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

Diplochromosome
See endorduplication.

Diplonema
(adj: diplotene) 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 beign to disjoin.

Diplophase
Phase with 2n chromosomes.

Dipole (electric)
A pair of equal yet opposite electrical charges that are separated by a small distance.

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).

Directed mutagenesis
The process of generation of nucleotide changes in cloned genes by anyone of several procedures, including site-specific 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 re-circularizing. See cloning.

Disarm
To delete from a plasmid or virus genes that are cytotoxic or tumour inducing.