The constant of proportionality between the diffusion flux and the
concentration gradient in Fick's first law. Its magnitude is
indicative of the rate of atomic diffusion.
Digital
Refers to systems employing only quantized (discrete) states to
convey information (also see"analog").
Dimer
A molecule formed by the joining of two identical monomers.
Diode
two-terminal device that conducts current well in one direction
and poorly in the other.
Dip
Dual In-line Package - common ceramic or plastic enclosure for an
integrated circuit.
Dipole (electric)
A pair of equal yet opposite electrical charges that are separated
by a small distance.
Dislocation
A linear crystalline defect around which there is atomic
misalignment. Plastic deformation corresponds to the motion of
dislocations in response to an applied shear stress. Edge, screw,
and mixed dislocations are possible.
DNA Probes
A DNA or nucleic acid probe is a short strand of DNA that locates
and binds to its complementary sequence in samples containing
single strands of DNA or RNA enabling identification of specific
sequences. Nucleic acid probe assays exploit the fundamental
hybridization reaction that occurs spontaneously between two
complementary DNA:DNA or DNA:RNA strands. As in immunoassays,
detection of the hybrid requires that the probe be labeled.
Various direct and indirect methods have been devised for the
detection of the hybrid. Direct labeling involves attaching the
label directly to the probe sequence; indirect labeling binds an
antibody to the DNA:DNA or DNA:RNA hybrid. As in immunoassays,
non-isotopically-labeled probes are preferred over radio-labeled
probes primarily because of radiation hazards, disposal problems,
and short reagent shelf life. In addition, the factors determining
the detection limits of hybridization assays based on labeled
probes are similar to those in immunoassays. Therefore, the
development of a simple, inexpensive and sensitive direct
detection system which eliminates the use of labels is highly
desirable.
DNA Sequencing
There are two main classical methods for sequencing DNA: The first
method, developed by Allan Maxam and Walter Gilbert, involves
chemicals used to cleave the DNA at certain positions, generating
a set of fragments that differ by one nucleotide. The second
method, developed by Fred Sanger and Alan Coulson, involves
enzymatic synthesis of DNA strands that terminate in a modified
nucleotide. Analysis of fragments is similar for both methods and
involves gel electrophoresis and autoradiography or
fluorescence. The enzymatic method has largely replaced the
chemical method as the technique of choice, although there are
some situations where chemical sequencing can provide data more
easily than the enzymatic method.
Domain
A region of a ferromagnetic or ferrimagnetic material in which all
atomic or ionic magnetic moments are aligned in the same
direction.
