A delicate, double membrane-bounded body found within the cytoplasm of most cells of plants and animals, but not bacteria. It contains the chromosomes that can be seen with the aid of a microscope during meiosis and mitosis. It functions as the control center of the cell. The double membrane, or nuclear membrane, contains many pores …
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Nucleoid
An aggregation of DNA found in prokaryotes and in the mitochondria and chloroblasts of eukaryotes, and which has some resemblance to a nucleus, but lacks a nuclear envelope and histones. A chloroblast is the place in a cell where photosynthesis takes place. See DNA (deoxyribonucleic acid), Eukaryote cell (or organism), Histone, Mitochondria, Nucleus (of a …
Nucleotide
An organic molecule consisting of a purine (adenine or guanine) or a pyramiding (thymine, uracil or cytosine) base linked to a sugar (deoxyribose or ribose) and a phosphate group. DNA and RNA are made of long chains of nucleotides. See Adenine, Adenosine triphosphate (ATP), Copy errors, Cytosine, Deoxyribose, DNA (deoxyribonucleic acid), DNA double helix, Guanine, …
Nucleic acid
A complex organic acid or macromolecule in living cells made up a long chain of nucleotides, of which there are two types, DNA and RNA. Friedrich Miescher (1844-1895) is credited with discovering nucleic acids in 1869, and went on to suggest they could play a role in heredity. Current research on nucleic acids forms a major …
NREM sleep
Abbreviation for ‘non-rapid-eye movement sleep’, also referred to as quiet or inactive sleep, and one of the five EEG stages. The period of NREM sleep, taking up about 75% of sleep time and lasting from four to six hours, consists of four such stages (which stages 3 and 4 combined since 2007). Stage 1, occurring …
Nuclear family
Family comprising parents and children (i.e., not members of the extended family such as grandparents, aunts and uncles). Taken to be the dominant family form in late industrial cultures in which the unit of analysis is the household consisting of mother, father and children. In fact, since the 1970s, this type of family represents a …
Novelty preference
The preference of infants for novel stimuli, characterized, for example, by a longer looking time at a novel stimulus when it is paired with a familiar stimulus in a familiarisation task. See Habituation, Moderate-discrepancy hypothesis, Numerical identity, Pupillometry, Visual acuity, Violation of expectation technique
Notochord
A term devised by Richard Owen (1804-1892) in 1848, it is an axial mesodermal tissue, adjacent to the neural tube, found in embryonic stages of all chordates and protochordates, and which often regresses as maturity is approached. Typically, it is a rod-shaped mass of vacuolated cells, lying immediately below the nerve cord and may provide …
Notch
A gene involved in cell fate specification. It is found in all metazoans and in mammals there are four different notch receptors (NOTCH1-NOTCH4). Notch signalling in conjunction with other gene encoded proteins plays a crucial role in the development and maintenance of a wide variety of tissues during both embryonic and adult life. For example, …
Nosology
The classification of disorders based on observable and detectable signs and symptoms on observation and interview, or more generally the study of disorders. See Disorder, Etiology, Oppositional defiant disorder (ODD), Pathogenesis (or pathogensy), Signs and symptoms