that carries male gametes to the ovum. See Appendix C, 1830, Amici. pollination the transfer of pollen from anther to stigma. See Appendix C; 1694, Camerarius; pollen grain, self-pollination.
Polycystic kidney disease
polocyte the small degenerate sister cell of the sec- ondary oocyte. This cell generally divides into two polar bodies, which disintegrate. See polar body. polyacrylamide gel a gel prepared by mixing a monomer (acrylamide) with a cross-linking agent (N,N′-methylenebisacrylamide) in the presence of a polymerizing agent. An insoluble three-dimensional network of monomer chains is formed. In water, the network becomes hydrated. Depending upon the relative proportions of the ingredients, it is possible to prepare gels with different pore sizes.
The gels can then be used to separate biological molecules like proteins of a given range of sizes. polyacrylamide gel electrophoresis See electro- phoresis. polyadenylation enzymatic addition of several ad- enine nucleotides to the 3′ end of mRNA molecules as part of the processing that primary RNA tran- 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 later be cleaved into individual messages, each of which is translated into a single protein, or a giant polypeptide chain may be translated that is later cleaved to yield the individual proteins.
Polycis- tronic mRNAs are common in prokaryotes. For ex- ample, the lac operon (q.v.) of E. coli generates a polycistronic mRNA. Contrast with monocistronic mRNA. See histone genes, polyprotein, retroviruses, transcription unit, trans-splicing, ubiquitin. polyclonal an adjective applied to cells or mole- cules arising from more than one clone; e.g., an anti- genic preparation (even a highly purified one) elicits the synthesis of various immunoglobulin molecules. These antibodies would react specifically with differ- ent components of the complex antigen molecule. Thus, the antibody preparation generated by such an antigen would be polyclonal in the sense that it would contain immunoglobulins synthesized by dif- ferent clones of B lymphocytes. polyclone See compartmentalization. Polycomb (Pc) a Drosophila mutation that pro- duces additional sex combs (q.v.) on the second and third pairs of legs in males. Pc is at 3-47.1 on the genetic map.
Proteins encoded by the normal allele inhibit Hox genes. Binding sites of Pc proteins have been visualized by immunochemical staining of gi- ant polytene chromosomes. The Pc protein and the heterochromatin-associated protein 1 (HP1) (q.v.) share a homologous domain 37 amino acids long near their N termini. The Pc proteins bind to histone 3 molecules (q.v.) that have been tagged by the ad- dition of methyl groups to their tails. See Appendix C, 1989, Zink and Paro; histones, SUMO proteins. polycomplex structures, observed in certain in- sects, within oocyte nuclei, formed by the fusion of components from synaptonemal complexes (q.v.) that have detached from the diplotene chromo- somes.
polycystic kidney disease one of the most com- mon genetic diseases in humans with about 1 in 1,000 individuals affected. The major feature of PKD is the development of fluid-filled cysts in the kidneys that damage or destroy them. The disease is due to dominant mutations in a gene that maps to 13.3 on the short arm of chromosome 16. The gene (PKD1) spans 52 kilobase pairs and generates a tran- script containing 14,148 among 46 exons. The predicted PKD1 protein, poly- cystin, is a glycoprotein with a carboxyl tail that con- tains about 225 amino acids and which protrudes into the cytoplasm. This is followed by about 1,500 amino acids containing transmembrane domains. The N-terminal extracellular portion of the protein contains about 2,500 amino acids, and these are sub- divided into domains that are thought to bind to a variety of proteins and carbohydrates in the extra- cellular matrix. Polycystin is thought to function in cellular signal transduction (q.v.) mediated through
its cytoplasmic tail. See Appendix C, 1995, Hughes et al. polycythemia vera a disease in humans character- ized by the overproduction of red blood cells. Eryth- roblasts in the bone marrow are hypersensitive to erythropoietin (q.v.). See Janis kinase 2. polydactyly the occurrence of more than the usual number of fingers or toes. polyembryony the formation of multiple embryos from a zygote by its fission at an early developmental stage. Monozygotic twins constitute the simplest ex- ample of polyembryony. Monozygotic quadruplets are commonly formed by armadillos. In certain para- sitic wasps, as many as 2,000 embryos can be formed by polyembryony from a single zygote. polyestrous mammal See estrous cycle. polyethylene glycol a chemical used to promote the fusion of tissue-cultured cells, as in the produc- tion of a hybridoma (q.v.). polyfusome a gelatinous mass assembled by the fusion of the adjacent fusomes (q.v.) formed at con- secutive cystocyte divisions in Drosophila. The dia-
gram illustrates a polyfusome in a cystocyte clone during the divison of 8 cells into 16. Cell 1 is ob- scured by the cells lying above it. In each of the other seven cells, a spindle and a ring canal (q.v.) can be seen. Pairs of centrioles lie at the spindle poles. The polyfusome protrudes through each ring canal and touches one pole of each spindle. As a re- sult of this orientation, one cell of each dividing pair will retain all previously formed ring canals, while the other will receive none. These spindle-fusome alignments during the cycle of cystocyte divisions (q.v.) produce a branched chain of interconnected cells. There are always two central cells, each with four ring canals. In female sterile mutations charac- terized by ovarian tumors, polyfusomes often fail to form properly, and the pattern of germ cell divisions and their differentiation are abnormal. See bag of marbles (bam), fusome, hu-li tai shao (hts), otu, pro- oocyte.
