A Dictionary of Genetics phần 6 pot

288 multifactorial tants replace healthy genes, the good ones never multiple factor hypothesis See quantitative inheri- tance.come back and so the process resembles a ratchet wheel, which moves in only one direction. Muller multiple genes See multiple factor hypothesis, proposed that sexuality arose because it allowed polygene, quantitative inheritance. crossing over to occur between homologous chro- mosomes from parents carrying different mutants. multiple infection simultaneous invasion of a bac- Recombinant offspring with mutant-free genomes terial cell by more than one phage, often of different could thus arise and halt the action of the rachet. genotypes in experiments designed to promote phage The term Muller ratchet was coined by J. Felsenstein recombination; superinfection. in 1974. multiple myeloma See myeloma. multifactorial polygenic. multiple neurofibromatosis See neurofibromato- multiforked chromosome a bacterial chromosome sis. containing more than one replication fork, due to the initiation of a second fork before completion of multiple transmembrane domain proteins pro- the first replication cycle. tein molecules that contain several segments that lie multigene family a set of genes descended by du- embedded in the cell membrane. These domains are plication and variation from some ancestral gene. connected by segments alternately at the cytoplas- Such genes may be clustered together on the same mic and extracellular surfaces. Rhodopsin (q.v.) and chromosome or dispersed on different chromosomes. the cystic fibrosis transmembrane-conductance regu- Examples of multigene families include those that lator are examples of multiple transmembrane do- encode the histones, hemoglobins, immunoglob- main proteins. See cystic fibrosis, opsin. ulins, histocompatibility antigens, actins, tubulins, keratins, collagens, heat shock proteins, salivary glue multiplex PCR a type of polymerase chain reac- proteins, chorion proteins, cuticle proteins, yolk tion (q.v.) that is used to sample various regions of proteins, and phaseolins. See isoforms, reiterated a large gene from one end to the other. For example, genes. to analyze the human dystrophin gene, which occu- pies over 2 million base pairs on the X chromosome, multimer a protein molecule made up of two or multiplex PCR might involve simultaneous amplifi- more polypeptide chains, each referred to as a mon- cation from nine different sets of primers, all within omer. The terms dimer, trimer, tetramer, pentamer, the same reaction test tube. Each set of primers is etc., are used if the number of monomers per multi- chosen to produce a different-sized amplification mer is known. Compare with monomer, oligomer, product from a different region of the dystrophin polymer. gene. Normal males will display nine characteristic bands after the amplification products are separated multiparous bearing or producing more than one by gel electrophoresis. Males with deletions in the offspring at a birth. See parity. dystrophin gene will be missing one or more of these multiple allelism See allele. bands. See muscular dystrophy. multiple choice mating referring to an experimen- multiplicity of infection the average number of tal design in studies of behavior genetics where a test phages that infect a bacterium in a specific experi- organism is allowed to choose between two (or ment. The fraction of bacteria infected with 0, 1, 2, more) genetically different mates. 3, ,n phage follows a Poisson distribution. multiple codon recognition See wobble hypoth- multiplicity reactivation the production of re- esis. combinant virus progeny following the simultaneous multiple-event curve a curve (relating relative infection of each host cell by two or more virus par- survival to radiation dose) that contains an initial flat ticles, all of which are incapable of multiplying be- portion. This finding indicates that there is little bio- cause they carry lethal mutations induced by expo- logical effect until a certain dose has accumulated, sure to a mutagen. and suggests that the sensitive target must be hit more than once (or that there must be multiple tar- multipolar spindle a spindle with several poles found in cells with multiple centrioles. Such cells aregets, each of which must be destroyed) to produce a biologically measurable effect. See single-event seen infrequently, but they can be produced in large numbers by irradiation. See mitotic apparatus.curve, target theory. mutation 289 multitarget survival curve See extrapolation num- total number of mapped genes is about 7,000. The mice housed in laboratories throughout the worldber. are all derived from three subspecies. These are all multivalent designating an association of more offshoots of an original population that began mi- than two chromosomes whose homologous regions grating from northern India about 900,000 years are synapsed by pairs (as in autotetraploids and ago. The mouse genome contains 2.5 gbp of DNA translocation heterozygotes). and about 30,000 structural genes. Roughly 80% of these have identifiable orthologs in the human ge- multivoltine producing more than one brood in a nome. See Appendix A, Chordata, Mammalia, Ro- year, as in certain birds and moths. dentia; Appendix C, 1905, Cue ´ not; 1909, 1914, Lit- Muntiacus