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