Dextrin - an intermediate polysaccharide compound resulting from the hydrolysis of starch to maltose by amylase enzymes.

Diabetes - a disease associated with the absence or reduced levels of insulin, which is a hormone essential for the transport of glucose to cells.

Diagnostic procedure a test or assay used to determine the presence of an organism, substance or nucleic acid sequence alteration.

Diakinesis a stage of meiosis just before metaphase I, in which the separation of homologous chromosomes is almost completed.

Diazotroph - an organism that can fIx atmospheric nitrogen.

Dicentric chromosome - a chromosome having two centromeres.

Dichogamy - the condition in which the male and the female reproducrive organs of a flower mature at different times, thereby making self fertilisarion improbable or impossible.

Dicot - see dicotyledon.

Dicotyledon - a plant with two cotyledons, or seed leaves. One of the two classes of plants in the Angiosperms (the other class is the monocotyledons). Colloquially called a dicot. Examples include many crop plants (potato, pea, beans), ornamentals (rose, ivy) and: timber trees (oak, beech, lime).

Di -deoxynucleotide - a deoxynucleotide that lacks a 3' - hydroxyl group, and is thus unable to form the 3' -5' phospho -diester bond necessary for chain elongation. Dideoxyucleotides are used in . DNA sequencing and the treatment of viral diseases. Also sometimes referred to as didN.

Differential centrifugation - a method of separating sub cellular particles according to their sedimentation coefficients, which are roughly proportional to their size. Cell extracts are subjected to a succession of centrifuge runs at progressively faster rotation speeds. Large particles, such as nuclei or mitochondria, will be precipitated at relatively slow speeds; higher G forces will be required to sediment small particles, such as ribosomes.

Differentially permeable - referring to a membrane, through which different substances diffuse at different rates; some substances may be unable to diffuse through such a membrane.

Differentiation - a process in which unspecialised cells develop structures and functions characteristic of a particular type of cell. Development from one cell to many cells, accompanied by a modification of the new cells for the performance of particular functions. In tissue culture, the term is used to describe the formation of different cell types.

Diffusion - the movement of molecules from a region of higher concentration to a region oflower concentration.

Digest to cut DNA molecules with one or more restriction endonucleases. See cleave.

Dihaploid - an individual which arises from a doubled haploid.

Dihybrid - an individual that is heterozygous for two pairs of alleles; the progeny of a cross between homozygous parents differing at two loci.

Dimer - association of two molecules.

Dimethyl sulphoxide - a highly hygroscopic liquid and powerful solvent with little odour or colour. It is an organic co -solvent used in small quantities to dissolve neutral organic substance in tissue culture media preparation. DMSO also has uses as a cryoprotectant.

Dimorphism - the existence of two distinctly different types of individuals within a species. An obvious example is the sexual dimorphism in certain animals.

Diploid
1. the status of having two complete sets of chromosomes, most commonly one set of paternal origin and the other of maternal origin.
2. an organism or cell with a double set (2n) of chromosomes (most commonly one of paternal origin, and the other of maternal origin), or referring to an individual conltaining a double set of chromosomes per cell. Somatic tissues of higher plants and animals are ordinarily diploid inhromosome constitution, in contrast with the haploid gametes.

Diploid cell - a cell which contains two sets of chromosomes. Cf haploid cell.

Diplonema - stage in prophase of meiosis I following the pachytene stage, but preceding diakinesis, in which one pair of sister chromatids begin to separate from the other pair, i.e., the centromeres begin to disjoin.

Diplophase - phase with 2n chromosomes.

Direct embryogenesis - embryoids form directly in culture, without an intervening callus phase, on the surface of zygotic or somatic embryos or on explant tissues (leaf section, root tip, etc.).

Direct organogenesis - formation of organs directly on the surface of cultured intact explants. The processdoes not involve callus formation.

Direct repeat - two or more stretches of DNA within a single molecule which have the same nucleotide sequence in the same orientation. Direct repeats may be either adjacent to one another or far apart on the same molecule. For example
TATTA...TATTA.
ATAAT...ATAAT.

Directed mutagenesis - the process of generation of nucleotide changes in cloned genes by any one of several procedures, including sitespecific and random mutagenesis. Also called in vitro mutagenesis.

Directional cloning - the technique by which DNA insert and vector molecules are digested with two different restriction enzymes to create non complementary sticky ends at either end of each restriction fragment, so allowing the insert to be ligated to the vector in a specific orientation and preventing the vector from recircularising.