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Bacterial artificial chromosome (BAC): Cloning vector that can incorporate large fragments of DNA. (see YACS)
Bacteriophage: A virus that infects bacteria. The bacteriophage DNA has served as a basis for cloning vectors, and is also utilized to create phage libraries containing human or other genes.
Baculovirus: An insect virus which forms the basis of a protein expression system
Base pair: A pair of nitrogenous bases (a purine and a pyrimidine), held together by hydrogen bonds, that form the core of DNA and RNA i.e the A:T, G:C and A:U interactions.
Beta sheet: A three dimensional arrangement taken up by polypeptide chains that consists of alternating strands linked by hydrogen bonds. The alternating strands together form a sheet that is frequently twisted. One of the secondary structural elements characteristic of proteins.
Bivalent: Having two binding sites; having 2 free electrons available for binding.
Blunt-end (ligation): The joining of DNA fragments that contain no overhang at either end and consequently no DNA bases available for hybridization (cf. sticky-end ligation).
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