Dispense - portion out a nutrient medium into containers, such as test a tubes, jars, Erlenmeyer flasks, Petri dishes, etc.

Dissection - separation of a tissue by cutting for analysis or observation.

Dissolve - pass chemicals into solution.

Distillation - the process of heating a mixture to separate the more volatile from the less volatile parts, and then cooling and condensing the resulting vapour so as to produce a more nearly pure or refined substance.

Di-sulphide bond - a chemical bond that stabilises the three dimensional structure of proteins, and hence the protein's normal function. They form between cysteine residues in the same or different peptide molecules. a.k.a. di-sulphide bridge.

Ditype - in fungi, a tetrad that contains. two kinds of meiotic products(spores), e.g., 2AB and 2ab.

Diurnal - term describing the occurrence of an event at least once every 24 hours. cf circadian.

Dizygotic twins - two egg twins, i.e., a pair of individuals that shared the same uterus at the same time, but which arose from separate and independent fertilisation of two ova

DNA - deoxyribonucleic acid; formerly spelt desoxyribonucleic acid. The long chain of molecules in most cells that carries the genetic message and controls all cellular functions in most forms of life. The information carrying genetic material that comprises the genes. DNA is a macromolecule composed of a long chain of deoxyribonucleotides joined by phosphodiester linkages.

Each deoxyribonucleotide contains a phosphate group, the five carbon sugar 2-deoxribose, and a nitrogen containing base. The genetic material of most organisms and organelles so far examined is double stranded DNA; a number of viral genomes consist of single stranded DNA or single or double stranded RNA. In double stranded DNA, the two strands run in opposite (anti parallel) directions and are coiled round one another in a double helix.

Purine bases on one strand specifically hydrogen bond with pyrimidine bases on the other strand, according to the Watson Crick rules (A pairs with T; G pairs With C). Hence a constant width for the double helix of 20 A˚ (2.0 nm) is maintained. In the B-form, DNA adopts a right handed helical conformation, with each chain making a complete turn every 34 A˚ (3.4 nm), or once every ten bases.

DNA amplification - multiplication of a piece of DNA in a test tube into many thousands of millions of copies. The most commonly used process is the polymerase chain reaction (PCR) system, but other systems are being developed, including ligase chain reaction (LCR), nucleic acids sequence dependent amplification, and the Q-b system.

DNA bank - In AnGR: storage of DNA, which may or may not be the complete genome, but should always be accompanied by inventory information. (Note: at the present time, animals cannot be reestablished from DNA alone.) (Source: FAO, 1999).

DNA construct - a DNA molecule inserted into a cloning vector, usually a plasmid.

DNA delivery system - a generic term for any procedure that transports DNA into a recipient cell.

DNA diagnosis - the use of DNA polymorphisms to detect the presence of a specific allele (often associated with a disease or syndrome) or DNA sequence.

DNA fingerprint - the unique pattern of DNA fragments identified originally by Southern hybridisation (using a probe that binds to a polymorphic region of DNA) or now by polymerase chain reaction (PCR) (using primers flanking the polymorphic region). See genetic fmgerprinting.

DNA helicase - an enzyme that catalyses the unwinding of the complementary strands of a DNA double helix.

DNA hybridisation - the pairing of two DNA molecules, often from different sources, by hydrogen bonding between complementary nucleotides. This technique is frequently used to detect the presence of a specific nucleotide sequence in a DNA sample.

DNA ligase - an enzyme that catalyses a reaction that links two DNA molecules via the formation of a phospho-diester bond between the 3' hydroxyl and 5' phosphate of adjacent nucleotides. It plays an important role in DNA repair and replication.

DNA ligase is one of the essential tools of recombinant DNA technology, enabling (among other things) the incorporation of foreign DNA into vectors. The ligase enzyme encoded by phage T4 : is commonly used in genecloning experiments. It requires ATP as a cofactor. T4 is used in vitro to join the vector and insert DNAs.

DNA micro array - a small glass surface to which has been fixed an array of DNA fragments, each with a defined location. A typical DNA chip would contain: 10,000 discrete spots (each containing a different DNA fragment) in an area of just a few square centimetres. When a solution of fluorescently labelled DNA fragments is hybridised to the chip, spots to which hybridisation occurs are visible as fluorescence. If the spots on the chip are genes (expressed sequence tags, q.v.), hybridisation with cDNA from a particular tissue shows which genes are expressed in that tissue.

If the spots are short, synthesised oligonucleotides (approximately 25 bases) corresponding to that part of a gene containing a single nucleotide polymorphisms (SNP) (q.v.), with a separate spot for each of the 4 possible bases at that site, hybridisation with genomic DNA from an individual plant or animal enables that individual to be genotyped at as many SNP loci as are represented on the chip. The big advantage of DNA chips is the extent to which the process of genotyping can be automated, thereby enabling huge numbers of plants or animals to be genotyped for a huge number of loci.