Nicotinamide-adenine dinucleotide phosphate (NADP)
nick in nucleic acid chemistry, the absence of a phosphodiester bond between adjacent nucleotides in one strand of duplex DNA. Compare with cut. nickase an enzyme that causes single-stranded breaks in duplex DNA, allowing it to unwind. nick-closing enzyme See topoisomerase. nick translation an in vitro procedure used to ra- dioactively label a DNA of interest uniformly to a high specific activity.
First, nicks are introduced into the unlabeled DNA by an endonuclease, generating 3′ hydroxyl termini. E. coli DNA polymerase I is then used to add radioactive residues to the 3′ hy- droxy terminus of the nick, with concomitant re- moval of the nucleotides from the 5′ side. The result is an identical DNA molecule with the nick dis- placed further along the duplex. See strand-specific hybridization probes. Nicotiana a genus containing about 60 species, many of which have been intensively studied geneti- cally.
Much interest has been generated from the finding that tumors arise spontaneously at high fre- quency in certain interspecific hybrids, such as those plants produced by the cross N. langsdorffii × N. glauca. The species of greatest commercial impor- tance is N. tabacum, the source of tobacco. N. taba- cum is an allotetraploid, and N. sylvestris and N. to- mentosiformis are its parental diploids. Analysis of chDNA and mtDNA reveal that tobacco inherited these cytoplasmic organelles from N. sylvestris. To- bacco genes that confer resistance to the tobacco mosaic virus (q.v.) have been cloned and sequenced utilizing transposon tagging (q.v.).
See Appendix C, 1761, Kolreuter; 1925, Goodspeed and Clausen; 1926, Clausen and Goodspeed; 1986, Shinozaki et al.; 1994, Whitham et al. nicotinamide-adenine dinucleotide (NAD) a co- enzyme (formerly called DPN or coenzyme 1) func- tioning as an electron carrier in many enzymatic oxi- dation-reduction reactions. The oxidized form is symbolized NAD+, the reduced form NADH (see structural formulas below). See citric acid cycle, cyto- chrome system, mitochondrial proton transport. nicotinamide-adenine dinucleotide phosphate (NADP) an electron carrier (formerly called TPN or coenzyme 2). The oxidized form is symbolized NADP+, the reduced form NADPH. See nicotine-ad- enine dinucleotide (NAD).
Nicotinamide-adenine dinucleotide (NAD)/nicotinamide-adenine dinucleotide phosphate (NADP)
nicotine a poisonous, volatile alkaloid present in the leaves of Nicotiana tabacum and responsible for many of the effects of tobacco smoking. It functions in the plant as a potent insecticide.
nicotinic acid one of the B vitamins. Also called niacin in the older literature.
Niemann-Pick disease a group of human disor- ders characterized by enlargement of the spleen and liver and by the accumulation of sphingomyelin (q.v.) and other lipids throughout the body. Two German pediatricians, Albert Niemann and Ludwig Pick, published accounts of the disease in 1914 and 1927, respectively. The syndrome is due to mutations in a gene at 18q11, q12 that encodes a lysosomal sphingomyelinease. Amniocentesis and testing of fe-
tal cells for sphingomyelinase activity permits moni- toring of pregnancies at risk. Heterozygotes can be identified, since their leukocytes contain about 60% the normal activity of sphingomyelinase. See sphin- gomyelin. nif (nitrogen-fixing) genes genes that enable the bacteria containing them to fix atmospheric nitro- gen. Such genes are generally carried by the plasmids of nodulating bacteria, and they encode the enzyme nitrogenase.
See nitrogen fixation, Rhizobium. nigericin See ionophore. Nile blue a mixture of two dyes: Nile blue A, a water-soluble basic dye; and Nile red, a lysochrome formed by spontaneous oxidation of Nile blue A (an example of allochromacy, q.v.). Structures are shown on page 299.
Nilsson-Pelger model of eye evolution a compu- tational model (shown below) designed to simulate the evolution of an eye. It starts with at flat sheet of photosensitive cells lying above a flat layer of pig- mented cells and below a monolayer of transparent cells. Using a sequence of small modifications in shape, the originally flat patch gradually changes into a cup, which then acquires a lens in its opening. The end result is a focused, light-imaging organ with the geometry typically seen in a fish eye. The mathe- matical modeling procedure chosen was such that each 1% increment of change produced a maximal
increase in visual acuity. Using conservative assump- tions as to heritabilities (q.v.) and selection pressures (q.v.), the times taken (in generations) to perform each evolutionary stage are shown in the diagram. To complete an eye for a species with one genera- tion per year, it would take a total of only 364,000 years, a relatively short time from a geological pro- spective. So it is not surprising that eyes have been produced independently at least 40 times during the evolution of Eumetazoa (q.v.). See Appendix C, 1994, Nilsson and Pelger. ninhydrin an organic reagent that reacts with and colors amino acids. Ninhydrin solutions are sprayed on chromatographs, and the separated amino acids and polypeptides are then rendered visible as ninhy- drin-positive spots.
