polyprotein a cistronic product that is posttransla- tionally cleaved into several independent proteins. For example, an enkephalin precursor protein con- tains six copies of met-enkephalin and one copy of leu-enkephalin. See enkephalins, retroviruses. polyribonucleotide phosphorylase See polynu- cleotide phosphorylase. polyribosome polysome (q.v.). polysaccharide a carbohydrate formed by the polymerization of many monosaccharide units. Starch, cellulose, and glycogen (q.v.) are examples of polysaccharides. polysomaticism the phenomenon where an indi- vidual contains diploid and polyploid cells in the same tissue. polysome a multiple structure containing a group of ribosomes held together by a molecule of messen- ger RNA. A contraction of polyribosome. See Appen- dix C, 1962, Warner et al. polysomy the reduplication of some but not all of the chromosomes of a set beyond the normal diploid number. A metafemale Drosophila is polysomic (tri- somic for the X). polyspermy the penetration of more than one sperm into one ovum at the time of fertilization. Polysphondylium pallidum See Acrasiomycota. polytene chromosome a giant cable-like chromo- some consisting of many identical chromatids lying in parallel.
The chromatin is hypercoiled in localized regions, and since the chromatids are in register, a pattern of bands is produced vertical to the long axis of the chromosome. Polytene chromosomes are found within a limited number of organisms. They are present in the macronucleus anlage of some ciliates, in the synergids and antipodal cells of the ovules of certain angiosperms, and in various tissues of dipter- ans. The Drosophila salivary gland chromosomes (q.v.) have been studied most extensively. See Ap- pendix C, 1881, Balbiani; 1912, Rambousek; 1934, Bauer; 1952, Beermann; 1959, Pelling; 1969, Am- mermann; 1980, Gronemeyer and Pongs; Anophe- les, Balbiani ring, Calliphora erythrocephala, Chiro- nomus, Culex pipiens, Glyptotendipes barbipes, insulator DNAs, otu mutation, Rhynchosciara, Sciara, Smittia. polytenization the continued replication of each interphase chromosome to produce giant chromo- somes made up of multiple chromatids lying in par- allel and forming a cable-like structure. See somatic pairing. polythetic group a group of organisms that share a large number of features, no single one of which is either essential for group membership or is sufficient to make an organism a member of the group. polytopic pertaining to the distribution of subspe- cies in two or more geographically discontinuous areas. polytrophic meroistic ovary See insect ovary types. polytypic species a species subdivided into a number of specialized races. pome a fleshy, many-seeded fruit such as the apple or pear, in which the enlarged end of the flower stalk forms much of the flesh.
Pompe disease a hereditary glycogen storage dis- ease in humans arising from a deficiency of the lyso- somal enzyme α-1,4-glucosidase, due to a recessive gene on chromosome 17. Prevalence of the disease is 1/100,000. Pongidae the family of primates containing all an- thropoid apes. Pongo pygmaeus the orangutan, a primate with a haploid chromosome number of 24. About 30 bio- chemical marker genes have been distributed among 20 linkage groups. See Hominoidea. popcorn See corn, quantitative inheritance. population a local (geographically defined) group of conspecific organisms sharing a common gene pool; also called a deme. population biology the study of the patterns in which organisms are related in space and time. Such disciplines as ecology, taxonomy, ethology, popula- tion genetics, and others that deal primarily with the interactions of organisms or groups of organisms (demes, species, etc.) are included under this term. population cage a special cage in which Drosoph- ila populations can be reared for many generations. The cage is designed so that samples of the popu- lation can be conveniently withdrawn and food supplies can be replenished. See Appendix C, 1934, L’He´ritier and Teissier. population density 1. in ecology, the number of individuals of a population per unit of living space (e.g., per acre of land, per cubic meter of water,
etc.). 2. in cell or tissue culture, the number of cells per unit area or volume of a culture vessel. See satu- ration density. population doubling level in cell or tissue cul- ture, the total number of population doublings of a cell line or strain since its initiation in vitro. population doubling time See doubling time. population genetics the study of the genetic com- position of populations. Population geneticists try to estimate gene frequencies and detect the selective influences that determine them in natural popu- lations. They also build mathematic models to elu- cidate the interaction of factors such as selection, population size, mutation, and migration upon the fixation and loss of linked and unlinked genes. See Appendix C, 1908, Hardy, Weinberg; 1930-32, Wright, Fisher, Haldane. population structure the manner in which a pop- ulation is subdivided into local breeding groups or demes, the sizes of such demes in terms of the num- ber of breeding individuals, and the amount of mi- gration or gene flow between demes. Populus a genus containing trees such as the trembling aspen (P. tremuloides), the black cotton- wood (P. trichocarpa), and the white poplar (P. alba.). Most species are dioecious (q.v.).
