All your cells contain proto-oncogenes, which can change into cancer-causing genes. Why do cells possess such potential time bombs?

The given question is incomplete. The complete question is:

Question: All your cells contain proto-oncogenes, which can change into cancer-causing genes. Why do cells possess such potential time bombs?

A. Cells produce proto-oncogenes as a by-product of mitosis.

B. Proto-oncogenes are necessary for the normal control of cell growth and division.

C. Proto-oncogenes protect cells from infection by cancer-causing viruses.

D. Proto-oncogenes are unavoidable environmental carcinogens.

E. Proto-oncogenes are genetic junk that has not yet been eliminated by natural selection.

Answer:

B. Proto-oncogenes are necessary for the normal control of cell growth and division.

Explanation:

The process of cell division is under tight regulation by a family of extracellular growth factors, specific proteins that trigger cell division and the tumor suppressor genes that normally inhibit cell division. Defects in these genes can lead to cancer.

For example, the protein encoded by tumor suppressor gene p53 stimulates the synthesis of p21 which in turn inhibits the cyclin-cdk2 to block entry of G1 cells into the S phase until the DNA is repaired. Mutations in tumor suppressor genes retinoblastoma and p53 allow cells to proceed to the S phase irrespective of DNA repair.

Likewise, a mutation in ras proto-oncogene encodes a protein with no GTPase activity. The mutant Ras protein is always in its activated form, regardless of the signals from normal receptors. This makes the cells to divide in an uncontrolled manner.