<prompt> If DNA is damaged in the cell cycle, which gene causes cell cycle arrest?RbMYCp53K-RAS  <response>: Ans. c p53Ref: Robbins and Cotran Pathologic Basic Disease 9th Ed; Page No-28P53 Gene in chromosome 17p 13.1 It is called as genome guardian protein. Encodes a53Ka protein p53 is universally expressed in all cells. p53 induces p21, which inhibits CDKs- hypo-phosphorylation activation ofRb- inhibition of G1 -S progression. It regulates cell cycle progressions, DNA repair, cellular senescence and apoptosis.Note The suppressor gene p53 apoptosis by depositing CD95 on the cell membrane surface. Mutations in tumor suppressor genes can result in unrestrained cell division eg, Li-Fraumeni syndrome.Extra Mile Mutated p53 protein inactivation promotes tumor development. Its gene product is broken down more slowly than normal protein, leading to intranuclear accumulation of p53 protein.How to rule out other options? Rb gene: Tumor suppressive pocket protein that binds E2F transcription factors in its hyperphosphorylated state. It prevents G1S transition. RAS oncogene It is a signal transduction protein. Most common oncogene mutation involved in human cancer 15-20 of human cancer. Normal RAS proteins alternate between activated GTP- bound signal transmitting and inactive GDP-bound forms. When the cells are stimulated by growth factors RAS becomes activated by exchanging GDP to GTP Activated RAS binds to farnesyl transferase to activate MAP kinase for induction of mitogenesis. The mutant RAS activates the mitogen-activated protein MAP kinase pathway leading to cell proliferation. MYC It is most common nuclear transcription factor affected in human tumors. MYC up regulates expression of telomerase. It reprogram somatic cell into pluripotent stem cells and contribute to cancer cell sternness. 