Post DNA precipitation while extraction it from bacteria, prior to dissolving the DNA in TE buffer, we give 70% ethanol wash. What happens if the pellet is left in the 70% ethanol for some duration
The DNA will remain precipitated and unharmed. You can keep it in 70% EtOH as long as you wish. Just spin it down again before removing the liquid.
During the prior step, precipitation, you typically add 2.2 volumes of EtOH to the aqueous solution containing DNA. This results in ~70% EtOH solution (68.75 % to be precise) that precipitates DNA from solution as DNA has to compete with EtOH for hydrogen bonds to water (to keep it soluble) and suddenly there are not enough of them. You then spin DNA down, remove the solution, keep the pellet, and add 70% EtOH - virtually the same concentration as in the previous step. So, the addition of 70% EtOH keeps DNA unsoluble and washes away leftowers from from the previous step (e. g. salts).
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