Question

Cellular growth and repair require a cell to go through the cell cycle. Like all biological functions, cellular growth and repair is intricately controlled through a series of checkpoints involving specific signaling molecules.

a. What are the checkpoints in the cell cycle? What do they do?

b. What is the basic mechanism of cell signaling that controls the checkpoints? Be specific!!

c. How is cancer related to these checkpoints and another inhibitory mechanism that stops normal cell growth?

Answer 1

Answer a):

There are three checkpoints: G1, G2, and metaphase. Each checkpoint has different significance. The G1 checkpoint prevents the coping of damaged parts of DNA during the DNA duplication. The G2 checkpoint prevents the cell by checking the presence of double stranded breaks in DNAs. In metaphase checkpoint, cell checks that all chromosomes should align in the spindle fiber correctly.

Answer b):

The mechanism behind this phenomenon is the presence of p53 gene and its protein. Under normal DNA the p53 gene is not active and hence it does not arrest in G1 phase, while in G1/G2 phase, if abnormality occurs the p53 protein causes binding of cyclin-kinase inhibitor to the DNA and hence the cell cycle gets arrested in G1 or G2 phase. The metaphase checkpoint occurs if anaphase-promoting complex is inhibited, the cell will be arrested in this phase.

Answer c):

Mutated p53 protein makes cell no protection by checking abnormalities occurred in DNA replication during G1 and G2 phase. Abnormal cyclin protein causes no protection in G1/G2/M phase.

The e cip/kip and INK4a/ARF family proteins are inhibitors for the cell cycle progression.

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