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- N -

N2  Free nitrogen gas. In liquid form, used as a cryopreservant. See cryobiological preservation.

naked bud  A bud not protected by bud scales.

nanometre  1 × 10-9 m. One millionth of a millimetre, a.k.a. a millimicron; equals ten angstroms.

narrow-host-range plasmid  A plasmid that can replicate in one, or at most a few, different bacterial species.

narrow-sense heritability  In quantitative genetics, the proportion of the phenotypic variance that is due to variation in breeding values.

native protein  The naturally occurring form of a protein.

natural selection  The differential survival and reproduction of organisms because of differences in characteristics that affect their ability to utilize environmental resources.

NDP  Ribonucleoside diphosphate. See nucleotide; ribonucleoside triphosphate (NTP).

necrosis  Death associated with discoloration and dehydration of all or some parts of organs.

negative autogenous regulation; negative self-regulation  Inhibition of the expression of a gene or set of coordinately regulated genes by the product of the gene or the product of one of the genes.

negative control system  A mechanism in which the regulatory protein(s) is required to turn off gene expression.

negative selection  Selection of individuals that do not possess a certain character. A method by which growing cells that do not carry a DNA insert integrated at a specific chromosomal location are selected. See positive selection.

negative self-regulation  See negative autogenous regulation.

nematodes  A class of slender, unsegmented worms, often parasitic. a.k.a. eelworms, especially when phytoparasitic.

neo-formation  Organogenesis; production of newly formed structures, such as tissues, meristems and embryos.

neoplasm  Localized cell multiplication. Generally it designates a collection of cells which have undergone genetic transformation, forming a tumour. Neoplasmic cells differ in structure and function from the original cell type.

neoteny  The retention of juvenile body characters in the adult state, or the occurrence of adult characters in the juvenile state.

net photosynthesis  Photosynthetic activity minus respiratory activity, as measured by carbon dioxide exchange.

neutral mutation  A mutation that changes the nucleotide sequence of a gene but has negligible effect on the fitness of the organism.

neutral theory  The theory that much of evolution has been primarily due to random drift of neutral mutations.

NFT  See nutrient film technique.

nick  Verb: To break a phospho-diester bond in the backbone of one of the strands of a duplex DNA molecule. cf cleave; cut.

nicked circle; relaxed circle  During extraction of plasmid DNA from the bacterial cell, one strand of the DNA becomes nicked. This relaxes the torsional strain needed to maintain supercoiling, producing the familiar form of plasmid. See plasmid.

nick translation  A procedure for labelling DNA. A DNA fragment is treated with DNase to produce single-stranded nicks. The nick is moved along the DNA molecule in the presence of labelled deoxyribonucleoside triphosphates by the concerted action of the 5´-> 3´ exonuclease and 5´-> 3´ polymerase activities of E. coli DNA polymerase I.

nitrification  A chemical process in which nitrogen in plant and animal wastes and dead remains is oxidized, first to nitrites and then to nitrates.

nitrate  The only form in which nitrogen can be used directly by plants; a component of chemical fertilizers.

nitrite  A salt or ester of nitrous acid.

nitrocellulose; cellulose nitrate  A nitrated derivative of cellulose. It is made into membrane filters of defined porosity, used to immobilize DNA, RNA or protein, which can then be probed with a labelled sequence or antibody. These filters have a variety of uses in molecular biology, particularly in nucleic acid hybridization experiments. Used extensively in the Southern and northern blotting procedures involving DNA and RNA.

nitrogen assimilation  The incorporation of nitrogen into organic cell substances by living organisms.

nitrogen fixation  The conversion of atmospheric nitrogen (N2) into oxidized forms that can be assimilated by plants. Biological nitrogen fixation is catalysed by the enzyme nitrogenase, which is found only in prokaryotes. Certain blue-green algae and some genera of bacteria (e.g., Rhizobium spp.; Azotobacter spp.) are capable of biochemically fixing nitrogen. Such bacteria are very important symbionts for plants growing in nitrogen-poor soils.

nitrogenous bases  The purines (adenine and guanine) and pyrimidines (thymine, cytosine and uracil) that form DNA and RNA molecules.

NMP  Ribonucleoside monophosphate. See nucleotide.

