Explain its normal role and why scientists would regard it as the "guardian of the genome."

The gene p53, also called the guardian of the genome is located on the 17th chromosome in human genomes.
It was given this alternative name because it has a role in response to damage of DNA molecule and in preventing cancer.
The protein product of this gene stops the replication of damaged DNA, activates proteins that repair the DNA damage and if the reparation proteins fail to fix the damage it activates the process of apoptosis or so-called cell death in order to prevent the damaged cell to proliferate and potentially develop into a tumor.