During primer extension, the DNA
   polymerase sometimes inserts a ddNTP instead of a dNTP.
<P>
   The  ddNTP  lacks  the  3'-OH  needed  to  attach the next nucleotide,
   terminating the chain.
<P>
   Gel  electrophoresis separates the new fragments that terminate at the
   ddNTP.
<P>
   By  tagging  the  ddNTP  with different fluorescent dyes, the original
   nucleotide sequence can be determined.
<P>
   Sanger sequencing tutorial:  <A ClAsS=MID Href="../../../med/Vid/Klug8e/ch19/19_6_1a.swf" oNClick="popurlt('../../../../med/Vid/Klug8e/ch19/19_6_1a.swf');txtcolor('termination1','gray');return false;" Id=termination1><iMg sRc="/images/shockwave.gif" aLt='' boRDer=0></A>