9. Which one of the following best defines an oncogene?
   (1) An oncogene never codes for a cell cycle protein, which promotes cell proliferation.
   (2) Oncogenes are always involved in inherited forms of cancer.
   (3) An oncogene codes for a protein that prevents a cell from undergoing apoptosis.
   (4) An oncogene is a dominantly expressed mutated gene that renders a cell advantageous towards survival.

Final Answer

The correct statement defining an oncogene is:

(4) An oncogene is a dominantly expressed mutated gene that renders a cell advantageous towards survival.

Explanation:

• An oncogene is a mutated or abnormally expressed form of a proto-oncogene, a normal gene involved in regulating cell growth, division, and survival. When mutated, oncogenes gain functions that allow cells to proliferate uncontrollably and evade normal cell death (apoptosis), which contributes to cancer formation.​

• Oncogenes act in a dominant fashion, meaning a mutation in a single allele can promote tumorigenesis by pushing cells towards survival and proliferation advantages.

• The statement that oncogenes never code for cell cycle proteins (Option 1) is incorrect; many oncogenes encode proteins regulating the cell cycle and proliferation.

• Oncogenes are not always involved in inherited cancers (Option 2). Most oncogene mutations arise sporadically during a person’s lifetime rather than being inherited.

• While oncogenes promote survival, the assertion that they code specifically for proteins that prevent apoptosis (Option 3) is a partial view; oncogenes can affect multiple cellular processes, including proliferation, survival, and growth signaling, not exclusively apoptosis suppression.

Summary

Oncogenes are dominant gain-of-function mutant genes that confer a survival and proliferative advantage to cells, playing a critical role in cancer development. Understanding oncogenes helps in developing targeted therapies aimed at these mutated genes or their protein products.