Affinity chromatography - a method for separating molecules by exploiting their ability to bind specifically to other molecules. There are several types of biological affinity chromatography. A biological molecule can be immobilised and a smaller molecule (ligand, q.v.,) to which it is to bind can be stuck to it, or the smaller ligand can be immobilised and the macromolecule stuck to it. A variant is to use an antibody as the immobilised molecule and use it to 'capture' its antigen: this is often called immuno-affinity chromatography

Aflatoxin - toxic compounds, produced by moulds (fungi) of the Aspergillus flavus group, that bind to DNA and prevent replication and transcription. Aflatoxins can cause acute liver damage and cancer. Animals may be poisoned by eating stored food or feed contaminated with the mould

Agar - a polysaccharide solidifying agent used in nutrient media preparations and obtained from certain types of red algae (Rhodophyta). Both the type of agar and its concentration can affect the growth and appearance of cultured explants

Agarose gel electrophoresis - a process in which a matrix composed of a highly purified form of agar is used to separate larger DNA and RNA molecules. See electrophoresis

Agonist - a drug, hormone or transmit ter substance that forms a complex with a receptor site that is capable of triggering an active response from a cell

Agrobacterium - a genus of bacteria that includes several plant pathogenic species, causing tumour like symptoms

Agrobacterium tumefaciens-mediated transformation - a naturally occurring process of DNA transfer from the bacterium A. tumefaciens to plants

Agrobiodiversity - that component of biodiversity that is relevant to food and agriculture production. The term agrobiodiversity encompasses within-species, species and ecosystem diversity. (Based on FAG, 1999)

AIDS - the usually fatal human disease in which the immune system is destroyed by retrovirus (Human Immuno deficiency Virus, HIV). The virus infects and destroys helper T cells, which are essential for combating infections

Airlift fermenter - a cylindrical fermentation vessel in which the cells are mixed by air introduced at the base of the vessel and that rises through the column of culture medium. The cell suspension circulates around the column as a consequence of the gradient of air bubbles in different parts of the reactor

Albinism - hereditary absence of pigment in an organism. Albino animals have no colour in their skin, hair and eyes. The term is also used for absence of chlorophyll in plants

Albino - 1. an organism lacking pigmentation, due to genetic factors. The condition is albinism, q.v
2. a conspicuous plastome (plastid) mutant involving loss of chlorophyll

Aleurone - the outermost layer of the endosperm in a seed

Algal biomass - single celled plants, such as Chlorella spp. and spirulina spp., are grown commercially in ponds to make feed materials. Chlorella is grown commercially to make into fish food: it is fed to zooplankton, and these in turn are harvested as feed for fish farms. This is a means of covert sunlight into food in a way more convenient and controllable than normal farming

Alginate - polysaccharide gelling agent

Alkylating agents - chemicals that transfer alkyl (methyl, ethyl, etc.) groups to the bases in DNA

Allele - one of a pair, or series, of variant forms of a gene that occur at a given locus in a chromosome. Alleles are symbolised with the same basis symbol (e.g., B for dominant and b for recessive); B1, B2, ..., Bn for n additive alleles
at a locus). In a normal diploid cell there are two alleles of anyone gene (one from each parent), which occupy the same relative position (locus) on homologous chromosomes. Within a population there may be more than two alleles of a gene

Allele frequency - the number of copies of an allele in a population, expressed as a proportion of the total number of copies of all alleles at a locus in a population

Allele specific amplification (ASA) - the use of polymerase chain reaction (PCR) at a sufficiently high stringency that only a primer with exactly the same sequence as the target DNA will be amplified. A powerful means of genotyping for single-locus disorders that have been characterised at the molecular level

Allelic exclusion - a phenomenon whereby only one functional allele of an antibody gene can be assembled in a given B lymphocyte. The "allele" on the other homologous chromosome in a diploid mammalian cell cannot undergo a functional re-arrangement, which would result in the production of two different antibodies by a single plasma cell

Allelopathy - the phenomenon by which the secretion of chemicals, such as phenolic and terpenoid compounds, by a plant inhibits the growth or reproduction of other plant species with which it is competing

Allergen - an antigen that provokes an immune response

Allogamy - cross fertilisation in plants

Allometric - when the growth rate of one part of an organism differs from that of another part or of the rest of the body

Allopatric speciation - speciation occurring at least in part because of geographic isolation

Allopolyploid - a polyploid organism (usually a plant) having multiple sets of chromosomes derived from different species. Hybrids are usally sterile, because they do not have sets of homologous chromosomes and there fore pairing cannot take place. However, if doubling of the chromosomes number occurs in a hybrid derived from two diploid (2n) species, the resulting tetraploid (4n) is a fertile plant, since it contains two sets of homologous chromosome and pairing may occur; this tetraploid is an allotetraploid

Allosteric enzyme - an enzyme that has two structurally distinct forms, one of which is active and other inactive. Active forms of allosteric enzymes tend to catalyse the initial step in a pathway leading to the synthesis of molecules. The end product of this synthesis can act as a feedback inhibitor, converting the enzyme to the inactive form, thus controlling the amount of product synthesized

Allosteric regulation - a catalysis regulating process in which the binding of a small effector molecule to one site on an enzyme affects the activity at another site