Researchers investigated the role of p53 in cell cycle regulation by genetically engineering cell lines with disrupted alleles of the p53 gene. They exposed the cell lines to DNA-damaging radiation and observed how this exposure impacted mitosis.
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Alleles of the p53 gene were selectively deleted in a line of human cells and then monitored after exposure to DNA-damaging gamma (γ) radiation to determine what proportion of the cells entered mitosis (cell division). The shaded squares represent cells with two normal alleles of the p53 gene. The half-shaded squares represent cells with one normal and one disrupted allele of the p53 gene. The unshaded squares represent cells in which both alleles of the p53 gene were disrupted. The mitotic index is the proportion of cells undergoing mitosis at a given time.
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Figure 2C from:
Bunz, F. et al. "Requirement for p53 and p21 to sustain G2 Arrest after DNA damage." Science 282, 5393 (1998): 1497–1501.