the genus containing various species of tle; 1936, Bittner; 1940, Earle; 1942, Snell; 1948, small deer living in India, Nepal, and southeast Asia. Gorer et al.; 1953, Snell; 1967, Mintz; 1972, Benac- The Indian muntjac, M. muntjac vaginalis, is remark- erraf and McDevitt; 1975, Mintz and Illmensee; able in having the smallest chromosome number 1976, Hozumi and Tonegawa; 1980, Gordon et al.; known for any mammal (6 per diploid female). The 1987, Kuehn et al.; 1988, Mansour, Thomas and Ca- Chinese muntjac (M. reevesi) has a larger diploid pecchi; 1994, Zhang et al., Arendt and Nu ¨ bler-Jung; value (46). During evolution, the chromosome Appendix E, Mouse Databases; Hox genes, mouse in- number of the Indian species appears to have de- bred lines, oncomouse, T complex. creased by end-to-end fusions between different mustard gas sulfur mustard (q.v.). chromosomes. The amount of DNA in diploid nu- clei from the two species is about the same. See Ap- Mustela the genus that includes M. erminea, the pendix C, 1997, Yang et al.; chromosome painting. ermine; M. lutreola, the European mink; M. vison, the North American mink. mu phage a phage “species” whose genetic mate- rial behaves like insertion sequences, being capable mutable gene in multicellular organisms, a gene of transposition, insertion, inactivation of host genes, that spontaneously mutates at a sufficiently high rate and causing rearrangements of host chromosomes. to produce mosaicism. murine belonging to the family of rodents that mutable site a site on a chromosome at which contains the mice and rats. mutations can occur. murine mammary tumor virus an oncogenic RNA mutagen a physical or chemical agent that raises virus. See mammary tumor agent. the frequency of mutation above the spontaneous rate. Musaceae the family of monocotyledons that con- tains the major food-producing species of bananas mutagenesis the production of mutations, gener- and plantains. From the culinary standpoint, ba- ally by the use of agents that interact with nucleic nanas refers to fruit eaten fresh and plantains to fruit acids. See alkylating agent, hot spot, oligonucleotide- eaten only after cooking. See bananas. directed mutagenesis, radiation genetics. Musca domestica the housefly. DDT resistance in mutagenic causing mutation. this species has been extensively studied by geneti- mutagenize to expose to a mutagenic agent. cists. mutant an organism bearing a mutant gene that muscular dystrophy a heterogeneous group of he- expresses itself in the phenotype of the organism. reditary diseases affecting humans and other mam- mals that cause progressive muscle weakness due to mutant hunt the isolation and accumulation of a defects in the biochemistry of muscle tissue. See large number of mutations affecting a given process, Becker muscular dystrophy (BMD), Duchenne muscu- in preparation for mutational dissection of the lar dystrophy (DMD). gene(s) governing that process. For example, one might select for mutations that confer phage resis- Mus musculus the laboratory mouse. Its diploid tance in E. coli. chromosome number is 20, and extensive genetic maps are available for the 19 autosomes and the X mutation 1. the process by which a gene under- goes a structural change. 2. a modified gene resultingchromosome. There are large collections of strains containing neurological mutants, loci associated with from mutation. 3. by extension, the individual man- ifesting the mutation. See Appendix C, 1901, deoncogenic viruses (especially retroviruses), loci that encode enzymes, and histocompatibility loci. The Vries; isocoding mutation, point mutation. 290 mutational dissection mutational dissection See genetic dissection. human chromosome 8. The viral gene is often sym- bolized v-myc and the cellular gene c-myc (pro- mutational hot spot See hot spot. nounced “see-mick”). The myc oncogene encodes a protein which is expressed in proliferating cells in mutational load the genetic disability sustained by normal embryonic and adult tissues. Its expression is a population due to the accumulation of deleterious abnormally high in human and rodent tumors. The genes generated by recurrent mutation. protein contains a helix-turn-helix motif (q.v.) and a mutation breeding induction of mutations by mu- leucine zipper (q.v.), and it binds to specific target tagens to develop new crop varieties that can in- genes when modulating cell proliferation. The gene crease agricultural productivity. is activated when it is placed next to certain immu- noglobulin genes as a result of a translocation. See mutation distance the smallest number of muta- Burkitt lymphoma, oncogene. tions required to derive one DNA sequence from an- other. mycelium the vegetative portion of a fungus com- posed of a network of filaments called hyphae. Tu- mutation event the actual origin of a mutation in bular hyphae are often divided into compartments time and space, as opposed to the phenotypic mani- by cross walls. However, since there are perforations festation of such an event, which may be generations in the septa, the cytoplasm is continuous. An aerial later. hypha can constrict to produce a conidium (q.v.). mutation frequency the proportion of mutants in Mycobacterium leprae See leprosy bacterium. a population. Mycobacterium tuberculosis the causative