N
Asparagine
see amino acids
N-end rule
The observed relationship between
the N-terminal amino acid of a protein and the half-life of that protein in
vivo. InEscherichia coli, for example, proteins whose N-terminal
amino acid is either arginine or lysine are characterized by a short halflife; early degradation of such proteins
may be avoided in strains mutant in the AAT gene.
n. gen.
see genus novum.
N-Serve
see nitrapyrin.
n. sp.
see species nova.
N-type starter
see lactic acid starters.
NA
Numerical aperture: see resolving
power.
Na+-ATPase
See ion transport and sodium motive force.
Na+-motive force
see sodium motive force.
Na+ pump
see ion transport.
nabam
(dithane D-14) Disodium ethylene BISDITHIOCARBAMATE,
an agricultural antifungal agent sometimes used in conjunction with zinc
sulphate and lime – when ZINEB is formed.
nactins
see macrotetralides.
Abbreviation
of nicotinamide adenine dinucleotide, a coenzyme present in all cells that
helps enzymes transfer electrons during the redox reactions of metabolism.
NADP
Abbreviation
of nicotinamide adenine dinucleotide phosphate, a coenzyme that functions as an
electron acceptor in the lightdependent reactions of photosynthesis.
natural
killer cell
A
nonspecific defensive cell that attacks tumor cells and destroys infected body
cells, especially those harboring viruses.
natural
logarithm
The
logarithm (log) to base e where e ≈ 2.718.
The natural logarithm is represented by the symbol ln.
natural
selection
Differential
success in the reproduction of different phenotypes resulting from the
interaction of organisms with their environment. Evolution occurs when natural
selection causes changes in relative frequencies of alleles in the gene pool.
nectar
[Gk. nektar, the
drink of the gods]
A sugary fluid that attracts insects to plants.
A sugary fluid that attracts insects to plants.
negative
feedback
A primary
mechanism of homeostasis, whereby a change in a physiological variable that is
being monitored triggers a response that counteracts the initial fluctuation.
nematocyst
[Gk. nema,
nematos, thread + kyst, bladder]
A threadlike stinger, containing a poisonous or paralyzing substance, found in the cnidocyte of cnidarians.
A threadlike stinger, containing a poisonous or paralyzing substance, found in the cnidocyte of cnidarians.
nephridium pl. nephridia
[Gk. nephros, kidney]
A tubular excretory structure found in many invertebrates.
A tubular excretory structure found in many invertebrates.
nephron
(nef-ron)
[Gk. nephros, kidney]
The tubular excretory unit of the vertebrate kidney.
The tubular excretory unit of the vertebrate kidney.
neritic
zone
(neh-rit-ik)
The shallow regions of the ocean overlying the continental shelves.
The shallow regions of the ocean overlying the continental shelves.
nerve
A
ropelike bundle of neuron fibers (axons and dendrites) tightly wrapped in
connective tissue.
nerve
fiber
A
filamentous process extending from the cell body of a neuron and conducting the
nerve impulse; an axon.
nerve
impulse
A rapid,
transient, self-propagating change in electric potential across the membrane of
an axon.
nervous
system
All the
nerve cells of an animal; the receptor-conductor-effector system; in humans,
the nervous system consists of the central nervous system (brain and spinal
cord) and the peripheral nervous system.
net
primary production
In a
community or an ecosystem, the increase in the amount of plant or algal
material between the beginning and end of a specified time period, such as a
growing season.
net
primary productivity (NPP)
The gross
primary productivity minus the energy used by the producers for cellular
respiration; represents the storage of chemical energy in an ecosystem
available to consumers.
net
productivity
In a
trophic level, a community, or an ecosystem, the amount of energy (in calories)
stored in chemical compounds or the increase in biomass (in grams or metric
tons) in a particular period of time; it is the difference between gross
productivity and the energy used by the organisms in respiration.
neural
crest
A band of
cells along the border where the neural tube pinches off from the ectoderm; the
cells migrate to various parts of the embryo and form the pigment cells in the
skin, bones of the skull, the teeth, the adrenal glands, and parts of the
peripheral nervous system.
neural
groove
Dorsal,
longitudinal groove that forms in a vertebrate embryo; bordered by two neural
folds; preceded by the neural-plate stage and followed by the neural-tube
stage.
neural
plate
Thickened
strip of ectoderm in early vertebrate embryos that forms along the dorsal side
of the body and gives rise to the central nervous system.
neural
tube
Primitive,
hollow, dorsal nervous system of the early vertebrate embryo; formed by fusion
of neural folds around the neural groove.
neuromodulator
A
chemical agent that is released by a neuron and diffuses through a local region
of the central nervous system, acting on neurons within that region; generally
has the effect of modulating the response to neurotransmitters.
neuromuscular
junction
The
junction between an axon terminal of a motor neuron and a muscle fiber
innervated by that motor neuron; the axon terminal of a motor neuron is
typically branched, forming neuromuscular junctions with a number of different
muscle fibers.
neuron
(noor-on)
[Gk. nerve]
A nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane.
A nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane.
neurosecretory
cells
Hypothalamus
cells that receive signals from other nerve cells, but instead of signaling to
an adjacent nerve cell or muscle, they release hormones into the bloodstream.
neurotransmitter
A
chemical messenger released from the synaptic terminal of a neuron at a
chemical synapse that diffuses across the synaptic cleft and binds to and
stimulates the postsynaptic cell.
neutral
variation
Genetic
diversity that confers no apparent selective advantage.
neutron
(new-tron)
An electrically neutral particle (a particle having no electrical charge), found in the atom.
