It strongly depends on the cells or cell lines which you are working on. In my expierence, you can observe p53/p21 induction starting from 4h (in MCF7 for example) and 8-12h (HepG2 or HCT116). Some cell lines go into apoptosis after 24/48h, so too long incubation post-treatment is not always beneficial. If you don't know, how your cells behave, do a time-course analysis (3h, 6h, 9h, 12h, 24h, 48h, 72h). You can use Nutlin-3 as a positive control, which leads to increased p53.

By the way, p53 is stabilized on the protein level, not on the mRNA (at least as an immediate response). This is different for p21, which is up-regulated in the transkriptional level (induced by p53).

If you are looking for some pictures from the literature, search for "p21 induction dna damage" in Google "pictures".

Thanks Ricahrd and Eugen for replying. I think doing a timeline experiment as you have suggested wud be of great help. Ok Thanks

Camptothecin is a topo I inhibitor, leading to regid binding to DNA and resulting in DNA breaks. In my opinion you can see first apoptosis induction after some hours, thus protein expression should be tested as soon as possible, since these reaction may be a result of indirect interaction (e.g. induced by apoptotic processes or other effects).

Alright guys. I found that at protein level i can see good upregulation in p53 and p21 levels after about 6 hrs. After 12 hrs they started to diminish as compared to the less treatment time. For mRNA levels i could not get the same results.

What a pity!