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