effecting {HEAD ~ hydrogen bonding} between {TAIL ~ mutually complementary portions} of the first and second series fragments to form a double stranded structure; and 
between
effecting { HEAD ~ hydrogen bonding } between { TAIL ~ mutually complementary portions } of the first and second series fragments to form a double stranded structure ; and
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH