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 fibroblast

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 non dividing 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).

Explanation:

From the given data of the group of the cells, present between four proteins, i.e. protein p16 which is not being seen to play the role in restricting the cell cycle entry by the fibroblasts. The proof of the same is present in group 7 in which the absence of the different proteins the p16 presence had helped the replication of the DNA is better with respect to growth when used with 10% serum.