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