13. You may draw the following conclusions (true or false):

A. Following substance X treatment cyclin E levels are reduced. Since CDK2 levels are also reduced the stichiometry of the complex is not altered. False CDK2 levels are not altered, thus the stichiometry of the complex is significantly changed. The absence of cyclin E from the CDK2 containing immunocomplex may explain the previously observed reduction in CDK2 kinase activity following exposure to substance X.

B. Following substance X treatment p27 levels in CDK2 immunocomplexes are slightly reduced. True however this reduction per-se cannot explain the  previously observed reduction in CDK2 kinase activity following exposure to substance X. Since p27 binds to CDK-cyclin complexes, it is possible that  the reduction in cyclin E in the CDK2 immunocomplexes leads to a econdary reduction in p27 levels in these immunocomplexes.

C. Following substance X treatment p21 levels are not changed whereas cyclin E levels are significantly reduced. That means that there is more p21 in each complex, and this causes the observed reduction in CDK2 kinase activity. False the observation is correct but the explanation is wrong. Since this immunocomplex was obtained using anti CDK2 antibodies, then CDK2 levels should serve as control. There is no change in CDK2 levels and in p21 levels following exposure to substance X. So increase in p21 levels cannot explain the previously observed reduction in CDK2 kinase activity following exposure to substance X. It is possible that a very small portion of CDK2 already binds cyclin A at this stage. p21 is bound to this complex , and substance X does not affect its stability.