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