agent mutation pressure the continued production of of human tuberculosis, a disease with an annual an allele by mutation. death toll of three million. This human pathogen mutation rate the number of mutation events per arose from a soil bacterium and may have subse- gene per unit time (e.g., per cell generation). quently moved to cows and then to humans, follow- ing the domestication of cattle. The H37 Rv strain mutator gene a mutant gene that increases the was isolated in 1905, and it is the DNA of bacteria spontaneous mutation rate of one or more other from this strain that was sequenced. The circular genes. Many of the early “mutator genes” have chromosome contains 4,411,529 base pairs and turned out to be transposable elements (q.v.). Oth- 3,924 ORFs. The demonstration that the DNA of ers are due to mutations in genes that encode heli- M. tuberculosis has a high content of GC relative to cases (q.v.) or proteins that function in proofreading AT disproved the tetranucleotide hypothesis (q.v.). (q.v.). See Activator-Dissociation system, Dotted, dna The tubercle bacillus is resistant to many antibiotics, mutations, mismatch repair. and this natural resistance is mainly due to its hydro- mutein a mutant protein, such as a CRM (q.v.). phobic cell envelope, which acts as a permeability barrier. Many of its genes are devoted to a synthesis muton the smallest unit of DNA in which a change and breakdown of the lipoproteins in this envelope. can result in a mutation (a single nucleotide). See The genome also contains at least two prophages Appendix C, 1955, Benzer. and over 50 insertion sequences (q.v.). See Appendix mutual exclusion a phenomenon observed among A, Bacteria, Actinobacteria; Appendix C, 1882, ciliary antigens of certain protozoans in which only 1905, Koch; 1998, Cole et al.; Appendix E; Chargaff one genetic locus for a serotype is active at a given rule, leprosy bacterium, lysogenic cycle. time. For example, in Paramecium primaurelia and Mycoplasma a genus of bacteria that is character- Tetrahymena pryriformis, mutual exclusion of sero- ized by the absence of a cell wall. M. capritolum is types in heterozygotes occurs with allelic genes as of interest because in this species UGA encodes well as with nonallelic genes. tryptophan rather than serving as a termination co- mutualism a symbiosis in which both species ben- don. M. genitalium, a parasite of the human genital efit. and respiratory tracts, has recently been shown to have a genome of only 580,070 base pairs. Since this mutually exclusive events a series of alternative is one of the smallest known genomes for any free- events in which only one can occur at a given time. living organism, the number of open reading frames reveals the minimal set of genes necessary for inde- myc a gene originally described in the avian MC29 myelocytomatosis virus, an oncovirus of the chicken. pendent life. There are only 470 genes (average size, 1040 base pairs), and these comprise 88% of the ge-A homologous gene is located on the long arm of myotonic dystrophy 291 nome. The related species M. pneumoniae has a Appendix C, 1958, Kendrew et al.; gene superfamily, hemoglobin genes.larger genome (820 kb), and the number of ORFs is 679. All 470 ORFs from the smaller species are found in the larger bacterium, and their protein se- myoglobin gene the gene that encodes myoglo- quences are 67% identical. See Appendix A, Prokary- bin. It is remarkable in that less than 5% of its struc- otae, Aphragmabacteria; Appendix C, 1985, Yamao; ture codes for message. All the genes of the alpha 1995, Fraser, Venter et al.; Appendix E; bacterial cell and beta hemoglobin families are made up of three wall, pleuropneumonia-like organisms, TIGR, univer- coding regions interrupted by two introns. The myo- sal code theory. globin gene contains four exons and three introns, and each of these introns is much longer than any of Mycostatin a trade name for nystatin (q.v.). those found in hemoglobin genes. myelin sheath the insulating covering of an axon myosin the hexameric protein that interacts with formed by the plasma membrane of a Schwann cell. actin (q.v.) to convert the energy from the hydrolysis myeloblasts cells that differentiate by aggregation of ATP into the force for muscle contraction. Actin to form multinucleated, striated muscle cells. functions both as a structural protein and an en- zyme. A myosin molecule can catalyze the hydroly- myeloid leukemia See Philadelphia (Ph 1 ) chromo- sis of 5 to 10 ATP molecules per second. Each myo- some. sin consists of a slender stem (about 135 nanometers long) and a globular head region (about 10 nanome- myeloma cancer of plasma cells, presumably due ters long). The molecule is formed from two identi- to clonal proliferation of a single plasma cell that es- cal heavy chains, each possessing about 2,000 amino capes the normal control of division. Such cells re- acids. In the tail region, the heavy chains twist to- produce and secrete a specific homogeneous protein gether to form an alpha helix, from which the two related to gamma globulins. See Bence-Jones pro- globular heads protrude. The C termini are distal to teins, HAT medium, hybridoma. the heads. Two light chain proteins, A 1 (190 amino acids) and A 2 (148 amino acids), attach to the globu- myeloma protein a partial or complete immuno- lar heads of each heavy chain. The light chain pro- globulin molecule secreted by a myeloma (q.v.). teins contain calcium-binding sites. The globular myeloproliferative disease any disease caused by head regions contain the ATPase activity and can the uncontrolled proliferation of blood cells pro- bind temporarily to actin to form a complex referred duced in the bone marrow. Leukemias result from to as actomyosin. In avian and mammalian species, proliferating lymphocytes. Lymphomas also contain numerous isoforms of both myosin heavy and light proliferating lymphocytes, but in this case the chains have been isolated from muscle and nonmus- sources are lymph nodes. Granulocytes, monocytes, cle tissues. and megakaryocytes (all of which see) are the sources of myeloid leukemias. The designation “acute” indi- myosin genes the genes encoding the isoforms of cates that the cancer cells remain immature, divide the heavy and light myosin chains. In Drosophila, rapidly, and are destined to overwhelm the body; two myosin heavy chain genes have been identified: whereas the “chronic” term is used for cells that di- one encoding a muscle myosin (Mhc) and one en- vide less frequently and mature enough to perform coding a cytoplasmic myosin (Mhc-c). The transcrip- some of their normal functions. See Abelson murine tion unit of Mhc is 22 kilobases long and contains 19 leukemia virus, acute myeloid leukemia 1 gene, Bur- different exons. Multiple transcripts are generated kitt lymphoma, Philadelphia (Ph 1 ) chromosome, poly- by alternative splicing (q.v.). Genes for the two light cythemia. chains are also known. In mammals, the muscle my- osin heavy chain isoforms are encoded by a family Myleran a trade name for busulfan (q.v.). containing at least 10 genes. myoglobin the monomeric heme (q.v.) protein that stores oxygen in vertebrate muscles. The myo- myotonic dystrophy an autosomal dominant dis- order due to an unstable trinucleotide repeat (q.v.).globin gene is thought to have been derived directly from the ancestral gene that by duplication pro- The gene involved encodes a muscle protein kinase, and the trinucleotide repeat is located in the 3′ un-duced an ORF that evolved into the alpha chain gene of hemoglobin (q.v.). The myoglobin and alpha translated region of the gene. In susceptible families, there is an increase in the severity of the disease inchain genes diverged 600 to 800 million years ago. Human myoglobin contains 152 amino acids. See successive generations. See genetic anticipation. myria a rarely used prefix meaning 10,000. Used myxomatosis a fatal virus disease affecting rabbits. The virus was introduced into wild populations ofwith metric units of measurement. rabbits in Australia as a means of controlling them. myriapod an arthropod belonging to the Myria- Myxomycota the phylum containing the plasmo- poda. Millipedes and centipedes were the first dial slime molds. These protoctists generate multi- animals to colonize the land. See Appendix A, Ar- nucleate plasmodia that feed by phagocytosis and thropoda, Mandibulata, Myriapoda; metamerism, Si- subsequently form stalked, funguslike fruiting struc- lurian. tures. From the standpoint of genetics, Physarum po- Mytilus edulis See Pelecypoda. lycephalum is the best-known species. 292 N nanos (nos) a Drosophila gene that maps to 3- 66.2, and it encodes an RNA-binding protein that blocks translation. This repressor is localized at the n neutron (q.v.). posterior pole of embryos where it functions to shut down the translation of mRNAs produced by hunch- N 1. the haploid chromosome number. 2. normal back (q.v.). solution. 3. nitrogen. narrow heritability See heritability. N -acetyl serine an acetylated serine thought to function in mammalian systems as N-formylmethio- nascent polypeptide chain the forming polypep- nine does in bacterial translation. tide chain that is attached to the 50 S subunit of a ribosome through a molecule of tRNA. The free end of the nascent polypeptide contains the N-terminal amino acid. See translation. nascent RNA an RNA molecule in the process of being synthesized (hence incomplete) or a complete, newly synthesized RNA molecule before any alter- ations have been made (e.g., prior to nuclear pro- cessing or RNA editing, both of which see). NAD nicotinamide-adenine dinucleotide (q.v.). Nasonia brevicornis another name for Mormon- NADP nicotinamide-adenine dinucleotide phos- iella vitripennis (q.v.). phate (q.v.). Nasonia vitripennis another name for Mormon- Naegleria a genus of soil amoebas capable of iella vitripennis (q.v.). transforming into flagellates. Species from this genus are often studied in terms of the morphogenesis of native indigenous. A native species is not intro- flagella. duced into an area by humans, either intentionally or accidentally. nail patella syndrome a hereditary disease in hu- mans. Individuals afflicted with this disorder have natural immunity an outmoded concept that some misshapen fingernails and small kneecaps, or lack immunities are inherited in the apparent absence of them. The disease is due to a dominant gene residing prior contact with an antigen. The prevailing para- on chromosome 9. digm is that all immunity ultimately requires contact with a sensitizing antigen and therefore is acquired. nalidixic acid an antibiotic that inhibits DNA rep- lication in growing bacteria. It specifically inhibits natural killer (NK) cells large leukocytes found in the DNA gyrase of E. coli. the blood (where they make up about 10% of the total lymphocytes) and in spleen and lymph nodes. They are activated by interferon (q.v.), and they at- tack tumor cells without prior immunization. NK cells are distinct from B lymphocytes and T lympho- cytes. natural selection the differential fecundity (q.v.) in nature between members of a species possessing adaptive characters and those without such advan-nanometer (nm) one-billionth (10 −9 ) meter. The preferred length unit for describing ultrastructural tages. See Appendix C, 1818, Wells; 1858, Darwin and Wallace; 1859, Darwin; 1934, 1937, L’He ´ ritierdimensions (e.g., a ribosome of 15 nm diameter). The nanometer replaces the millimicron (mµ), an and Teissier; 1952, Bradshaw; 1954, Allison; artifi- cial selection, evolution, fundamental theorem of nat-equivalent length, found in the earlier literature. Ten nm equals one Angstrom unit. ural selection, heavy metals, selection. 293 294 Nautilus Nautilus See living fossil. negative staining a staining technique for high- resolution electron microscopy of viruses. A virus n D refractive index. suspension is mixed with a phosphotungstic acid so- lution and poured into an atomizer sprayer. The Ndj 1 See bouquet configuration. mixture is then sprayed upon electron microscope Neandertal a race of humans that roamed through grids previously coated with a film of carbon. The Europe, North Africa, the Near East, Iraq, and Cen- phosphotungstic acid enters the contours of the tral Asia in the middle and upper Pleistocene specimen, which is viewed as a light object against a (300,000 to 30,000 years ago). The fossils are named dark background. See Appendix C, 1959, Brenner after the valley in western Germany where they were and Horne. first discovered. The ranges of Homo neandertalensis and Homo sapiens overlapped in Europe during re- negative supercoiling See supercoiling. cent millennia, but there seems to have been little neobiogenesis the concept that life has been gen- interbreeding. Sequence comparisons of the D loop erated from inorganic material repeatedly in nature. (q.v.) region of mtDNA from Neandertal fossilized bones and modern humans show that Neandertals neo-Darwinism the post-Darwinian concept that became extinct without leaving a trace of their species evolve by the natural selection of adaptive mtDNAs in modern humans. See Appendix C, 1997, phenotypes caused by mutant genes. Krings et al., Homo. Neogene a subdivision of the Tertiary period, in- Nearctic one of the six biogeographic realms (q.v.) corporating the Pliocene and Miocene epochs. See of the earth, comprising North America, Greenland, geologic time divisions. and extending to the Mexican plateau. Neolithic pertaining to the later Stone Age, during nebenkern a two-stranded helical structure sur- which agriculture and animal husbandry originated rounding the proximal region of the tail filament of and flourished. a spermatozoon. The nebenkern is derived from clumped mitochondria. neomorph a mutant gene producing a qualita- tively new effect that is not produced by the normal negative complementation suppression of the allele. wild-type activity of one subunit of a multimeric protein by a mutant allelic subunit. neomycin an antibiotic produced by Streptomyces fradiae. negative contrast technique See negative stain- ing. neontology the study of living (extant) species, as negative eugenics See eugenics. opposed to paleontology (the study of extinct spe- cies). negative feedback the suppression or diminution of an effect by its own influence on the process that neoplasm a localized population of proliferating gives rise to it. cells in an animal that are not governed by the usual limitations of normal growth. The neoplasm is said negative gene control prevention of gene expres- to be benign if it does not undergo metastasis and sion by the binding of a specific controlling factor malignant if it undergoes metastasis. to DNA. For example, in bacterial operons (either inducible or repressible), the binding of a repressor neotenin synonym for allatum hormone (q.v.). protein to the operator prevents transcription of neoteny the retention of larval characteristics structural genes in that operon. See regulator gene. throughout life with reproduction occurring during Compare with positive gene control. the larval period. In Ambystoma mexicanum, for ex- negative interference a situation in which the co- ample, the gill-breathing, water-dwelling larval sala- efficient of coincidence is greater than 1. In such mander matures and reproduces sexually without cases, the occurrence of one exchange between ho- undergoing metamorphosis to a lung-breathing, land- mologous chromosomes appears to increase the like- dwelling, adult form. See axolotl. lihood of another in its vicinity. Neotropical one of the six biogeographic realms negative regulation See negative gene control. (q.v.) of the earth, comprising Central and South America (south of the Mexican plateau) and thenegative sense ssDNA or RNA See plus (+) and minus (−) viral strands. West Indies. neurosecretory spheres 295 neuraminic acid a nine-carbon amino sugar widely