You have ligated a foreign DNA fragment into the EcoRI site in the multiple cloning site of pUC18. You then transform host E. coli with the ligation mixture. Some of the bacterial colonies growing on the nutrient agar plate that contains ampicillin and X-gal are white and some are blue. Bacterial colonies that are blue

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Bacterial colonies that are blue contain nonrecombinant plasmid DNA. This is because the insertion of foreign DNA into the multiple cloning site of pUC18 disrupts the lacZ gene, preventing the production of β-galactosidase, which is necessary for the hydrolysis of X-gal into a blue product. Therefore, colonies that are blue have not had the lacZ gene disrupted, indicating they contain nonrecombinant plasmid DNA.

You Have Ligated a Foreign DNA Fragment into the EcoRI