polygamy polandry and/or polygyny. Compare with monogamy. polygene one of a group of genes that together control a quantative character. See Appendix C, 1941, Mather; oligogene, quantitative inheritance. polygenic character a quantitatively variable phe- notype dependent on the interaction of numerous genes. polyglucosan a polymer such as glycogen made up of a chain of glucose units. polygyny the mating of a male with more than one female during a single reproductive cycle. Com- pare with monogamy, polyandry. polyhedrin See baculoviruses. polylinker site a stretch of DNA engineered to have multiple sites for cleavage by specific restric- 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- action mixture containing the target DNA is heated to separate complementary DNA strands. Second, the mixture is cooled, and synthetic primers with nucleotide sequences complementary to each end of the DNA are allowed to anneal to the separated strands. Finally, the temperature is raised again, and a heat-stable DNA polymerase (q.v.) in the reaction mixture synthesizes new DNA strands by adding nu- cleotide bases to the primers. These steps are re- peated for a series of replication cycles, each lasting a few minutes, in an automated cycler that controls the required temperature variations. The number of DNA strands doubles with each successive cycle, re-
sulting in an exponential increase in the number of copies of the target DNA. Twenty cycles yield a mil- lionfold amplification; 30 cycles yield an amplifica- tion factor of 1 billion. The ability of PCR to quickly and accurately generate billions of copies of the min- utest amount of DNA has created a wealth of new practical applications in numerous areas, including DNA cloning and sequencing, screening for genetic disorders, detection of disease-causing organisms, DNA fingerprinting, and examination of species dif- ferences. See Appendix C, 1985, Saiki, Mullis et al.; 1993, Smith and Mullis; ligase chain reaction, reverse transcription-polymerase chain reaction, Taq DNA polymerase. polymerization the formation of a polymer from a population of monomeric molecules. polymerization start site the nucleotide in a DNA promoter sequence from which the first nucle- otide of an RNA transcript is synthesized.
polymorphic locus a genetic locus, in a popula- tion, at which the most common allele has a fre- quency less than 0.95. Compare with monomorphic locus. polymorphism the existence of two or more ge- netically different classes in the same interbreeding population (Rh-positive and Rh-negative humans, for example). The polymorphism may be transient, or the proportions of the different classes may re- main the same for many generations. In the latter case, the phenomenon is referred to as balanced poly- morphism. If the classes are located in different re- gions, geographic polymorphism exists. See Appendix C, 1954, Allison; 1966, Lewontin and Hubby. polymorphonuclear leukocyte See granulocyte. polyneme hypothesis the concept that a newly formed chromatid contains more than one DNA du- plex. Contrast with unineme hypothesis. polynucleotide a linear sequence of 20 or more joined nucleotides. See oligonucleotide.
polynucleotide kinase an enzyme that phospho- rylates the 5′ hydroxyl termini produced by endonu- cleases (q.v.). polynucleotide phosphorylase the first enzyme shown to catalyze the synthesis of polynucleotides. It was isolated from Azotobacter vinelandii in 1955, and it linked ribonucleotides together in a random fashion. Subsequently this enzyme was used to pro- duce artificial messenger RNA molecules. See Ap- pendix C, 1955, Grunberg-Manago and Ochoa; 1961, Nirenberg and Matthaei. polyoma virus a virus that induces tumors in new- born mice, rats, and hamsters and can also transform cultured mouse or rat cells. The genome of the vi- rus is a double-stranded, supercoiled, circular DNA molecule containing about 5,300 base pairs. See Ap- pendix C, 1983, Rassoulzadegan et al.; oncogenic vi- rus, transformation. polyp 1. the sedentary form of a coelenterate. 2. a small stalked neoplasm projecting from a mucous surface (for example, an intestinal polyp). polypeptide a polymer made up of less than 50 amino acids. See amino acid, peptide bond. polyphasic lethal a mutation characterized by two or more lethal phases separated by develop- mental periods in which it produces no deaths. polyphenism the occurrence of several pheno- types in a population that are not due to genetic dif- ferences between the individuals in question. polypheny pleiotropy (q.v.). polyphyletic group a group of species classified together, some members of which are descended from different ancestral populations. Contrast with monophyletic group. polyploid designating a cell or an individual that has more than two sets of chromosomes.
polyploidy the situation where the number of chromosome sets is greater than two. If N is the value for one set of chromosomes, a somatic cell can be 2N (diploid), 3N (triploid), 4N (tetraploid), 5N (pentaploid), 6N (hexaploid), and so forth. When compared to diploids, polyploid cells are generally larger and metabolically more active. Most genes continue to be expressed at the same relative levels. However, a few genes seem to sense increasing gene dosage and raise or lower their levels of transcription appropriately. Polyploidy is a dominant factor in plant evolution, where rounds of large scale genomic duplication have been followed by selective gene loss. This conclusion arises from observations of an- notated genomes where genes commonly occur in duplicate copies. The frequency of polyploidy varies across plant groups. It is rare in conifers, whereas 95% of fern species and 70% of angiosperms are polyploid. See Appendix A, Plantae; Appendix C, 1917, Winge; 1937, Blakeslee and Avery; 1999, Gal- itski et al.; allopolyploid, Arabidopsis thaliana, auto- polyploid, bananas, colchicine, DNA chip, euploid, Gossypium, haploid or haploidy, Nicotiana, -ploid, Raphanobrassica, wheat.