related memory loss. See epidermal growth factor (EGF).distributed in living organisms. One of the distinc- tions between eubacteria and archaebacteria is the neurofibroma a fibrous tumor of peripheral nerves. presence of neuraminic acid in the cell walls of the former and its absence in the latter. In animals, neu- neurofibromatosis one of the most common sin- raminic acid is found in mucolipids, mucopolysac- gle gene disorders affecting the human nervous sys- charides, and glycoproteins. Neuraminic acid-con- tem. The disease is characterized by the presence in taining membrane components play a role in the the skin, or along the course of peripheral nerves, attachment and penetration of virus particles into of multiple neurofibromas that gradually increase in animal cells. See ganglioside. number and size. There are two types of neurofi- bromatosis, abbreviated NF1 and NF2. NF1 (also called von Recklinghausen disease) is one of the most common autosomal dominant disorders of hu- mans, affecting 1/3,000 individuals. The NF1 gene is located on the long arm of chromosome 17 at 11.2. The gene spans 3 × 10 5 nucleotides and en- codes a protein (neurofibromin) containing 2,818 amino acids. The spontaneous mutation rate of the NF1 gene is high, and 30–50% of patients carry new NF1 mutations. Neurofibromatosis 2 is a rarer con- dition, affecting about 1 in 37,000 individuals. The NF2 gene is at 22q12, and it encodes a protein (mer- lin) containing 590 amino acids. Neurofibromin is located in the cytoplasm and apparently functions in signal transduction (q.v.), whereas merlin is believed neuregulins (NRGs) a family of structurally re- to link the cell membrane to certain cytoskeletal lated growth and differentiation factors found in the proteins. See anti-oncogenes. http://www.nf.org central and peripheral nervous systems, which in- neurofibromin See neurofibromatosis. cludes products of the Nrg1, Nrg2, Nrg3, and Nrg4 genes. NRG1, the most widely studied neuregulin, neurohormone a hormone synthesized and se- has 14 different isoforms, produced by alternative creted by specialized nerve cells; e.g., gonadotropin- splicing of its mRNA, and 7 isoforms of NRG2 have releasing hormone produced by neurosecretory cells been identified. A variety of proteins identified in located in the hypothalamus. various independent studies, including the neu dif- ferentiation factor (NDF), heregulin (HRG), glial neurohypophysis the portion of the hypophysis growth factor 2 (GGF2), and acetylcholine recep- that develops from the floor of the diencephalon. tor-inducing activity (ARIA), are all isoforms of neurological mutant a mutant producing malfor- NRG1, produced by alternatively spliced mRNA. mations of the sense organs or the central nervous All NRG1 isoforms have in common with each system or striking abnormalities in locomotion or other and with other neuregulins an epidermal behavior. Hundreds of neurological mutants have growth factor (EGF)-like sequence, which is essen- been collected in Drosophila, Caenorhabditis, and tial for their function. Some neuregulins have in the mouse. See Appendix C, 1969, Hotta and Benzer; common a transmembrane domain, followed by a 1971, Suzuki et al.; 1981, Chalfie and Sulston; 1986, variable intracellular domain, while others differ Tomlinson and Ready. from one another in their N-terminal domains. These structural features suggest functional similari- neuron a nerve cell. ties as well as distinctions between the neuregulin family members. NRGs interact with a family of re- neuropathy a collective term for a great variety of ceptor tyrosine kinases on target cells to influence a behavioral disorders that may have hereditary com- number of cellular processes, including the synthesis ponents. of acetylcholine receptors at neuromuscular junc- tions, the proliferation and survival of oligodendro- neurosecretory spheres electron-dense spheres 0.1–0.2 micrometers in diameter, synthesized bycytes, and the proliferation and myelination of Schwann cells. Nrg1 is a candidate gene for schizo- and transported in the axoplasm of specialized neu- rons.phrenia (q.v.) and is also thought to be linked to age- 296 Neurospora crassa Neurospora crassa the ascomycete fungus upon tained in a population by the balance between muta- tional input and random extinction. Neutral muta-which many of the classical studies of biochemical genetics were performed. In Neurospora each set of tions are not functionless; they are simply equally effective to the ancestral alleles in promoting themeiotic products is arranged in a linear fashion, and therefore the particular meiotic division at which survival and reproduction of the organisms that car- ry them. However, such neutral mutations can spreadgenetic exchange occurs can be determined by dis- secting open the ascus and growing the individual in a population purely by chance because only a rel- atively small number of gametes are “sampled” fromascospores (see ordered tetrad). The haploid chro- mosome number of this species is 7, and seven de- the vast supply produced in each generation and therefore are represented in the individuals of thetailed linkage maps are available. Neurospora is esti- mated to have a genome of 38.6 million nucleotide next generation. See Appendix C, 1968, Kimura. base pairs. It