In an experiment, yeast cells were cultured identically, and media changed every three days in order to prevent nutrient depletion, separately in two flask, flask A and flask B. After 1 week, you realized that you forgot to change the media in flask B. You noticed that there are less cells in the flask B and that they look smaller. At which checkpoint in the cell cycle did the cells in flask B stop at? Name the checkpoint, and what the cell is checking for.
Please explain!