The prog- eny of crosses between certain related species are typically grown on plantations. These hybrids are fertile and have exceptional growth and vigor. Popu- lus trichocarpa is the first tree species selected for DNA sequencing because of its small genome size. It is 550 mbp, which is only 4× larger than Arabi- dopsis, but 40× smaller than pine. See Appendix A, Plantae, Angiospermae, Dicotyledonae, Salicales. porcine referring to members of the pig family, es- pecially the domestic pig Sus scrofa. porphyrias diseases caused by toxic accumulations of porphyrins (q.v.) and related compounds in tis- sues. Inherited porphyrias are due to mutations in genes that encode enzymes which catalyze steps in the biosynthesis of heme (q.v.). There are at least eight enzymes that control steps in the heme biosyn- thetic pathway. One is protoporphyrinogen oxidase (PPOX) which is encoded by a gene that maps to 1q22.
This gene has 13 exons and spans about 8 kb. Mutations in the PPOX gene cause variegate por- phyria (VP). The symptoms include photosensitiv- ity, abdominal pain, and mental disturbances includ- ing hallucinations, depression, and paranoia. Attacks of VP are often triggered by drugs such as barbitu- rates. The disease has a very high prevalence in South African populations of Dutch descent, pre- sumably because of a founder effect (q.v.). porphyrin any of a class of organic compounds in which four pyrrole nuclei are connected in a ring structure usually associated with metals (like iron or magnesium). Porphyrins form parts of the hemoglo- bin, cytochrome, and chlorophyll molecules. See heme. positional candidate approach in human genet- ics, a strategy for identifying the gene responsible for a disease by mapping the mutant gene to a specific chromosomal region and then looking for an appro- priate candidate among the genes already localized in that region.
Individuals suffering from the disease are then tested for mutations in the candidate gene. For example, a gene encoding a fibroblast growth factor receptor (FGFR3) protein was discovered dur- ing a chromosome walk (q.v.) toward the Hunting- ton disease gene.
Next, the gene for achondroplasia (q.v.) was mapped to the same chromosomal region. Finally, the FGFR3 genes of dwarfs were found to contain missense mutations, proving that mutations of the FGFR3 gene were responsible for the retarded growth characterizing the disease. positional cloning a strategy for identifying and cloning a gene based on a knowledge of its position in the genome (q.v.), with little or no information about the function or product of the gene at the out- set. This strategy has been applied in a variety of plant and animal species. In humans, the chromo- somal position of the gene of interest is usually de- termined by linkage analyses of families affected by a particular disease. A search is then made for genet- ically linked molecular markers, and the closest ones flanking the gene are used to start chromosome walking (q.v.) in order to identify additional markers with the closest possible linkage to the gene.
The DNA defined by these molecular markers is then cloned and the gene residing between them identi- fied by a variety of means, such as by searching data- bases for genes within the identified genomic region, by sequencing the gene-containing region and look- ing for an open reading frame(s), by comparing the suspected gene’s sequence and expression patterns in mutant and wild-type individuals, and where pos- sible, by the ability of the putative gene to rescue (q.v.) a mutant phenotype (q.v.). Once identified, the gene is cloned for further analysis. Human dis- ease genes identified by positional cloning include cystic fibrosis, Duchenne muscular dystrophy, fragile X syndrome, and Huntington disease (all of which
Positional Information Hypothesis
See). Also called map-based cloning. Compare with functional cloning. See marker, open reading frame, positional candidate approach. Positional Information Hypothesis a model de- veloped by Louis Wolpert to explain pattern speci- fication (q.v.) during development. His idea is that each population of cells in an embryonic structure lies in a field that contains a gradient of a chemical morphogen (q.v.).
