Glossary

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NADH/NAD⁺Nicotinamide adenine dinucleotide is an important electron shuttle in cells. NADH/NAD⁺ interconverts between its oxidizing form (NAD⁺) and its reducing form (NADH) and participates in many redox reactions involved in cellular metabolism.

natural selection
Natural selection is an essential mechanism of evolution proposed by Charles Darwin and generally accepted by the scientific community as the best explanation of speciation as evidenced in the fossil record.

The basic concept is that environmental conditions (or "nature") determine (or "select") how well particular traits of organisms can serve the survival and reproduction of the organism; organisms lacking these traits might die before reproducing, or be less fecund. As long as environmental conditions remain the same, or similar enough that these traits continue to be adaptive, such traits will become more common within populations.

negative regulation
The downregulation of a constitutive gene, often occurring by feedback inhibition (ex. trp operon).

non-competitive inhibitors
Inhibition of an enzyme where the inhibitor binds a site other than the active site of the enzyme, resulting in a conformational change and the enzyme's inability to effectively bind substrate.

non-polar
Substances that do not combine (mix) with water.

nonpolar covalent bond
A nonpolar covalent bond is one in which the electrons are shared more or less equally between the two atoms involved, and the electrical charge is also distributed equally around the molecule.

nucleic acid
The nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are one of the basic classes of macromolecules studied in biochemistry. Nucleic acid, so called because of its prevalence in cellular nuclei, is the generic name of a family of biopolymers. The monomers are called nucleotides, and each consists of three components: a nitrogenous base (either a purine or a pyrimidine), a pentose sugar, and a phosphate group. Different nucleic acid types differ in the specific sugar found in their chain.

nucleotide
A nucleotide is an organic molecule consisting of a nitrogenous base (a purine or a pyrimidine), a pentose sugar (deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. (A nucleoside is similar, except that it contains only the sugar and base, without a phosphate.)

Nucleotides are the monomers of nucleic acids and also play important roles in cellular energy transport and transformations (notably ATP and NAD+/NADH) and in enzyme regulation.