testing the mutagenic potential of a nucleoside analog, thereby determining whether said {TAIL ~ analog} causes {HEAD ~ incorrect base} pairing 
testing the mutagenic potential of a nucleoside analog , thereby determining whether said { TAIL ~ analog } causes { HEAD ~ incorrect base } pairing
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH