There is {HEAD ~ no limitation} on {TAIL ~ the type} of other chemical group that can be incorporated on the individual nucleotides 
There is { HEAD ~ no limitation } on { TAIL ~ the type } of other chemical group that can be incorporated on the individual nucleotides
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH