B

Back Cross:

It is another term for a test-cross.

Back Mutation:

Reverse the effect of a point or frame-shift mutation that had altered a gene; thus it restores the wild-type phenotype.

Bacterial Artificial Chromosome (BAC):

A chromosome-like structure, constructed by genetic engineering. BAC is a cloning vector capable of carrying between 100 and 300 kilobases of target sequence. They are propagated as a mini-chromosome in a bacterial host. The size of the typical BAC is ideal for use as an intermediate in large-scale genome sequencing projects. Entire genomes can be cloned into BAC libraries, and entire BAC clones can be shotgun-sequenced fairly rapidly.

Bacteriophage:

(simply phage) A virus that infects a bacterium and which is often used in molecular genetics experiments as a vector, or cloning vehicle. Recombinant phages can be made in which certain non-essential l DNA is removed and replaced with the DNA of interest. The phage can accommodate a DNA "insert" of about 15-20 kb. Replication of that virus will thus replicate the investigator's DNA.

Balbiani Ring:

It is an extremely large puff at a band of a polytene chromosome.

Band:

A pattern of light and dark regions by Giemsa staining that can serve as landmarks on chromosomes.

Base:

In molecular biology, this term refers to the purine bases adenine and guanine, and the pyrimidine bases uracil, thymine, and cytosine, or modification of these bases.

Baseline:

The initial cycles of PCR during which there is little change in fluorescence signal (usually cycles 3 to 15).

Baseline value: During PCR, changing reaction conditions and environment can influence fluorescence. In general, the level of fluorescence in any one well corresponds to the amount of target present. Fluorescence levels may fluctuate due to changes in the reaction medium creating a background signal. The background signal is most evident during the initial cycles of PCR prior to significant accumulation of the target amplicon. During these early PCR cycles, the background signal in all wells is used to determine the ‘baseline fluorescence’ across the entire reaction plate. The goal of data analysis is to determine when target amplification is sufficiently above the background signal, facilitating more accurate measurement of fluorescence.

Base Pair (bp):

One pair of complementary nucleotides within a duplex strand of a nucleic acid. Under Watson-Crick rules, these pairs consist of one pyrimidine and one purine: i.e., C-G, A-T (DNA) or A-U (RNA).

Base Sequence:

The order of nucleotide bases in a DNA molecule.

Base Sequence Analysis:

A method, sometimes automated, for determining the base sequence.

B-DNA:

Conformation of the Watson-Crick double helix in which the two strands form a right-handed double helix.

Bidirectional Replication:

It is accomplished when two replication forks move away from the same origin in different directions.

Binding Site:

A place on cellular DNA to which a protein (such as a transcription factor) can bind.

Biotin:

A coenzyme which is essential for carboxylation reactions.

Bioassay:

The determination of the activity or concentration of a chemical by its effect on the growth of an organism under experimental conditions.

Bivalent:

It is the structure containing all the four chromatids at the start of meiosis.

Biolistics:

In molecular biology, a method developed to inject DNA into cells by mixing the DNA with small metal particles and then firing the particles into the host cell at very high sped.

Blotting:

A technique for detecting one RNA within a mixture of RNAs (a Northern blot) or one type of DNA within of a mixture of DNAs (a Southern blot). Blotting involves gel electrophoresis, transfer to a blotting membrane (typically nitrocellulose or activated nylon), and incubating with a radioactive probe. Exposing the membrane to X-ray film produces darkening at a spot correlating with the position of the DNA or RNA of interest. The darker the spot, the more nucleic acid was present there.

Blunt End:

A terminus of a duplex DNA molecule which ends precisely at a base pair, with no overhang in either strand. Some but not all restriction endonucleases leave blunt ends after cleaving DNA. Blunt-ended DNA can be ligated nonspecifically to other blunt-ended DNA molecules.

Blunt End Ligation:

It is a reaction that joins two DNA duplex molecules directly at their ends.

Box:

Refers to a short nucleic acid consensus sequence or motif that is universal within kingdoms of organisms. Examples of DNA boxes are the Pribow box (TATAAT) for RNA polymerase, the Hogness box (TATA) that has a similar function in eukaryotic organisms, and the homeo box.

Branch Migration:

It descibes the ability of a DNA strand partially paired with its complement in a duplex to extend its pairing by displacing the resident strand with which it is homologous.