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11 months ago in Molecular Biology By Shabir Ahmed

How is plasmid DNA measured for a molecular biology experiment?

When prepping for a critical experiment like transfection or cloning, I need to ensure my plasmid DNA is both quantitatively sufficient and qualitatively pure. Standard spectrophotometry can be misleading due to contaminant absorption. I'm seeking the current best-practice methodologies that experienced labs rely on to get accurate, reproducible measurements before proceeding.

 

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By Joshna Answered 3 months ago

In my lab, we always cross-verify measurements. While a Nanodrop spectrophotometer gives a quick A260/A280 ratio for purity, I have seen it skewed by residual phenol or RNA. For any quantitative downstream application, I would recommend using a fluorescence-based assay like Qubit. It's specific for double-stranded DNA and is unaffected by common contaminants. Always run an agarose gel, too it visually confirms the plasmid's integrity and supercoiled state, which spectrophotometry cannot tell you. This two-pronged approach prevents experimental failure.

 

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