nitrocellulose filter a very thin filter composed of nitrocellulose fibers that selectively bind single- stranded DNA strongly, but not double-stranded DNA or RNA. The ssDNA binds along its sugar- phosphate backbone, leaving its bases free to pair with complementary bases contained in labeled ssDNA or RNA probes. See DNA hybridization. nitrogen the fourth most abundant of the biologi- cally important elements. Atomic number 7; atomic weight 14.0067; valence 3−, 5+; most abundant iso- tope 14N; heavy isotope 15N. This heavy isotope was 1958. See Appendix C.
nitrogen fixation the enzymatic incorporation of nitrogen from the atmosphere into organic com- pounds. The ability to fix nitrogen is limited to cer- tain bacteria. Sixty five million years ago nodulating legumes (q.v.) discovered a way to utilize atmo- spheric nitrogen directly, through symbiosis with ni- trogen-fixing bacteria. See Azotobacteria, nif genes, Rhizobium. nitrogen mustard di(2-chloroethyl) methylamine; an alkylating agent (q.v.) that is a potent mutagen and chromosome-breaking agent. See sulfur mustard.
nitrogenous base a purine or pyrimidine; more generally an aromatic, nitrogen-containing molecule that has basic properties (is a proton acceptor). nitrous acid HNO2, a mutagen that converts the NH2 groups of the purines and pyrimidines to OH groups. NK cells See natural killer (NK) cells. N6-methyladenine. See 5-methylcytosine. NMR spectroscopy nuclear magnetic resonance spectroscopy (q.v.). node 1. in vascular plants, a slightly enlarged por- tion of a stem where leaves and buds arise and where branches originate. 2. in a circular DNA superhelix, the point of contact in a figure-of-eight; if the left strand in the upper part of 8 is closest to the viewer at the node, it is called a positive node; if the left strand in the upper part of 8 is in back of the other strand at the node, it is called a negative node. 3. in a cladogram (q.v.), a point where one branch splits
off from another. Each node represents a common ancestor, and the branches are the lineages derived from it. Also called a divergence node. See PhyloCode.
nodulating legumes a large family of dicotyledon- ous species that form nitrogen-fixing nodules on their roots or stems. Examples are Glycine max and Phaseolus vulgaris (both of which see). See Dicotyledo- neae, Rhizobium. noise in colloquial usage, variation in an experi- ment attributed to uncontrolled effects, usually asso- ciated with a variance component called experimen- tal error. Nomarski differential interference microscope an optical system that, like the phase contrast mi- croscope, permits the visualization of transparent structures in a living cell. However, in the Nomarski system the field is quite shallow so that there is free- dom from phase disturbances from structures above and below the plane of focus. The observation meth- od is comparable to that with extreme oblique illu- mination, and the specimen therefore appears in re- lief. nomenclature the naming of species according to rules developed by international associations of tax- onomists.
Several codes have been published, and these continue to be revised. There are five: the In- ternational Code of Zoological Nomenclature (ICZN), the International Code of Botanical No- menclature (ICBN), the International Code of No- menclature for Cultivated Plants (ICNCP), the In- ternational Code for the Nomenclature of Bacteria (ICNB), and the International Code of Virus Classi- fication and Nomenclature (ICVCN). See Appendix C, 1735, Linne´. nonautogenous anautogenous (q.v.). nonbasic chromosomal proteins acidic or neutral proteins (therefore not histones) associated with chromosomes: e.g., certain enzymes such as DNA polymerases. noncoding (of a section of a nucleic acid mole- cule) not directing the production of a peptide se- quence. See skeletal DNA hypothesis. non-Darwinian evolution genetic changes in pop- ulations produced by forces other than natural selec- tion; a term usually associated with the neutralist view of evolution.
