[Chimera-users] ribbon problem

Dear all, I'd like to use chimera to do a picture of a circular DNA with ribbon representation. I found the two following problems: 1.If I load the .pdb files, chimera is unable to recognize the topology and shows extra-bond connecting the last base of a strand with the previous of the other strand. 2.If I load the AMBER topology, the connectivity is correctly recognized, but the first base and last are not connected by the ribbon. Is there any solution?