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blue/white colonies - blue colonies turned white after over-incubation (Mar/31/2005 )

Hi All,
i did some cloning stuff and got a couple white colonies on LB+Amp+IPTG+X-Gal plate, i picked two and got plasmid, however when i left them at 37 C one more day, all my white colonies were blue, even the ones i picked, could someone explain this, did they lose insert overtime?

One more problem, i did some ligation reactions which included vector only and vector+insert, after transformation, i got all white colonies for both these reactions, could someone help me with this ?

thanx a lot


When working with Amp, you should always pay attention to the size of colonies, and where on the plate you find them - in addition to the blue or white criteria, especially if the plates have been incubatedmore than 16-20 hours.
Small white colonies relatively close to a blue colony are very often satelites.
Tricky part is that inserts sometimes (but not at all always) slows down growth, leading to smaller colonies also for the white colonies we really want.

If you have a choice - pick the whites that is not very close to a blue colony.