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a Related Biological Terms:

framework region

(see complementarity-determining region (CDR))

free base

The purine or pyrimidine moiety of a nucleoside, nucleotide or nucleic acid that is not attached to the pentose or pentose phosphate moiety. (see also base)

free radical

A molecular species containing an unpaired electron, e.g. the hydroxyl (OH•), superoxide (O2•-) and nitric oxide (NO•) radicals. (see also antioxidant; lipid peroxidation; oxidative stress)

freeze-fracture

A technique to prepare cells for scanning electron microscopy; cleavage of a solidly frozen cell, followed by freeze-etching, i.e. sublimation of some of the frozen water of the cell and coating the exposed subcellular structures with a film of platinum or carbon.

freezing at reactive centres of enzymes

(see entropy effect)

frictional coefficient

A measure of the size and asymmetry of a molecule in solution, derived from hydrodynamic measurements, e.g. diffusion, ultracentrifugation, mobility in electrophoresis; equal to RT/ND, qE/v, and M(1-)/NS, where R is the gas constant, T is the absolute temperature, N is Avogadro's number, D is the diffusion coefficient, q is the macromolecule's charge, E is the electric field strength, v is the rate of electrophoretic movement, M is the molecular mass, is the partial specific volume, is the solution density and S is the sedimentation coefficient. Related tool: real time pcr

functional cloning

(see positional candidate approach)

functional divergence ratio (FDR)

A measure of the evolutionary selective pressure on a protein. In a family of homologous proteins, or the genes encoding them, the amino acid residues are categorized as functional (e.g. those at the binding site and catalytic site) or non-functional. The FDR is the ratio of mutations found among functional residues to those found among non-functional residues. Learn more about restriction enzymes.

furanose

The form of a sugar when it is condensed into a five-membered ring. It consists of four carbon atoms and the oxygen atom that is the link to the anomeric carbon atom. (see also pyranose)

futile cycle (substrate cycle)

The coupling through common substrate/product pairs of an energy-requiring enzymic reaction with another energy-producing reaction that regenerates one of the products of the first reaction. The net result is the expenditure of energy, e.g. the phosphofructokinase and fructose phosphate phosphatase reactions that together generate and hydrolyse fructose bisphosphate with the net consumption of ATP. Such a cycle is potentially useful in the rapid response of metabolism to regulatory factors. Learn more about restriction enzymes.

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