nucleosome A beadlike subunit of chromatin, the
material chromosomes are made of, consisting of about
150–200 base pairs of DNA wrapped around a core
complex of eight histone proteins (two molecules each
ofhistones H2a, H2b, H3, and H4). Histones are a
type of protein associated with DNA in chromosomes
in the nucleus, and they function to coil the DNA into
nucleosomes.
nucleotides Nucleosides with one or more phos-
phate groups esterified mainly to the 3- or the 5- posi-
tion of the sugar moiety. Nucleotides found in cells are
adenylic acid, guanylic acid, uridylic acid, cytidylic
acid, deoxyadenylic acid, deoxyguanylic acid, deoxy-
cytidylic acid, and thymidylic acid. A nucleotide is a
nucleoside in which the primary hydroxy group of
either 2-deoxy-D-ribose or of D-ribose is esterified by
orthophosphoric acid.
See also ADENOSINE 5’-TRIPHOSPHATE; NAD+;
NADP+;NUCLEOSIDES.
nucleus There arefour definitions:
1.The round structure within a cell enclosed in a
double membrane containing chromosomes in
which DNA replication and transcription take place.
- A collection of nerve cells in the brain.
3. A positively charged core of an atom containing
protons and neutrons.
4. The frozen center of a comet’s head that contains
most of the comet’s mass.
null cell Aclass of lymphocytes characterized by a
lack of surface markers specific for either T or B lym-
phocytes.numerical taxonomy SeePHENETICS.nuptial flight The mating flight of insects (e.g.,
queen with males).nut A simple, dry, indehiscent fruit with a bony shell.nyctinasty Amovement made by a plant partly in
response to a nondirectional stimulus such as day and
night conditions.nymph A development stage in insects that is imma-
ture and undergoes incomplete metamorphosis. Does
not have a pupal stage.nymph 251