I extracted my plasmid with QIAGEN midi kit. Yesterday I measured the concentration and it was decreasing than the first time. Today I also measured to become sure that it is decreasing. And it's true that the concentration decreased than the yesterday's concentration. What could be reason behind it?
Are you using nanodrop machine for checking the concentration....? or manually observing the OD? Md Rashidur Rahman
This sounds strange indeed. Even if your plasmid were to be fully hydrolyzed, the nucleotides would still absorb at 260 nm (absorbance would actually increase). Excluding experimental mistake, the only explanation I can think of for a decreasing A260 is if your preparation were to be contaminated with some chemical that would attack purine or pyrimidine bases, such as bleach (hypochlorite) originating from poorly rinsed glassware.
Can you give us some numbers? Slight changes in measured concentration can be due to the accuracy of your measuring methods.
Are you thoroughly thawing and mixing your samples before measuring? DNA can precipitate out of solution.
Or do you mean that you extracted plasmid from one bacteria culture over two different days and saw a decrease in plasmid yield? That would just mean that your bacteria aren't in log phase anymore.
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