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I am using pet21a as a carrier vector for my gene, with R.E BamHI and XhoI,in reverse primer I haven't added any extra nucleotide sequence so that it should code for the histag towards C-terminal. Am I doing so correctly?

Another question: after transformation to DH5alpha, I am getting only white colony(X gal stock was 20mg/ml about 200ul and IPTG 100mM stock about 40ulon surface of plate)... why I am not getting blue colonies?

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pET21A doesn't encode the lacZ α-peptide necessary for blue-white screening by complementation. Since DH5α has the lacZΔM15 background, only the lacZ ω-peptide is produced and thus β-galactosidase is inactive.

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