MCAT Content / Control Of Gene Expression In Eukaryotes / Cancer As A Failure Of Normal Cellular Controls Oncogenes Tumor Suppressor Genes

Cancer as a failure of normal cellular controls, oncogenes, tumor suppressor genes

Topic: Control Of Gene Expression In Eukaryotes

Cancer, a disease of altered gene expression, is the result of gene mutations or dramatic changes in gene regulation. Oncogenes are involved in causing cancer. The tumor suppressor gene regulates the cell cycle and prevents uncontrolled cell growth i.e. cancer.

Cancer can be described as a disease of altered gene expression. Many proteins are turned on or off (gene activation or gene silencing) that dramatically alter the overall activity of the cell. In normal cells, some genes function to prevent excess, inappropriate cell growth. These are tumor suppressor genes, which are active in normal cells to prevent uncontrolled cell growth. There are many tumor suppressor genes in cells, but the one most studied is p53, which is mutated in over 50 per cent of all cancer types. The p53 protein itself functions as a transcription factor. It can bind to sites in the promoters of genes to initiate transcription. Therefore, the mutation of p53 in cancer will dramatically alter the transcriptional activity of its target genes.

Another type of gene often deregulated in cancers is proto-oncogenes which are positive cell-cycle regulators. When mutated, proto-oncogenes can become oncogenes and cause cancer. Overexpression of the oncogene can lead to uncontrolled cell growth because oncogenes can alter transcriptional activity, stability, or protein translation of another gene that directly or indirectly controls cell growth. An example of an oncogene involved in cancer is a protein called Myc. Myc is a transcription factor that is aberrantly activated in Burkett’s Lymphoma, a cancer of the lymph system. Overexpression of Myc transforms normal B cells into cancerous cells that continue to grow uncontrollably. High B-cell numbers can result in tumors that can interfere with normal bodily functions.

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Key Points

• Cancer results from a gene that is not normally expressed in a cell, but is switched on and expressed at high levels due to mutations or alterations in gene regulation.

• Tumor suppressor genes, active in normal cells, work to prevent uncontrolled cell growth.

• Proto-oncogenes, which are positive cell-cycle regulators, can become oncogenes and cause cancer when mutated.

Key Terms

Oncogene: any gene that contributes to the conversion of a normal cell into a cancerous cell when mutated or expressed at high levels

Proto-oncogene: a gene that promotes the specialization and division of normal cells that becomes an oncogene following mutation

Cancer: a disease in which the cells of a tissue undergo uncontrolled (and often rapid) proliferation

Burkett’s lymphoma: a cancer of the lymphatic system due to uncontrolled growth of B lymphocytes

Tumor suppressor genes: genes that slow down cell division, repair DNA mistakes and begin cell apoptosis

P53: a gene that codes for a protein that regulates the cell cycle

Transcription factor: a protein that controls the rate of transcription of genetic information from DNA to messenger RNA

Promoter: a region of DNA where transcription of a gene is initiated

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