NO  See nucleolar organizer.

nod box  A DNA sequence that controls the transcriptional regulation of Rhizobium nodulation genes.

nodal culture  The culture of a lateral bud and a section of adjacent stem tissue.

node (L. nodus, a knot)  Slightly enlarged portion of the stem where leaves and buds arise and where branches originate. Stems have nodes but roots do not.

nodular  Term commonly used to describe a pebbly (rough) texture of a callus.

nodulation  The formation of nodules by symbiotic bacteria on the roots of plants.

nodule  The enlargement or swelling on roots of nitrogen-fixing plants. The nodules contain symbiotic nitrogen-fixing bacteria. See nitrogen fixation.

non-autonomous  A term referring to biological units that cannot function by themselves; such units require the assistance of another unit, or "helper". See autonomous.

non-disjunction  Failure of disjunction or separation of homologous chromosomes or chromatids in mitosis or meiosis, resulting in too many chromosomes in some daughter cells and too few in others. Examples: In meiosis, both members of a pair of chromosomes go to one pole so that the other pole does not receive either of them; in mitosis, both sister chromatids go to the same pole.

non-histone chromosomal proteins  In chromosomes, all of the proteins except the histones.

nonsense mutation  A mutation which converts an amino-acid-specifying codon into a stop codon, e.g., a change from UAU (tyr) to UAG (amber) would lead to the premature termination of a polypeptide chain at the place where a tyrosine was inserted in the wild-type. See stop codon; suppressor.

non-target organism  An organism which is affected by an interaction for which it was not the intended recipient.

non-template strand  In transcription, the non-transcribed strand of DNA. a.k.a. sense strand or coding strand. It will have the same sequence as the RNA transcript, except that T is present at positions where U is present in the RNA transcript.

non-virulent agent. See attenuated vaccine.

NOR  See nucleolar organizer region.

northern blot  A cellulose or nylon membrane to which RNA molecules have been attached by capillary action. The transferred RNA is hybridized to single-stranded DNA probes. northern blot technique is often used to measure expression (transcription) of a gene for which a specific cDNA is available for use as a probe. See Southern blot, western blot.

northern blotting  Similar to Southern blotting, except that RNA is transferred onto a matrix and the presence of a specific RNA molecule is detected by DNA-RNA hybridization. See northern hybridization.

northern hybridization  Hybridization of a labelled DNA probe to RNA fragments that have been transferred from an agarose gel to a nitrocellulose filter. See northern blotting; hybridization (3).

NTP  Ribonucleoside triphosphate See nucleotide.

nucellar embryo  An embryo which has developed vegetatively from somatic tissue surrounding the embryo sac, rather than by fertilization of the egg cell.

nucellus (L. nucella, a small nut)  Tissue composing the chief part of the young ovule in which the embryo sac develops; megasporangium.

nuclear transfer  A technology by which animals are created by cloning a single diploid somatic cell. It involves taking a single diploid cell from a culture of cells, and inserting it into an enucleated ovum, i.e., an ovum from which the haploid nucleus has been removed. The resultant diploid ovum develops into an embryo that is placed in a recipient female, which gives birth to the cloned animal in the normal manner. Note that the term is somewhat of a misnomer, since it is a whole cell that is transferred, not just the nucleus.

nuclease  A class of enzymes that degrade DNA or RNA molecules by cleaving the phospho-diester bonds that link adjacent nucleotides. In deoxyribonuclease (DNase), the substrate is DNA. In endonuclease, it cleaves at internal sites in the substrate molecule. Exonuclease progressively cleaves from the end of the substrate molecule. In ribonuclease (RNase), the substrate is RNA. In the S1 nuclease, the substrate is single-stranded DNA or RNA. Nucleases have varying degrees of base-sequence specificity, the most specific being the restriction endonucleases.

nucleic acid  A macromolecule composed of phosphoric acid, pentose sugar, and organic bases. The two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are made up of long chains of molecules called nucleotides (q.v.). They were first isolated as part of a protein complex in 1871, and were separated from the protein moiety in 1889. See DNA; RNA; nucleotides.

nucleic acid probe  See DNA probe.

nuclein  The term used by Friedrich Miescher to describe the nuclear material he discovered in 1869, which today is known as DNA.

nucleo-cytoplasmic ratio  In a cell, the ratio of nuclear to cytoplasmic volume. This ratio is high in meristematic cells and low in differentiated cells.

