Abjad N

JEJAK MAHASISWA	..:: KAMUS BIOLOGI JEJAK MAHASISWA ::..															Dibuat 12/19/2011
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Glosarium ini diambil dari Buku Biologi Campbell | Jejak Mahasiswa

NAD⁺

(nicotinamide adenine dinucleotide) A coenzyme present in all cells that helps enzymes transfer electrons during the redox reactions of metabolism.

natural killer cell

A nonspecific defensive cell that attacks tumor cells and destroys infected body cells, especially those harboring viruses.

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.

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.

nephron

(NEF-ron) The tubular excretory unit of the vertebrate kidney.

neritic zone

(neh-RIT-ik) 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.

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.

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.

neuron

(NOOR-on) 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

An electrically neutral particle (a particle having no electrical charge), found in the atom.

niche

See ecological niche.

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.

nitrogenous base

(nigh-TRA-jen-us) An organic base that contains the element nitrogen.

node

A point along the stem of a plant at which leaves are attached.

nodes of Ranvier

(Ron-vee-AYE) 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.

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.

nondisjunction

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.

norm of reaction

The range of phenotypic possibilities for a single genotype, as influenced by the environment.

notochord

(NO-toh-kord) A longitudinal, flexible rod formed from dorsal mesoderm and located between the gut and the nerve cord in all chordate embryos.

nuclear envelope

The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.

nucleic acid

(polynucleotide) (PAHL-ee-NOO-klee-o-tide) 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.

nucleoid region

The region in a prokaryotic cell consisting of a concentrated mass of DNA.

nucleoid

(NOO-klee-oid) A dense region of DNA in a prokaryotic cell.

nucleolus

(noo-KLEE-oh-lus) (plural, nucleoli) 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.

nucleosome

(NOO-klee-oh-some) 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) The building block of a nucleic acid, consisting of a five-carbon sugar covently bonded to a nitrogenous base and a phosphate group.

nucleus

(1) An atom's central core, containing protons and neutrons. (2) The chromosome-containing organelle of a eukaryotic cell. (3) A cluster of neurons.