A student asked us: "What do the pH markers on the figure indicate? I understand that migration of the proteins depends upon the interaction between their charge, and the charges of the anode and cathode, but I fail to understand the purpose of the pH's given."
In an electrophoretic gel molecules migrate based on their charge: those with a positive charge will move towards the cathode, those with a negative charge will move towards the anode. The charge of a protein changes based on the pH, so if you add a pH gradient to a gel then proteins will migrate towards the pH where they have a charge of zero, i.e. their isoelectric point.
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