The loss of p53 function has other repercussions for the cell cycle.
Mutated p53 might lose its ability to trigger p21 production.
Without adequate levels of p21, there is no effective block on Cdk activation. Essentially, without a fully functional p53, the G1 checkpoint is severely compromised and the cell proceeds directly from G1 to S regardless of internal and external conditions.
At the completion of this shortened cell cycle, two daughter cells are produced that have inherited the mutated p53 gene. Given the non-optimal conditions under which the parent cell reproduced, it is likely that the daughter cells will have acquired other mutations in addition to the faulty tumor suppressor gene.
Cells such as these daughter cells quickly accumulate both oncogenes and non-functional tumor suppressor genes.
Again, the result is tumor growth.