(a) Based on the given 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.

The correct answer is "p18, p21 and p27".

Explanation:

In this experiment, four proteins were blocked and the level of DNA replication was tested in different cell cultures to see which proteins may play a role in cell cycle. Based on the given data we can conclude that p16 is the only protein that appear to have a pivotal role in cell cycle, while p18, p21 and p27 apparently does not. We can conclude this by observing group 7 of cells, were cells have only p16 active and were able to have around 80% of DNA replication.

The proteins tested that do not appear to play a role in blocking cell cycle entry by fibroblasts are ;  P18,  P21, and P27

Based on the data provided It can seen/observed that protein P16 has a pivotal role in the cell cycle, while other proteins ( P18, P21 and P27 ) do not hold any pivotal role.

The pivotal role of P16 is evident by observing group 7 of the cells, because the cells in group 7 contain only P16 active and they  also posses  around 80% of DNA replication.

Hence we can conclude that the proteins tested that do not appear to play a role in blocking cell cycle entry by fibroblasts are ; P18 , P21 and P27

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