at Related Biological Terms

A technique for characterization or preparation of subcellular particles. A swinging-bucket-type centrifuge tube is filled with a sucrose gradient, the bottom of which is most dense and the top least dense. A suspension of the particles is layered over the top of the solution, and centrifugation separates the particles within the gradient according to their density. At the termination of centrifugation the bottom of the tube is pierced and the sucrose, with particles of equal density, drips into a series of receiving tubes.

A sulphate ester of a ceramide or related compound, e.g. galactosyl-3-sulphate ceramide, ceramide-dihexoside sulphate.

The unsedimented portion that remains after centrifugation. (see also pellet)

A mutation that reverses the effect of an earlier mutation, e.g. a mutation in a gene for a tRNA that permits it to read and override an amber mutation.

(see Z-DNA)

A strategy for determination of the specificity of a soluble peptide. random collections of all the possible oligopeptides of a certain length, preceded by all the possible dipeptide sequences, are prepared; for example, if hexamers, there will be 400 (202) sets, each with the two known N-terminal residues followed by a random tetrapeptide; each of the 400 sets will consist of 160000 (204) hexapeptides. Each of the 400 is tested in an assay or bioassay, e.g. binding, inhibition, vasoconstriction. The successful set is refined by placing at the first of the random positions each of the amino acids, so there will be 20 sets of hexapeptides, each with a known tripeptide sequence followed by random tripeptide sequences. These are again tested in the assay system to further narrow the selection. Eventually the most favoured hexapeptide sequence is identified. A related strategy is positional scanning SPCL, in which a single amino acid is placed in each of the positions of the oligopeptide, a hexapeptide in the example, with each of the other five positions being filled at random by other amino acids. Each of these sub-libraries is assayed for selection and further refinement as in the original strategy. Houghten, R.A. (1994) Methods Companion Methods Enzymol. 6, 354-360

(see cyclic amplification and selection of targets (CASTing))

A consensus sequence of eukaryotic DNA that is part of the promoter region, and defines the transcription start site downstream from it; also known as a Hogness box or a Pribnow box.

(see Monod-Wyman-Changeux (MWC) model)

A virus that infects a bacterium and inserts its genome into that of the bacterium, rather than lysing it.