Cellular  response  to  a signal can include
   both cytoplasmic and nuclear activity.
<P>
<BR><LI> Multistep pathways may exhibit signal amplification.
<BR><LI> Some responses involve turning genes on or off in the nucleus.
<P>
   Review:  <A ClAsS=ACT Href="http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?11&amp;D" oNClick="popact('http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/activities/load.html?11&amp;D');txtcolor('response3','gray');return false;" Id=response3><iMg sRc="/images/tutorial.gif" aLt='' boRDer=0></A>