Thank you for using the Jiskha Homework Help Forum. If you do a SEARCH (to the upper right) look for another bio today because I posted sites explaining that earlier this afternoon.
P.S. I could not find that post so here is some information:
2. (tutorial): http://www.biology.arizona.edu/Cell_BIO/tutorials/pev/main.html
3. (part II of #2): http://www.biology.arizona.edu/Cell_BIO/tutorials/pev/page2.html