Amino acid: the building blocks from which all proteins are made - 20 different sorts occur naturally.
Bacterium: a simple micro-organism consisting of one cell in which there is no nucleus and where the DNA floats freely in a loop.
Base: one of the chemical sub- units of DNA which carries the genetic code. Cells 'read' the genetic message three bases at a time: each triplet corresponds to an amino acid.
Cell: the small sub-units from which living organisms are composed. In humans, higher plants and animals, most cells have a complex structure consisting of a nucleus surrounded by cytoplasm.
Chromosome: Humans have 23 pairs of chromosomes carrying their entire genetic specification.
DNA (deoxyribonucleic acid): in most living things, DNA is the molecule carrying primary genetic information. It consists of long chains of nucleotides each of which is a base linked to a sugar (deoxyribose) and phosphate molecule.
Double helix: the three dimensional structure of DNA in which two strands twist together in a spiral.
Gene: the unit of inheritance, consisting of a sequence of bases of DNA occupying a specific position within the genome.
Gene therapy: the deliberate repair or replacement of damaged genes.
Genome: the full set of chromosomes - basically all the genes in an organism's DNA.
Nucleus: one of the inner compartments of a cell, separated from the cytoplasm by a membrane and containing the chromosomes.
Polygenic: controlled by or associated with more than one gene
Probe: a short single strand of DNA, usually labelled with a radioactive isotope, which can be used to detect a complementary segment.
Transgenic: an organism containing genetic material artificially inserted from from another species
Virus: a sub-microscopic organism which has to invade another cell to replicate itself.