Bacteria -- Single-celled microorganisms without a nuclear membrane, distinct nucleus, or DNA organized in chromosomes. Bacteria vary in terms of morphology, oxygen and nutritional requirements, and motility. They may be free-living, living in decaying organic matter, or cause disease in plants or animals.

Bacteriophage, or phage -- A virus that infects and lyses certain bacteria, such as E. Coli. Bacteriophages were discovered by Felix d'Herelle and Frederick Twort in the 1910s. In the 1940s, Max Delbruck encouraged the phage group at Cold Spring Harbor to concentrate their research on seven specific bacteriophages (T1-T7), so that they could readily compare results. T2, T4, and T6 are serologically related and have large genomes. T3 and T7 are also related to each other serologically. T1 and T5 are not related to any other bacteriophages.

Colicins -- Any of various antibacterial substances produced by strains of intestinal bacteria, such as of E. coli, having a specific plasmid. These substances often act to inhibit macromolecular synthesis in related strains.

DNA, or deoxyribonucleic acid -- The primary genetic material of all cellular organisms and the DNA viruses. Located predominantly in cell nuclei, it is composed of two chains of nucleotides--deoxyribose and phosphate backbones with side chains of purine (adenine or guanine) or pyrimidine (cytosine and thymine) bases projecting inward. Hydrogen bonds link adenine to guanine, and cytosine to thymine. The two linked strands are twisted in a double helix.

Endonuclease -- Any of a group of enzymes catalyzing the hydrolysis of bonds between nucleic acids in the interior of a DNA or RNA molecule.

Escherichia coli, or E. coli -- Common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of culture in the laboratory.

Lysis -- A process of disintegration or dissolution of cells.

Lysogeny -- The phenomenon in which a bacterium is infected by a bacteriophage (virus), but instead of destroying the host cell, the viral DNA is integrated into the host's chromosome and replicated along with it for many generations; then (often induced by chemicals or ultraviolet light) the virus starts to replicate independently again, and lyses the host cell.

Mutation -- The process that produces a gene or a chromosome that differs from the wild-type. This process, either through an alteration in the nucleotide sequence of the DNA coding for a gene or through a change in the physical arrangement of a chromosome, results in the creation of a new character or trait not found in the parental type.

Oncogenes -- Genes with the potential to cause normal cells to become cancerous. The term may be used to describe viral or cellular genes with this potential.

Plasmid -- An extrachromosomal, circular, self-replicating DNA found in bacterial cells that carries genes for a variety of functions not essential for cell growth, e.g., antibiotic resistance, production of enzymes or toxins, or ability to metabolize certain nutrients.

Quantum theory -- The theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level.

Radiation biology -- The study of the ways in which radiation affects biological tissue and organisms.

Recombinant DNA, rDNA, hybrid strings -- Genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments, usually from more than one type of organism, e.g., inserting pieces of virus DNA into a bacterial plasmid.

Restriction enzyme -- An endonuclease which recognizes a specific sequence of bases in a DNA molecule. Each restriction enzyme has a single, specific recognition sequence, and binds to a DNA molecule at a specific site. As a result, treatment of a particular DNA molecule with a particular restriction enzyme will always produce the same set of DNA fragments.

RNA, or ribonucleic acid -- A single-stranded nucleic acid found in the cell nucleus and cytoplasm, which plays a key role in protein synthesis. (It also constitutes the genetic material of the RNA viruses.) It is similar to DNA but has ribose sugar, rather than deoxyribose sugar, and uracil, rather than thymine, as one of the pyrimidine bases. There are several classes of RNA molecules, including messenger RNA, transfer RNA, and ribosomal RNA, each serving a different purpose in the cell.