Read each question carefully. Write your response in the space provided for each part of each question. Answers must be written out in paragraph form. Outlines, bulleted lists, or diagrams alone are not acceptable and will not be scored.

Scientists used cells called fibroblasts to study factors that cause nondividing cells in the G0 stage of the cell cycle to reenter the cell cycle and undergo mitosis. In a laboratory, fibroblasts typically grow and divide when they are cultured in a growth medium that contains 10% serum (the liquid part of blood that remains after blood clots). If fibroblasts are cultured for 48 hours in the absence of serum (serum-starvation), they stop dividing and enter the G0 stage of the cell cycle.

The scientists divided serum-starved fibroblasts into groups to which they added no serum, 10% serum, or 0.1% serum. Each of the groups with 0.1% serum was also treated to block the production of only one or combinations of four proteins (p16, p18, p21, and p27) that are inhibitors of cyclin-dependent kinases. The scientists grew the fibroblasts for 48 hours and then measured the amount of DNA replication taking place to determine whether the cells had reentered the cell cycle. The scientists compared each group of cells to the group cultured with 10% serum and plotted the data (Figure 1).


(a) Based on the data for the groups of cells cultured with 0.1% serum (groups 3 to 7), identify which of the four proteins tested do(does) not appear to play a role in blocking cell cycle entry by fibroblasts.

Question 2

(b) Based on the data for the groups of cells cultured with 0.1% serum (groups 3 to 7), identify the groups(s) most likely to include cells undergoing mitosis.

Question 3

(c) Neurons (nerve cells) of the central nervous system (brain and spinal cord) are in G0 of the cell cycle and generally do not divide in response to an injury. The scientists predict that they can use the data from this experiment to help them treat individuals who suffer from spinal cord injuries. Provide reasoning to support the scientists’ prediction.

Question 4

(d) Interactions between cyclins and cyclin-dependent kinases control the cell cycle. Explain how the presence or absence of inhibitors of cyclin-dependent kinases might play a role in normal cells becoming cancer cells.

Public Answer

RSADU4 The First Answerer