Question

1. Imagine an experiment where you only compared cdc25ts grown at 26 °C vs. 37 °C and you did not include the wild-type grown at these temperatures. In this experiment you noticed that the cdc25ts cells grown at 37 °C were very long relative to cdc25ts cells grown at 26 °C. Thus, you concluded that cdc25 is important for cell division. Explain why without the wild-type counterpart, you can NOT make this conclusion. State what the wild-type control at these two temperatures allows you to conclude!

Answer 1

Temperature-sensitive mutants in cdc25 i.e. cdc25ts have been shown to arrest at G2⧸M prior to entry into mitosis. The cell cycle timing of entry into mitosis is governed by expression of the cdc25+(wild type) and mitotic inducer genes and the inhibitor genes. The products of these genes operate in a control network that determines cell size at mitosis and thereby coordinates growth with division. The mitotic inducer encoded by cdc25+ plays a key role in this control. In wild-type cells, cdc25+ gene function is required for entry into mitosis, p34cdc2 dephosphorylation, and kinase activation. Incremental increases in cdc25+ gene cause a corresponding decrease in cell size at mitosis, showing that the level of cdc25+ expression is rate-limiting for entry into M phase in wild-type cell.