Loading buffer is added at the end of the digestion because it is used as a marker for migration rates and also to give the sample density so that it sinks to the bottom of the well.
Buffers having low concentrations of EDTA (1mM) are often used to protect DNA from nuclease degradation during storage, but EDTA can interfere with restriction enzyme digestion if the final concentration in the reaction is too high.
Thus, it is used as a marker for migration rates and also to give the sample density so that it sinks to the bottom.
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