Question

Why is a defect in the cell-cycle check point that prevents a cell from entering the s phase least likely to result in the inhibition of a cancerous tumor?

Answer 1

Checkpoints are important to prevent damaged cells to operate. The checkpoint between G1 to S phase is monitored by proteins encoded from tumor suppressor genes. An example is p53 and Rb protein (Retinoblastoma protein). In this checkpoint, the correctness of the cell size, amount of nutrient and growth factors, and integrity of DNA is checked thoroughly.

Mutation in p53 and Rb proteins may lead to cancer development. But these mutations are recessive, so for cancer to occur, the cell should possess both defective alleles.

ANSWER: A defective checkpoint protein before entering the S phase may lead to the inability to check thoroughly the correctness of the cell size, the amount of nutrient and growth factors needed, and the integrity of DNA. But for cancer to occur, the cell should possess mutations in both alleles for cell cycle regulation.