An electrically neutral particle (a particle having no electrical charge), found in the atom.
niche
See Ecological niche.
nitrification
The
oxidation of ammonia or ammonium to nitrites and nitrates, as by nitrifying
bacteria.
nitrogen
cycle
Worldwide
circulation and reutilization of nitrogen atoms, chiefly due to metabolic
processes of living organisms; plants take up inorganic nitrogen and convert it
into organic compounds (chiefly proteins), which are assimilated into the
bodies of one or more animals; bacterial and fungal action on nitrogenous waste
products and dead organisms return nitrogen atoms to the inorganic state.
nitrogen
fixation
The
assimilation of atmospheric nitrogen by certain prokaryotes into nitrogenous
compounds that can be directly used by plants.
nitrogenase
(nih-trah-juh-nayz)
An enzyme, unique to certain prokaryotes, that reduces N2 to NH3.
An enzyme, unique to certain prokaryotes, that reduces N2 to NH3.
nitrogenous
base
(nigh-tra-jen-us)
An organic base that contains the element nitrogen.
An organic base that contains the element nitrogen.
nocturnal
[L. nocturnus, of
night]
Applied to organisms that are active during the hours of darkness.
Applied to organisms that are active during the hours of darkness.
node
[L. nodus, knot]
A point along the stem of a plant at which leaves are attached.
A point along the stem of a plant at which leaves are attached.
nodes of
Ranvier
(ran-veer)
The small gaps in the myelin sheath between successive glial cells along the axon of a neuron; also, the site of high concentration of voltage-gated ion channels.
The small gaps in the myelin sheath between successive glial cells along the axon of a neuron; also, the site of high concentration of voltage-gated ion channels.
nomograph
A graph
that allows a third variable to be measured when the values of two related
variables are known.
noncompetitive
inhibitor
A
substance that reduces the activity of an enzyme by binding to a location
remote from the active site, changing its conformation so that it no longer binds
to the substrate.
noncyclic
electron flow
A route
of electron flow during the light reactions of photosynthesis that involves
both photosystems and produces ATP, NADPH, and oxygen; the net electron flow is
from water to NADP+.
noncyclic
photophosphorylation
(fo-toh-fos-for-eh-lay-shun)
The production of ATP by noncyclic electron flow.
The production of ATP by noncyclic electron flow.
nondisjunction
[L. non, not
+ disjungere, to separate]
An accident of meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
An accident of meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
nonpolar
covalent bond
A type of
covalent bond in which electrons are shared equally between two atoms of
similar electronegativity.
nonsense
mutation
A
mutation that changes an amino acid codon to one of the three stop codons,
resulting in a shorter and usually nonfunctional protein.
noradrenaline
A
hormone, produced by the medulla of the adrenal gland, that increases the
concentration of glucose in the blood, raises blood pressure and heartbeat
rate, and increases muscular power and resistance to fatigue; also one of the
principal neurotransmitters; also called norepinephrine.
norepinephrine
See noradrenaline.
norm of
reaction
The range
of phenotypic possibilities for a single genotype, as influenced by the
environment.
notochord
(no-toh-kord)
[Gk. noto, back + L. chorda, cord]
A longitudinal, flexible rod formed from dorsal mesoderm and located between the gut and the nerve cord in all chordate embryos.
A longitudinal, flexible rod formed from dorsal mesoderm and located between the gut and the nerve cord in all chordate embryos.
nuclear
envelope
[L. nucleus, a
kernel]
The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
nucleic
acid
(new-clay-ick)
A polymer consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
A polymer consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
nucleic
acid probe
In DNA
technology, a labeled single-stranded nucleic acid molecule used to tag a
specific nucleotide sequence in a nucleic acid sample. Molecules of the probe
hydrogen-bond to the complementary sequence wherever it occurs; radioactive or
other labeling of the probe allows its location to be detected.
nucleoid
(noo-klee-oid)
A dense region of DNA in a prokaryotic cell.
A dense region of DNA in a prokaryotic cell.
nucleoid
region
The
region in a prokaryotic cell consisting of a concentrated mass of DNA.
nucleolus
(noo-klee-oh-lus)
[L. a small kernel]
A specialized structure in the nucleus, formed from various chromosomes and active in the synthesis of ribosomes.
A specialized structure in the nucleus, formed from various chromosomes and active in the synthesis of ribosomes.
nucleoside
(noo-klee-oh-side)
An organic molecule consisting of a nitrogenous base joined to a five-carbon sugar.
An organic molecule consisting of a nitrogenous base joined to a five-carbon sugar.
nucleosome
(noo-klee-oh-some)
[L. nucleus, a kernel + Gk. soma, body]
The basic, beadlike unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.
The basic, beadlike unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.
nucleotide
(noo-klee-oh-tide)
[L. nucleus, a kernel]
The building block of a nucleic acid, consisting of a five-carbon sugar covently bonded to a nitrogenous base and a phosphate group.
The building block of a nucleic acid, consisting of a five-carbon sugar covently bonded to a nitrogenous base and a phosphate group.
nucleus
[L. a
kernel]
(1) An atom's central core, containing protons and neutrons. (2) The chromosome-containing organelle of a eukaryotic cell. (3) A cluster of neurons.
(1) An atom's central core, containing protons and neutrons. (2) The chromosome-containing organelle of a eukaryotic cell. (3) A cluster of neurons.
null
hypothesis
In
statistical analysis, a hypothesis proposing that there is no statistically
significant difference between the observed results of an experiment and the
expected results.