has around 10,000 genes, but only neutron an elementary nuclear particle with a about 1,400 have counterparts in Drosophila, Caeno- mass approximately the same as that of a hydrogen rhabditis, or humans. More than half of its genes atom and electrically neutral; its mass is 1.0087 mass have no similarity to those in the other fungi that units. have been sequenced (Saccharomyces and Schizosac- charomyces). There are about 1.7 introns per gene, neutron contrast matching technique a tech- with an average intron size of 134 nucleotides. Neu- nique that involves determining the neutron-scatter- rospora has a lower proportion of genes in multigene ing densities of particles irradiated in solutions con- families than any other species for which data are taining various concentrations of light and heavy available. This is because it has evolved repeat-in- water. This technique was used on nucleosomes duced point mutation (RIP) (q.v.), a mechanism for (q.v.), and it was found that under conditions where detecting and mutationally inactivating DNA dupli- neutron scattering from DNA dominated the reac- cations. Dispersed throughout the genome are 424 tion, the radius of gyration was 50 A ˚ ngstroms. When tRNA genes and 74 5S rRNA genes. There are also scattering from the histone proteins was dominant, 175–200 copies of a tandem repeat that contains the the radius was 30 A ˚ ngstroms. The larger radius for 17S, 5.8S, and the 25S rRNA genes. These are local- DNA proved that it was located on the surface of ized in the nucleolus organizer which somehow pro- the nucleosome. See Appendix C, 1977, Pardon et al. tects them from RIP. The Neurospora mitochondrial N -formylmethionine a modified methionine mol- DNA contains 60,000 nucleotide pairs. See Appen- ecule that has a formyl group attached to its termi- dix A, Fungi, Ascomycota; Appendix C, 1927, Dodge; nal amino group. Such an amino acid is “blocked” 1941, Beadle and Tatum; 1944, Tatum et al.; 1948, in the sense that the absence of a free amino group Mitchell and Lein; 2003, Galagan et al.; Appendix E; prevents the amino acid from being inserted into a Appendix F. growing polypeptide chain. N-formylmethionine is neurula the stage of development of a vertebrate the starting amino acid in the synthesis of all bacte- embryo at which the neural axis is fully formed and rial polypeptides. See Appendix C, 1966, Adams and histogenesis is proceeding rapidly. Cappecchi; initiator tRNA, start codon. neutral equilibrium See passive equilibrium. neutral mutation 1. a genetic alteration whose phenotypic expression results in no change in the or- ganism’s adaptive value or fitness for present envi- ronmental conditions. 2. a mutation that has no measurable phenotypic effect as far as the study in niacin an early name for nicotinic acid (q.v.). question is concerned. See silent mutation. niche from the standpoint of a species, its behav- neutral mutation–random drift theory of molecu- ioral, morphological, and physiological adaptations lar evolution a theory according to which the ma- to its habitat. From the standpoint of the environ- jority of the nucleotide substitutions in the course of ment, the ecological conditions under which the evolution are the result of the random fixation of species survives and multiplies. See ecological niche, neutral or nearly neutral mutations, rather than the extremophiles. result of positive Darwinian selection. Many protein polymorphisms are selectively neutral and are main- niche preclusion See first-arriver principle. nicotinamide-adenine dinucleotide phosphate (NADP) 297 nick in nucleic acid chemistry, the absence of a glauca. The species of greatest commercial impor- tance is N. tabacum, the source of tobacco. N. taba-phosphodiester bond between adjacent nucleotides in one strand of duplex DNA. Compare with cut. cum is an allotetraploid, and N. sylvestris and N. to- mentosiformis are its parental diploids. Analysis of nickase an enzyme that causes single-stranded chDNA and mtDNA reveal that tobacco inherited breaks in duplex DNA, allowing it to unwind. these cytoplasmic organelles from N. sylvestris. To- bacco genes that confer resistance to the tobacco nick-closing enzyme See topoisomerase. mosaic virus (q.v.) have been cloned and sequenced utilizing transposon tagging (q.v.). See Appendix C, nick translation an in vitro procedure used to ra- 1761, Ko ¨ lreuter; 1925, Goodspeed and Clausen; dioactively label a DNA of interest uniformly to a 1926, Clausen and Goodspeed; 1986, Shinozaki et high specific activity. First, nicks are introduced into al.; 1994, Whitham et al. the unlabeled DNA by an endonuclease, generating 3′ hydroxyl termini. E. coli DNA polymerase I is nicotinamide-adenine dinucleotide (NAD) a co- then used to add radioactive residues to the 3′ hy- enzyme (formerly called DPN or coenzyme 1) func- droxy terminus of the nick, with concomitant re- tioning as an electron carrier in many enzymatic oxi- moval of the nucleotides from the 5′ side. The result dation-reduction reactions. The oxidized form is is an identical DNA molecule with the nick dis- symbolized NAD + , the reduced form NADH (see placed further along the duplex. See strand-specific structural formulas below). See citric acid cycle, cyto- hybridization probes. chrome system, mitochondrial proton transport. Nicotiana a genus containing about 60 species, many of which have been intensively studied geneti- nicotinamide-adenine dinucleotide phosphate (NADP) an electron carrier (formerly called TPNcally. Much interest has been generated from the finding that tumors arise spontaneously at high fre- or coenzyme 2). The oxidized form is symbolized NADP + , the reduced form NADPH. See nicotine-ad-quency in certain interspecific hybrids, such as those plants produced by the cross N. langsdorffii × N. enine dinucleotide (NAD). Nicotinamide-adenine dinucleotide (NAD)/nicotinamide-adenine dinucleotide phosphate (NADP) [...]