1-2.2.8Screening clones having desired DNA inserts with the help of biological properties:

• After selection of colonies having the vector, the next step is to screen the colonies having the recombinant vector (vector containing foreign DNA insert).
  
  
• Bacterial cloning vectors (e.g. pUC19, pGEM vectors) use the blue-white screening system based on lacZ system to distinguish transgenic cells from those that contain the parental vector (i.e. vector DNA with no recombinant sequence inserted). The recombinant colonies are grown in presence of X-gal.
  
  
• In these vectors, foreign DNA is inserted into a sequence that encodes an essential part of beta-galactosidase (an enzyme which cleaves galactose). Its activity results in formation of a blue-colour colony on the culture medium.
  
  
• Insertion of the foreign DNA into the beta-galactosidase coding sequence disrupts the function of the enzyme, and colonies containing recombinant plasmids give no blue colour (white).
  
  
• Using this colour phenotype, transgenic bacterial clones can be easily identified from those that do not contain recombinant DNA.
  
  
  

Fig 1-2.2.8: Screening for clones with desired DNA inserts