Systems and methods are provided for labelling nucleic acids in a cell of a cell culture or of a tissue. The systems and methods include contacting the cell culture or tissue with a first label and applying an electrical field to the cell, thereby increasing the permeability of the cell membrane, allowing the first label to be introduced into the cell. The cell culture or tissue can be disposed on a multielectrode array, and the electrical field can be applied by operating one or more electrodes of the array that are proximate to (e.g., beneath) the cell in order to control the location of the cell whose permeability is increased and/or to control the timing of such permeability increase.