Suppose you are a director of a ballet theater. After reading a book about the cell cycle, you are intrigued by the similarity between your night show and the cell cycle. Each night the theater fills, the lights go out, the curtain rises, the dancers perform, the curtain falls, the lights go on, and the spectators leave, just like the cell cycle consisting of a defined sequence of events. So you decided to improve theater operation by incorporating checkpoint mechanisms similar to the cell cycle control system. First, you designed a system that raises the curtains automatically when all the seats are filled. In each seat, sensors were installed to detect a seated person. But you have not decided how to connect the sensors to the curtains.

a) You can set the sensors to send out a positive signal that adds up to be enough to raise the curtain or set each sensor to send out a negative signal that will stop the curtain from going up until every seat is filled. Which system do you think a cell would use for cell cycle control?

b) Compare advantages and disadvantages of the two modes and explain your answer. (Your shows are so popular that all the seats are filled in each show. You don't have to worry about a block in the system due to empty seats.)