Question

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).

DNADNA+−

(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.

Answer 1

Answer:

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.
• Based on the data for the group of cells, among the four proteins, protein p16 does not appear to paly ar role in blocking cell cycle entry by fibroblasts.
• The evidence for this is in group 7, where, in the absence of the other proteins the p16 presence has helped the replication of DNA almost as good as the growth in the 10% serum.
• In the other groups (3 to 6) the presence of any other protein has always inhibited the DNA replication compared to the fibroblast in 10% serum.

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.
• Groups 4 to 7 have higher chance of including cells undergoing mitosis as they are showing more DNA replication compared to the group 1 which has 0% serum.
• In this group since it has been cultured for 48 hours in the absence of serum (serum-starvation), they stop dividing and enter the G0 stage of the cell cycle.

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.
• The data here would provide the crucial information on which protein is required to induce the cells to enter the cell cycle.
• To recover from an injury the cells should divide and repair the wound, this will happen through stimulating the cell with the required protein.
• It would also shed light on which protein would inhibit the cells from entry into the cell cycle.

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.
• Cyclin dependent kinases play a major role in regulating cell cycle and progression of the cell from one cell cycle stage to the next.
• In a normal cell, in the presence of inhibitors of cyclin-dependent kinases the cell cycle stops or the progression of the cell cycle is delayed.
• The G1 phase where the cell cycle arrest happens is due to inhibitors (CDI).
• The absence of inhibitors will increase cell proliferation and less number of cell will be seen in the G1 stage.
• So in the absence of inhibitors there will not be cell regulation but uncontrolled cell proliferation which will push the normal cells to behave like cancer cells.
• Inhibitors of cyclin dependent kinases are now being used in the treatment of cancer cells as well, as they will help in arresting the cell cycle and prevent their division and spread to other tissues.

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