I’m typically not a fan of McGraw-Hill’s animations because they are rather dry and un-engaging, however, this one does a good job of describing conditions under which the lac operon will be either ‘on’ or ‘off’ and illustrating how positive and negative regulatory elements work together so that the operon is on when needed, but conserves energy by remaining off when it is not needed.

http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html

more on this later….