Cell Cycle Regulation
Quality control inspectors typically do not limit their product testing to the final product at the end of the
assembly line. They monitor all aspects of production in hopes of preventing larger problems down the line.
Likewise, when cells are progressing through the cell cycle there are processes in place that check on the cell’s
progress. Is everything happening according to plan? Are there sufficient resources to complete the task of cell
division? Tightly regulating the cell cycle keeps a multicellular organism healthy by conserving materials. This
ensures that new cells receive accurate genetic information, and also prevents uncontrolled growth that may
lead to diseases like cancer.
Model 1 – The Cell Cycle
1. Model 1 displays the four major phases of the cell cycle. Identify each of these phases and briefly
describe the purpose of each phase.
2. There are three regulatory checkpoints built into the cell cycle. Indicate the phase in which each
checkpoint occurs and in which part of the phase (early or later) it occurs.
3. Progression through the cell cycle is dependent on both extra- and intra- cellular conditions. Consider
the following conditions that the cell checks for before proceeding further in its cycle. Indicate which
checkpoint(s) most likely responds to that condition and explain your reasoning.
a. The DNA has been completely replicated and checked for errors.
b. All chromosomes are attached to the spindle fibers.
c. There is adequate space in the environment for more cells.
d. There is a large amount of ATP available.
4. Some cells, like mature nerve or muscle cells, do not divide. Once they have grown to their “adult” size,
they enter into a phase called G0. In this phase, they perform their required functions, but do not grow
and divide.
a. According to Model 1, at what checkpoint would a nerve cell enter into the G0 phase?
b. Propose an explanation for why the cell enters into the G0 phase at the checkpoint identified in
part a, rather than either of the other two checkpoints.
Model 2 – Cyclin and Kinase
5. Using the terms enzyme, substrate, and enzyme-substrate complex, describe the relationship between
Cdk, cyclin, and MPF.
6. Consider the graph in Model 2.
a. Describe the changes in the concentration of cyclin dependent kinase (Cdk) as the cell moves
through different phases of the cell cycle.
b. Describe the changes in the concentration of cyclin as the cell moves through different phases of the
cell cycle.
c. Propose an explanation for the change in the maturation promoting factor (MPF) concentration
throughout the cell cycle based on how the concentration of Cdk and cyclin change.
d. A high concentration of MPF causes the cell to move through a specific checkpoint. Considering the
point in Model 2, which checkpoint of the cell cycle, according to Model 1, is regulated by the
concentration of MPF? Explain your reasoning.
e. Based on your answer to question d, how will a cell behave when it contains a high concentration of
MPF? Explain your reasoning.
7. The cyclin concentration decreases rapidly during a specific phase of the cell cycle. This occurs because
the cyclin molecule is broken down by other enzymes in the cell.
a. Propose an explanation for this change in concentration.
b. Some malfunctioning cells have cyclin always available at high concentrations. What effect
would this have on the cell cycle? Explain your reasoning.