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1. A method of synthesizing a plurality of polynucleotides having predetermined sequences, the method comprising the steps of: (a) providing a spatially addressable array of reaction sites, wherein each reaction site is operationally associated with at least one working electrode and has disposed thereon initiators attached by their 5′-ends and having a 3′-O-electrochemically labile protecting group; (b) performing for each kind of nucleotide a cycle of (i) deprotecting initiators or elongated fragments at electrodes at predetermined addresses by generating a voltage difference between each of the electrodes at the predetermined addresses and a reference electrode so that the electrochemically labile protecting group is cleaved, thereby generating free 3′-hydroxyls on the initiators or elongated fragments at the electrodes of the predetermined addresses, (ii) contacting under elongation conditions the electrodes with a 3′-O-electrochemically labile-protected nucleoside triphosphate and a template-independent DNA polymerase so that the initiators or elongated fragments at the predetermined addresses are elongated by the incorporation of a 3′-electrochemically labile-protected nucleoside triphosphate to form 3′-O-electrochemically labile-protected elongated fragments; and (c) repeating step (b) until the array of polynucleotides of predetermined sequences is completed, wherein said electrochemically labile protecting group is an amino group and wherein said spatially addressable array of reaction sites is integrated in a semiconductor device.
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