Acentric chromosome - chromosome fragment lacking a centromere

Acridine dyes - a class of positively charged polycyclic molecules that intercalate into DNA and induce frameshift mutations

Acrocentric - a chromosome that has its centromere near the end

Acropetal - 1. developing or blooming in succession towards the apex, such as leaves or flowers developing acropetally
2. transport or movement of substances towards the apex, such as the movement of water through the plant. The opposite tendency is termed basipetal

Acrosome - an apical organelle in the head of a spermatozoon, q.v

Actin - one of the two contractile proteins in muscle (the other being myosin). Actin is also found in the micro filaments that form part of the cytoskeleton of all cells

Activated charcoal - charcoal which has been treated to remove hydrocarbons and to increase its adsorptive properties. It acts by condensing and holding a gas or solute onto its surface; thus inhibitory substances in nutrient medium may be adsorbed to charcoal included in the medium. Rooting factors such as phenolamines present as contaminants in charcoal may stimulate growth in vitro. Its addition to rooting medium may stimulate root initiation in some plant species. Activated charcoal may differ in origin and in composition. cf charcoal; phenolic oxidation.

Activator - 1. a substance or physical agent that stimulates transcription of a specific gene or operon
2. a compound that, by binding to an allosteric site on an enzyme, enables the active site of the enzyme to bind to the substrate

Active collection - in PGR: Defined in the International Undertaking on Plant Genetic Resources (FAO, 1983) as a collection which complements a base collection (q.v.) and is a collection from which seed samples are drawn for distribution, exchange and other purposes such as multiplication and evaluation

Active site - 1. a site on the surface of a catalyst at which activity occurs
2. the site on the surface of an enzyme molecule that binds the substrate molecule

Adaptation - adjustment of a population to changed environment over generations, associated (at least in part) with genetic changes resulting from selection imposed by the changed environment. Not acclimatisation, q.v

Adaptation traits - the complex of traits related to reproduction and survival of the individual in a particular production environment. Adaptation traits contribute to individual fitness; they are the traits subjected to selection during the evolution of animal genetic resources. By definition, these traits are also important to the ability of the animal genetic resource to be sustained in the production environment

Adaptive radiation - the evolution of new forms, sub-species or species from one species of plant or animal in order to exploit new habitats or food sources. a.k.a. divergent evolution

Adaptor - 1. a synthetic double-stranded oligonucleotide that has a blunt end, while the other end has a nucleotide extension that can base pair with a cohesive end created by cleavage of a DNA molecule with a specific type II restriction endonuclease. The blunt end of the adaptor can be ligated to the ends of a target DNA molecule and the construct can be cloned into a vector by using the cohesive ends of the adaptor

2. a synthetic single-stranded oligonucleotide that, after self hybridisation, produces a molecule with cohesive ends and an internal restriction endonuclease site. When the adaptor is inserted into a cloning vector by means of the cohesive ends, the internal sequence provides a new restriction endonuclease site

Addendum - in formulation of tissue culture media: an item or a constituent substance to be added

Additive allelic effects - effects of alleles at a locus, where the heterozygote is exactly intermediate between the two homozygotes

Additive gene effects - additive allelic effects summed across all the loci that contribute to genetic variation in a quantitative trait

Adenine - a white crystalline purine base. A constituent of DNA and RNA and nucleotides such as ADP and ATP. A B-group Vitamin (B4) generally available as C5H5N5.3H2O, m.p. 189.13. It is added to some tissue culture media, as adenine sulphate, to promote shoot formation and for its weak cytokinin effect. It is present in plant tissues combined with aminoamide, phosphoric acids and D-ribose

Adenovirus - a group of DNA viruses which cause disease in animals. In man, they produce acute respiratory tract infections with symptoms resembling common cold. They are used in gene cloning, as vectors for expressing large amounts of recombinant proteins in animal cells. They are also used to make live virus vaccines against more dangerous pathogens. See viral vaccines
Adenylate cyclase - the enzyme that catalyses the formation of cyclic AMP