... amount of material per ml of solution that produces an absorbance of 1 in a 1.0-centimeter light path at a wavelength of 260 nanometers See absorbance odorant any one of a wide variety of molecules that produces an odor and that binds with an odorant receptor (q.v.) to trigger a cascade of signals that 3 06 O eventually allows the brain to recognize the particular odor Usually, several different odorants... paramutagenic allele is present paranemic spiral a spiral made up of two parallel threads coiled in opposite directions The threads can be easily separated without uncoiling See plectonemic spiral, relational coiling parapatric referring to populations or species that occupy adjacent areas with a narrow zone of overlap within which hybridization commonly occurs parapatric speciation a mode of gradual... Oryctolagus cuniculus the rabbit, a mammal commonly reared in the laboratory and the subject of intensive genetic research An extensive collection of mutations is available influencing a wide variety of morphological and physiological traits The haploid chromosome number is 22, and about 60 genes have been distributed among 16 linkage groups See Appendix A, Chordata, Mammalia, Lagomorpha; WHHL rabbit Oryza... See lamins nuclear family a pair of parents and their children nuclear fission a transformation of atomic nuclei characterized by the splitting of a nucleus into at least two other nuclei and the release of amounts of energy far greater than those generated by conventional chemical reactions nuclear fusion the coalescence of two or more atomic nuclei with the release of relatively vast amounts of energy... specifying an amino acid is converted to UAA, which signals chain termination UAA appears to be the codon primarily used for chain termination in E coli See amber mutation, nonsense mutation ochre suppressor any mutant gene coding for a mutant tRNA whose anticodon can respond to the UAA stop codon by the insertion of an amino acid See amber suppressor Ockham razor a variant spelling of Occam See Occam’s octad... to a known example or incident that serves as a model or provides a pattern for a more general phenomenon In a still more restricted sense, paradigm may refer to a ruling model that has replaced all others Darwin’s theory of evolution by natural selection is an example of such a paradigm As time passes, the matching of new discoveries with the model may lead to a revision of the paradigm An example of. .. et al.; 1 966 , Ernst and Anderson; 1991, Ernst; 2002, Wuthrich et ¨ al.; Appendix E, Individual Databases; Antennapedia, proteomics nucleolus nuclear pore complex (NPC) an octagonally symmetrical organelle that allows controlled passage of molecules from nucleus to cytoplasm and vice versa A typical mammalian nucleus contains between 3,000 and 4,000 NPCs Each complex is made up of a central core that... cells of the parathyroid gland paratope the site within an immunoglobulin Fab that specifically interacts with an antigenic determinant (epitope) Parazoa a subdivision of the animal kingdom containing organisms, like sponges, that lack tissues organized into organs, and have an indeterminate shape See Appendix A parenchyma 1 plant tissues composed of thinwalled cells that fit loosely together, leaving... Range in China, Africa north of the Sahara, Iceland, Spitzbergen, and the islands north of Siberia Paleocene the most ancient of the Tertiary epochs The placental mammals expanded at the expense of the marsupials Early primates and grasses made their first appearance The drifting of continents continued See geological time divisions Paleogene a subdivision of the Tertiary period, incorporating the Oligocene,... proteins of an agriculturally important plant Animal proteins, such as those in milk and beef, have a better balance of certain essential amino acids (like tryptophan and lysine) than do plant proteins Mutants like opaque 2 are of potential use in combating kwashiorkor (q.v.) See Appendix C, 1 964 , Mertz et al., zein open population a population that is freely exposed to gene flow (q.v.) open reading frame . less than 5% of its struc- otae, Aphragmabacteria; Appendix C, 1985, Yamao; ture codes for message. All the genes of the alpha 1995, Fraser, Venter et al.; Appendix E; bacterial cell and beta hemoglobin. counterparts in Drosophila, Caeno- mass approximately the same as that of a hydrogen rhabditis, or humans. More than half of its genes atom and electrically neutral; its mass is 1.0087 mass have. of of human tuberculosis, a disease with an annual an allele by mutation. death toll of three million. This human pathogen mutation rate the number of mutation events per arose from a soil bacterium