Alternatively, {HEAD ~ the second primer sequence} may be based upon {TAIL ~ sequences} derived from the cloning vector 
Alternatively , { HEAD ~ the second primer sequence } may be based upon { TAIL ~ sequences } derived from the cloning vector
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH