Question

The picture below represents a cell during metaphase of mitosis. The chromosomes are represented in red and the mitotic spindle is represented in green. BO ! A. The cell is stuck in metaphase because a checkpoint has been activated. Why was the checkpoint triggered? B. What could have happened to this cell without this checkpoint? There are three main checkpoints during the cell cycle. One checkpoint occurs at the end of G1 phase and involves the protein p53. If p53 is activated, the active form of p53 blocks D cyclin/Cdk activity. C. What event in the cell activates p53 protein? D. Why is this checkpoint important? What does this checkpoint prevent? E. If p53 protein was active in a cell, would you expect the cell to divide or not? F. Tumor cells are fast dividing cells. Would you expect tumor cells to have inactive or active p53 protein? G. Based on your answer to (F), explain why p53 is classified as a tumor suppressor protein.

Answer 1

Ans (A): Checkpoint is triggered because one chromosome is not arranged on the metaphase plate. This checkpoint will block the entry of the cell in the anaphase.

Ans (B): Without this checkpoint, daughter cells will have unequal numbers of chromosomes. And thus the equal distribution of chromosomes will not be possible.

Ans(C): p53 protein is activated in case of DNA damage.

Ans (D): This checkpoint allows the cell to decide whether to divide or not. This checkpoint allows the cell to repair the damage hence it is important.

It prevents the entry of cells in the S phase with damaged DNA.

Ans (E): No, the cell will not divide. The cell will try to repair the damage. If damage can't be repaired the cell will undergo apoptosis.

Ans (F): The tumor cells have inactive p53 protein.

Ans (G):

• After synthesis, p53 protein binds to the DNA.
• This binding stimulates the synthesis of p21 protein.
• This p21 protein interacts with cdk2 (cyclin-dependent kinase).
• This complex blocks the entry of the cell in the next phase.
• In the case of inactive p53, p21 protein will no longer be available to stop the cell division.
• Thus uncontrolled cell division will cause tumors.
• Hence p53 is classified as the tumor suppressor gene.