10.4A: Proto-oncogenes
- Explain regulation of the cell cycle by proto-oncogenes
Proto-oncogenes
The genes that code for the positive cell cycle regulators are called proto-oncogenes. Proto-oncogenes are normal genes that, when mutated in certain ways, become oncogenes: genes that cause a cell to become cancerous. There are several ways by which a proto-oncogene can be converted into an oncogene. Consider what might happen to the cell cycle in a cell with a recently-acquired oncogene. In most instances, the alteration of the DNA sequence will result in a less functional (or non-functional) protein. The result is detrimental to the cell and will likely prevent the cell from completing the cell cycle; however, the organism is not harmed because the mutation will not be carried forward. If a cell cannot reproduce, the mutation is not propagated and the damage is minimal.
Occasionally, however, a gene mutation causes a change that increases the activity of a positive regulator. For example, a mutation that allows the Cdk gene to be activated without being partnered with cyclin could push the cell cycle past a checkpoint before all of the required conditions are met. If the resulting daughter cells are too damaged to undergo further cell divisions, the mutation would not be propagated and no harm would come to the organism. However, if the atypical daughter cells are able to undergo further cell divisions, subsequent generations of cells will probably accumulate even more mutations, some possibly in additional genes that regulate the cell cycle.
The Cdk gene in the above example is only one of many genes that are considered proto-oncogenes. In addition to the cell cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to override cell cycle checkpoints. An oncogene is any gene that, when altered, leads to an increase in the rate of cell cycle progression.
Key Points
- Proto- oncogenes positively regulate the cell cycle.
- Mutations may cause proto-oncogenes to become oncogenes, disrupting normal cell division and causing cancers to form.
- Some mutations prevent the cell from reproducing, which keeps the mutations from being passed on.
- If a mutated cell is able to reproduce because the cell division regulators are damaged, then the mutation will be passed on, possibly accumulating more mutations with successive divisions.
Key Terms
- proto-oncogene : a gene that promotes the specialization and division of normal cells that becomes an oncogene following mutation
- mutation : any heritable change of the base-pair sequence of genetic material
- oncogene : any gene that contributes to the conversion of a normal cell into a cancerous cell when mutated or expressed at high levels