Blue-White Screening (Alpha-Complementation)
Definition
A colony selection method for pUC-family vectors in which intact lacZ-alpha produces functional beta-galactosidase that cleaves X-gal to a blue indigo product. Insertion of foreign DNA into the MCS disrupts the reading frame, giving white colonies.
- Vector family
- pUC-family plasmid vectors
- Reporter gene
- lacZ-alpha
- Substrate
- X-gal cleaved to a blue indigo product
- Positive clone colour
- White (insert disrupts lacZ-alpha reading frame)
Common questions
Why does a successful insertion produce white colonies instead of blue?+
The multiple cloning site sits inside the lacZ-alpha coding sequence. When foreign DNA is ligated into that site, it disrupts the reading frame of lacZ-alpha, so functional beta-galactosidase is not produced, X-gal is not cleaved, and the colony stays white instead of turning blue.
Does a white colony guarantee the intended insert is present?+
No. A white colony only shows that lacZ-alpha has been disrupted, which can also happen from an unrelated deletion, a frameshift mutation, or a non-target DNA fragment ligating into the site. Screening by colony PCR or restriction digest is still needed to confirm the correct insert is actually present.
Related terms
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- Multiple Cloning Site (MCS)
- A short stretch of synthetic DNA containing a cluster of unique restriction sites, engineered into cloning vectors such as pUC19 and pBlueScript...
- Plasmid
- A small, circular, double-stranded DNA molecule that replicates autonomously inside a bacterial cell. Used as a cloning vector when equipped with a...
- RFLP (Restriction Fragment Length Polymorphism)
- The first DNA profiling method used in forensic casework, developed by Alec Jeffreys in 1984. Restriction enzymes cut DNA at specific sequences,...
- Sticky End (Cohesive End)
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- Type II Restriction Endonuclease
- A bacterial enzyme that cleaves double-stranded DNA at a specific palindromic recognition sequence, generating fragments whose ends reflect the exact geometry of...