at Related Biological Terms

A technique to separate and/or characterize a macromolecule by high-speed centrifugation in a density gradient formed by the concentration gradient of a solute such as CsCl or sucrose. The macromolecule sediments until it reaches the zone of its own density. Also known as isopycnic centrifugation or zonal centrifugation.

The cleavage of N-glycosidic bonds of DNA to form apurinic DNA.

The loss of responsiveness of an enzyme to allosteric regulation while retaining its catalytic activity.

The chemical modification by oxidation, methylation, glycosylation, etc. of a xenobiotic to render it innocuous.

(see optical rotation)

An interim stage in the solution of a molecular structure from X-ray crystallographic data. Using only the intensities due to heavy atom reflections obtained by subtraction of the reflections of the protein from those of an isomorphous replacement, Fourier analysis calculates the vectors between heavy atoms. Their display is the difference Patterson map. This map may then be used to calculate the relative positions of the heavy atoms. (see also Fourier transformation; phase problem) Related tool: molecular biology tools

The fractionation of subcellular components according to their sedimentation behaviour; separation into nuclei, mitochondria, lysosomes, microsomes (endoplasmic reticulum), ribosomes, cytosol, etc. by removal of sedimenting material after cycles of processing at progressively increasing centrifugal force.

(= competitive labelling (differential chemical modification))

A method for comparison of different organs, or physiological or disease states by the genes that are uniquely expressed in them. The mRNAs of two types of cell to be compared, e.g. normal and cancer cells, are isolated, differently labelled for detection and hybridized to an appropriate cDNA library, preferably a normalized library. The library can be in the form of clones blotted onto duplicate filter membranes and the two sets of mRNAs, or can be labelled with two different radioisotopes, so that a clones that are equally labelled on both membranes identify a gene that is expressed equally in both kinds of cell, and a clone that fails to appear on one membrane will indicate transcription in only the other type of cell. A more sophisticated version labels the mRNAs with two different fluorophores, e.g. red emission for cancer cells and green emission for normal cells, so that when they are hybridized to the cDNA library that is arrayed in a relative excess on a single glass surface, the hybridization of both fluorophores to a single cDNA fragment will appear yellow, while the hybridization of only one, e.g. the normal cell mRNA, will appear green.Ito, T. and Sakaki, Y. (1996) Essays Biochem. 31, 11-21 Learn more about sgRNA.

(see subtractive DNA cloning)