Bacteriophage A virus that infects bacteria. Also called phage. Altered forms are useful as cloning vectors. These include vectors derived from wild type bacteriophage lambda (λ) and M 13. Lambda phages are capable of cloning DNA segments in the size range of 10-20 kb. These are lytic phages, and phage infection results in clear areas or plaques in a bacterial lawn of cells. Some lambda vectors have also been developed as expression vectors.

M13, a single stranded DNA phage can grow within a bacterium without destroying the cell but induces continuous phage multiplication in the host. It is useful as a cloning vector and for the Sanger dideoxy DNA  sequencing method. Both phage λ and M13 grow on E.coli as a host bacterium.

Bacteriophage lambda A DNA virus that infects E.coli and enters either into a lytic cycle or the lysogenic pathway. If the virus enters the lytic pathway, it forms progeny viral particles that are released by host cell lysis. Lysogeny involves integration of the viral genome into the host genome. Derivatives of phage lambda are used as cloning vectors.

Bacteriophage Qβ a single –stranded RNA phage.

Bacteriophage T7 A DNA virus with a strong promoter for T& RNA polymerase.

Bacteriophage fX174 A single – stranded DNA virus used to study DNA replication.

Bacteriophage μ A DNA virus capable of transposition or random insertion of DNA into the host genome. This virus is used for insertional mutagenesis.

Bacteriorhodopsin A pigmented protein present in the plasma membrane of Halobacterium halobium, a salt-loving  bacterium that pumps protons out of the cell in response to light.

Bacteriostat A substance that inhibits or slows down the growth and reproduction of bacteria.

Bacteriotropin An immune opsonins that stimulates Phagocytosis of a bacterium, other cell type or particle.

Bacterium A common name for the class Schizomycetes. These are minute (0.5-5μm), unicellular prokaryotic organisms that do not have a distinct nucleus. They can be identified by Gram staining. They are classified based on their oxygen requirement (aerobic and anaerobic) and shape (rodlike = bascillus; spherical = coccus; spiral = spirillum; comma shaped  = vibrio; corkscrew –shaped = spirochete; filamentous). Most bacteria reproduce asexually, by simple cell division, while a few undergo a type of sexual reproduction, called conjugation. Some bacteria such as green-blue cyanobacteria can photosynthesise. While some bacteria are saprophytes, others are parasites and can cause diseases. Bacteria are major agents of fermentation, putrefaction and decay, and a frequent source of contamination in tissue culture.

Bacteroid A group of anaerobic, Gram – negative, rod-shaped bacteria.

Baculovirus A class of insect viruses that infects arthropods including insects. It is used as a cloning vector for the expression of genes in eukaryotic cells. Baculoviruses contain a gene that is overexpressed late in their infection cycle, which fills the nucleus of the cell with many-sided bodies full of a protein. This protein is required for the spread of the virus in the wild. The gene for this protein is replaced in a vector cloning system by the gene to be cloned. Production  of the protein can be up to 50% of the cell’s protein content, and several proteins can be produced simultaneously. This facilitates production of multisubunit enzymes. Baculoviruses can be used to produce proteins that are glycosylated, which render them useful in producing biopharmaceuticals. In addition, baculoviruses are noninfective and nonpathogenic to vertebrates.

Bad Bcl-xL /Bcl-2 associated death promoter. It is a heterodimeric partner   for Bcl-xL and Bcl -2 that displaces Bax and promotes cell death.

Baffles Structures on the bottom of some culture flasks that increase aeration.

Bait A protein fused with a transcription factor which interacts with a prey fused with a related transcription factor. Interaction of the bait and prey proteins results in fusion of the two factors to these two proteins which are brought into proximity with each other. As a result a specific signal is produced, indicating an interaction has taken place. This is the basis of the yeast –two hybrid system.

Baker’s yeast A strain of Saccharomyces cerevisiae used for making bread.

Bal 31 nuclease An exonuclease that degrades both strands of a DNA duplex simultaneously.

Balanced lethal system A system that maintains a recessive lethal allele at each of two loci on the same pair of chromosomes. In a closed population where there is no crossing-over between the loci, only double heterozygotes for the lethal mutations survive.

Balanced polymorphism Two or more types of individuals present in the same breeding population.

Balbiani ring Extremely large puff at a band of a polytene chromosome.

Baltimore classification of viruses A universal classification of viruses devised by David Baltimore in 1971 based only on replication strategy and ignoring morphological properties and host range.

Baltimore, David (b. 1938) A molecular biologist who won the Nobel Prize for the discovery of reverse transcriptase in retroviruses together with Howard Temin.