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Three pathways for intermolecular DNA strand transfer by topoisomerase IB. See the text for a discussion. Topoisomerase I is depicted as an ellipse. The site of transesterification of topoisomerase to one strand of the DNA duplex is denoted by an arrowhead. The incoming DNA acceptor molecules are colored gray. The top strands of the DNAs are oriented 5′ (left) to 3′ (right).