nucleolar organizer (NO); nucleolar organizer region (NOR)  A chromosomal segment containing genes that encode ribosomal RNA; located at the secondary constriction of some chromosomes.

nucleolar zone  Any chromosome region, irrespective of whether or not it is a secondary-constriction, that is associated with the formation of the nucleolus during telophase.

nucleolus (L. nucleolus, a small nucleus)  An RNA-rich intranuclear organelle in the nucleus of eukaryotic cells, produced by a nucleolar organizer. It represents the storage place for ribosomes and ribosome precursors. The nucleolus consists primarily of ribosomal precursor RNA, ribosomal RNA, their associated proteins, and some, perhaps all, of the enzymatic equipment (RNA polymerase, RNA methylase, RNA cleavage enzymes) required for synthesis, conversion and assembly of ribosomes. Subsequently the ribosomes are transported to the cytoplasm.

nucleoplasm  The non-staining or slightly chromophilic, liquid or semi-liquid, ground substance of the interphase nucleus and which fills the nuclear space around the chromosomes and the nucleoli. Little is known of the chemical composition of this ground substance, which is not easily defined. It may be called "karyoplasm" when it is gel-like, and "karyolymph" when it is a colloidal fluid, but generally the terms are synonymous.

nucleoprotein  Conjugated protein composed of nucleic acid and protein; the material of which the chromosomes are made.

nucleoside  A base (purine or pyrimidine) that is covalently linked to a 5-carbon (pentose) sugar. When the sugar is ribose, the nucleoside is a ribonucleoside; when it is deoxyribose, the nucleoside is a deoxyribonucleoside. Adenine, guanine and cytosine occur in both DNA and RNA; thymine occurs in DNA; and uracil in RNA. They are the building blocks of DNA and RNA. See nucleoside analogue.

nucleoside analogue  A synthetic molecule that resembles a naturally occurring nucleoside, but that lacks the bond site needed to link it to an adjacent nucleotide. See nucleoside.

nucleosome  Spherical sub-units of eukaryotic chromatin that are composed of a core particle consisting of an octamer of histones (two molecules each of histones H2a, H2b, H3 and H4) and 146 nucleotide pairs.

nucleotide  A nucleoside with one or more phosphate groups linked to the 5´ carbon of the pentose sugar. Ribose-containing nucleosides include ribonucleoside monophosphate (NMP), ribonucleoside diphosphate (NDP), and ribonucleoside triphosphate (NTP). When the nucleoside contains the sugar deoxyribose, the nucleotides are called deoxyribonucleoside mono-, di-, or tri-phosphates (dNMP, dNDP, or dNTP). A building block of DNA and RNA. See chromosome; codon; complementary nucleotides; di-deoxynucleotide; DNA; gene; oligonucleotide; RNA.

nucleus (L. nucleus, kernel of a nut)  A dense protoplasmic-membrane-bound region of a eukaryotic cell that contains the chromosomes separated from the cytoplasm by a membrane; present in all eukaryotic cells except mature sieve-tube elements.

null mutation  See amorph.

nullisomy  (adj: nullisomic)  An otherwise diploid cell or organism lacking both members of a chromosome pair (chromosome formula 2n -2). See also  disomy.

nurse culture  Planting a cell from a suspension culture on a raft of filter paper above a callus tissue piece (nurse tissue). The filter paper serves to prevent tissue union but allows the flow of essential substances from the nurse to the isolated cell. In such culture, a piece of callus is first placed on nutrient agar. Over this tissue is laid a strip of filter paper.

nutrient cycle  The passage of a nutrient or element through an ecosystem, including its assimilation and release by various organisms and its transformation into various organic or inorganic chemical forms.

nutrient deficiency  Absence or insufficiency of some factor needed for normal growth and development.

nutrient film technique (NFT)  Hydroponic technique used to grow plants. NFT delivers a film of water or nutrient solution either continuously or through on-off cycles (e.g., on 8 minutes and off 7 minutes).

nutrient gradient  A diffusion gradient of nutrients and gases that develops in tissues where only a portion of the tissue is in contact with the medium. Gradients are less likely to form in liquid media than in callus cultures.

nutrient medium (pl: nutrient media)  A solid, semi-solid or liquid combination of: major and minor salts; an energy source (sucrose); vitamins; plant growth regulators; and occasionally other defined or undefined supplements. Often made from stock solutions, then sterilized by autoclaving or filtering through a micropore filter.

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