See neutral mutation-random drift theory of molecular evolution. nondisjunction the failure of homologous chro- mosomes (in meiosis I, primary nondisjunction) or sister chromatids (in meiosis II, secondary nondis- junction; or mitosis) to separate properly and to move to opposite poles. Nondisjunction results in one daughter cell receiving both and the other daughter cell none of the chromosomes in question. See Ap- pendix C, 1914, Bridges. nonessential amino acids See essential amino acids. nonhomologous chromosomes chromosomes that do not synapse during meiosis. noninducible enzyme constitutive enzyme (q.v.).
nonlinear tetrad a group of four meiotic products that are randomly arranged in the ascus. See linear tetrad. non-Mendelian ratio in the progeny of a cross, unusual phenotypic ratios that fail to follow Men- del’s laws, suggesting that gene conversion (q.v.) or another aberrant mechanism is responsible. nonparametric statistics See statistics. nonparental ditype See tetrad segregation types. nonpermissive cells See permissive cells. nonpermissive conditions environmental settings in which conditional lethal mutants fail to survive. nonpolar referring to water-insoluble chemical groups, such as the hydrophobic side chains of amino acids. nonrandom mating See assortative mating, in- breeding, outbreeding. nonreciprocal recombination See unequal cross- ing over. nonreciprocal translocation See translocation. nonrecurrent parent the parent of a hybrid that is not again used as a parent in backcrossing. nonrepetitive DNA segments of DNA exhibiting the reassociation kinetics expected of unique se- quences; single sequence DNA. nonselective medium a growth medium that al- lows growth of all genotypes present in a recombina- tion or mutation experiment. Compare with selective medium.
nonsense codon synonymous with stop codon (q.v.). nonsense mutation a mutation that converts a sense codon to a chain-terminating codon or vice versa. The results following translation are abnor- mally short or long polypeptides, generally with al- tered functional properties. Contrast with missense mutation. nonsense suppressor a gene coding for a tRNA that is mutant in its anticodon and therefore able to recognize a nonsense (stop) codon; nonsense sup- pressors cause extension of polypeptide chain syn- thesis through stop codons. See Appendix C, 1969, Abelson et al.; amber suppressor, ochre suppressor, readthrough. nonspherocytic hemolytic anemia See glucose-6- phosphate dehydrogenase deficiency. nopaline See opine. NOR nucleolar organizer region. noradrenaline norepinephrine. norepinephrine a hormone of the adrenal medulla that causes vasoconstriction and raises the blood pressure.
n orientation one of two possible orientations for inserting a target DNA fragment into a vector; in the n orientation, the genetic map of both target and vector have the same orientation; in the u orienta- tion, the target and the vector are in different orien- tations. normal distribution the most commonly used probability distribution in statistics. The formula for the normal curve is
where µ = the mean, σ = the standard deviation, e = the base of natural logarithms, π = 3.1416, and Y = the height of the ordinate for a given value of X. The graph of this formula, the normal curve, also called Laplacian or Gaussian, is bell shaped. The value of m locates the curve along the abscissa and that of σ determines its shape. The larger the standard devia- tion, the broader the curve. In nature, a vast number of continuous distributions are normally distributed.
normalizing selection the removal of those alleles that produce deviations from the average population phenotype by selection against all deviant individu- als. Such selection will reduce the variance in subse- quent generations. Also called stabilizing selection, centripetal selection. normal solution one containing a gram-equivalent weight of solute dissolved in sufficient water to make a liter of solution. norm of reaction the phenotypic variability pro- duced by a given genotype under the range of envi- ronmental conditions common to the natural habitat of the species or under the standard culture or ex- perimental conditons. See adaptive norm. northern blotting See Southern blotting. Nosema See microsporidia.
Notch a series of overlapping deficiencies of the X chromosomes of Drosophila melanogaster. All defi- ciencies lack the 3C7 band, and females heterozy- gous for the deficiency show distal notches of the wing. Hemizygous males die as embryos. The wild- type allele of Notch is essential for the proper dif- ferentiation of ectoderm. Mutations at the Notch locus result in a hypertrophy of the embryonic ner- vous system at the expense of hypodermal struc- tures. N+ encodes a transmembrane protein contain- ing 2,703 amino acids. Built into the molecule are 36 EGF repeats, some of which bind calcium while others facilitate the formation of Notch protein di- mers. See Appendix C, 1938, Slizynska; epidermal growth factor.
Notophthalmus viridescens the common spotted newt of the eastern United States. The amplification of rDNA has been extensively studied using the oo- cytes of this species. See Appendix A, Chordata, Am- phibia, Urodela; histone genes, lampbrush chromo- some, Triturus. novobiocin an antibiotic produced by Streptomyces niveus. (See structure on page 302.) np nucleotide pair. See base pair. NRG234 See sym-plasmid. nRNA nuclear RNA (q.v.). nt nucleotide. Compare with bp. N-terminal end proteins are conventionally writ- ten with the amino (NH2) end to the left. The as- sembly of amino acids into a polypeptide starts at the N-terminal end. See translation. N terminus N-terminal end.