ß-galactosidase
An enzyme that catalyzes the cleavage of lactose into glucose + galactose. In E. coli this enzyme is encoded by the lacZ gene. Often used as a reporter for assaying gene expression (see X-gal).

ß-lactam antibiotics
Antibiotics taht contain a ß-lactam ring and act by inhibiting peptidoglycan synthesis. For example, penicillins, cephalosporins, and related antibiotics.

ß-lactamase
An enzyme that cleaves the ß-lactam ring of ß-lactam antibiotics, thus inactivating the antibiotics. The ampicillin resistance encoded by many common plasmids is due to a secreted ß-lactamase.

BAC
An acronym for Bacterial Artifical Chromosome. Low copy number plasmid vectors that allow stable cloning very large DNA fragments (often 100 Kb or more).

Backcross
A genetic cross between an offspring and one of its parents or an organism genetically identical to one of its parents.

Back mutation
A reversion event which restores the original DNA sequence.

Bacteriophage (or simply Phage)
A virus that infects a bacterium.

Bacteriostatic
A condition which prevents the growth of bacteria without killing them.

Baculovirus
A virus that has been used as a cloning vector for the production of recombinant protein in insect cells.

Base analog
A purine or pyrimidine base that differs slightly in structure from the normal bases found in DNA or RNA. Some analogs may be incorporated into nucleic acids in place of the normal base, often resulting in a base substitution mutation. Some examples include aminopurine, azaguanine, azauracil, and 5-bromodeoxyuridine.

Base pair
A complementary purine and pyrimidine that are hydrogen-bonded to form double-stranded DNA or RNA.

Base substitution mutation
A mutation resulting in the replacement of one base for a different base.

Batch culture
Growth of bacteria in a fixed volume of liquid medium in a closed vessel, with no additions or removals made during the period of incubation.

Bidirectional replication
Two replication forks proceed in opposite directions from the same origin of replication. Sometimes called omega-replication.

Bioinformatics
The management and analysis of data (especially DNA sequence data) using advanced computing techniques. Bioinformatics is an important field of genomics research, because of the complexity of searching and comparing the large number of DNA sequences generated.

Biolistics
A means of introducing DNA into cells that involves bombardment with high-velocity microprojectiles coated with DNA.

Biological containment
One of the precautionary measures taken to prevent the replication of recombinant DNA molecules in microorganisms in the natural environment. Biological containment involves the use of vectors and host organisms that have been modified so that they will not survive outside of the laboratory.

Biotechnology
The use of living organisms (often microbes) in industrial processes.

Biotin
A molecule that can be incorporated into dUTP and used as a non-radioactive label for a DNA probe.

Blunt (flush) end
The end of a DNA molecule at which both strands terminate at the same nucleotide position with no single-stranded extension.

Branch migration
A "zipper-like" pairing of two homologous DNA strands during genetic recombination.

Broad host range
The ability to infect and reproduce in a wide variety of different organisms. For example, a phage or plasmid that can grow in many different bacterial species, or a bacterium that can produce disease in many different eukaryotic hosts.

Broth culture
Microorganisms grown in a liquid medium.

Buoyant density
The density possessed by a molecule or particle when suspended in an aqueous salt or sugar solution.

Burst size
The average number of phage released from a single infected bacterium during lytic growth. The burst size depends upon both the phage itself and the growth conditions of the bacteria. The burst size for characterized phage varies from only a few progeny phage per bacterium to over a hundred progeny phage per bacterium.

Bypass suppressor
A second site mutation that activates a new pathway that eliminates the need for the original mutant pathway. For an example of bypass suppression, see the argE suppression of proline biosynthesis mutants.