The position in the field deter- mines the concentration of morphogen, and the cells are programmed to enter one of a number of devel- opmental pathways depending on the concentration level of the morphogen to which they are exposed. See Appendix C, 1969, Wolpert. position effects the change in the expression of a gene accompanying a change in the position of the gene with respect to neighboring genes. The change in position may result from crossing over or from a chromosomal aberration. Position effects are of two types: the stable (S) type and the variegated (V) type. S-type position effects are also called cis-trans position effects. S-type position effects involve cis- trons that possess at least two mutated sites separa- ble by intragenic recombination. In the cis configu- ration (m1 m2/++) a normal phenotype is observed, whereas in the trans configuration (m1+/+m2) a mu- tant phenotype is produced.
A reasonable explana- tion for such an observation would be that the mRNA transcribed from a (++) chromatid would function normally, whereas the mRNAs transcribed from (m1 m2), (m1+), or (+m2) chromatids would not. V-type position effects generally involve the sup- pression of activity of a wild-type gene when it is placed in contact with heterochromatin because of a chromosome aberration. Under some conditions, the gene may escape suppression, and consequently the final phenotype may be variegated, with patches of normal and mutant tissues.
See Appendix C, 1925, Sturtevant; 1936, Schultz; 1945, Lewis; heterochro- matization, transvection. positive assortative mating See assortative mat- ing. positive control control by a regulatory protein that must bind to an operator before translation can take place. positive eugenics See eugenics. positive feedback the enhancement or amplifica- tion of an effect by its own influence on the process that gives rise to it. positive gene control enhancement of DNA tran- scription through binding of specific expressor mole- cules to promoter sites.
For example, the binding of CAP-cAMP complexes to promoters of bacterial genes involved in catabolism of sugars other than glucose facilitates binding of RNA polymerase to these operons when glucose is absent. See glucose- sensitive operons. Compare with negative gene con- trol. positive interference the interaction between crossovers such that the occurrence of one exchange between homologous chromosomes reduces the likelihood of another in its vicinity. Compare with negative interference. See Appendix C, 1916, Muller. positive sense ssDNA or RNA See plus (+) and mi- nus (−) viral strands. positive supercoiling See supercoiling. positron a particle of the atomic nucleus equal in mass to the electron and having an equal but oppo- site (positive) charge. postcoitum after mating. postmating isolation mechanism See postzygotic isolation mechanism. postmeiotic fusion a method for restoring dip- loidy in eggs produced by parthenogenesis, involving union of two identical haploid nuclei formed by a mitotic division of the egg nucleus.
postmeiotic segregation in ascomycete fungi such as Neurospora, the formation of heteroduplex regions (by meiotic crossing over) that results in ab- errant 4 : 4 pattern of asci in which adjacent pairs of ascospores produced by mitotic division after meio- sis have different genetic compositions. See tetrad segregation types. postreductional disjunction referring to the sepa- ration of alleles at particular heterozygous loci dur- ing the first meiotic division. If the loci are repre- sented by A and A′, in the case of postreductional disjunction the two chromatics that enter one sister nucleus have one A and one A′ allele, whereas in the case of prereductional disjunction both have A al- leles or both A′ alleles. postreplication repair repair to a DNA region after a replication fork has passed that region or in nonreplicating DNA. posttranscriptional processing those modifica- tions made to pre-mRNA molecules before they leave the nucleus; also called nuclear processing. A gene containing three exons (E1, E2, and E3) and two introns (I1 and I2) is diagrammed (page 351). RNA polymerase II transcribes the 3′-5′ strand of the gene