A method of preparing an immobilized oligonucleotide having a free 3'-end comprises the steps of: i) preparing an oligonucleotide attached in a first position to a solid support via its 3'-end and having a free 5'-end; ii) binding said oligonucleotide in a second position remote from the 3'-end to the solid support; and iii) selectively releasing the 3'-end of the oligonucleotide from the solid support to obtain the oligonucleotide attached to the support in said second position in a reversed orientation with a free 3'-end.
