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