Question

Define tumor-suppressor genes. Why is a mutated single copy of a tumor- suppressor gene expected to behave as a recessive gene?

Short answer

Answer: Tumor-suppressor genes play a vital role in preventing uncontrolled cell growth and division by producing proteins that counteract the effects of oncogenes. A mutated single copy of a tumor-suppressor gene is considered recessive because one functional copy of the gene is enough to prevent uncontrolled cell growth. An individual must inherit a mutated copy of the gene from both parents to exhibit a significantly higher risk of developing cancer, making it a recessive trait. However, heterozygous mutations can still increase cancer risk but to a lesser extent than homozygous mutations.

Step-by-step solution

Definition of Tumor-suppressor genes

Tumor-suppressor genes are genes that encode proteins responsible for preventing uncontrolled cell growth and division. They play a crucial role in maintaining the stability and integrity of the cell, preventing the development of tumors. When these genes function properly, they help to control cell growth and division, repair damaged DNA, and induce apoptosis (programmed cell death) if necessary.

Role of tumor-suppressor genes in cell growth and division

Tumor-suppressor genes have a vital role in controlling cell growth and division, as they act as a safeguard against the development of cancer. When these genes are functioning correctly, they produce proteins that counteract the effects of oncogenes, which are genes that promote cell growth and division. The balance between tumor-suppressor genes and oncogenes keeps cell growth and division under control, ensuring that cells divide only when needed, and that damaged cells are eliminated.

Mutated single copy of a tumor-suppressor gene and recessive behavior

In our body, we have two copies of each gene, one from each parent. When there is a mutation in a single copy of a tumor-suppressor gene, it is called a heterozygous mutation. In this case, the individual still carries one normal copy of the gene, which can still produce the functional protein that suppresses tumor formation. Since just one functional copy of the tumor-suppressor gene is enough to prevent uncontrolled cell growth, the mutated gene is considered recessive to the normal gene. In other words, an individual must inherit a mutated copy of the tumor-suppressor gene from both parents (in a homozygous state) to exhibit a significantly higher risk of developing cancer, which makes it a recessive trait. But, it is essential to mention that even heterozygous mutations can still increase cancer risk, but to a lesser extent than homozygous mutations.

Key concepts

Oncogenes

Oncogenes are genes that have the potential to cause cancer. In normal cells, these genes are involved in the regulation of cell growth and division. However, when these genes are mutated or expressed at high levels due to factors like genetic mutation or viral infection, they can become hyperactive.

This hyperactivity can lead to uncontrolled cell proliferation, which is characteristic of cancer. Genetic mutations that activate oncogenes can be inherited or acquired over a person's lifetime. It is important to understand that oncogenes are essentially normal genes, known as proto-oncogenes, which have gone awry. Their study is essential for developing targeted cancer therapies.

Cell Growth and Division

Cell growth and division are fundamental biological processes that enable organisms to develop, repair tissue, and reproduce. This intricate process is tightly regulated by numerous genes, including oncogenes and tumor-suppressor genes.

While oncogenes can stimulate cell growth and division, tumor-suppressor genes act as brakes, preventing cells from dividing too rapidly or uncontrollably. An imbalance in this regulation can lead to diseases, such as cancer. Understanding the mechanistic details of how cells regulate their growth and division is crucial for comprehending how cancers develop and for designing effective treatments.

DNA Repair

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. It is fundamental to cell viability as damage to DNA can result in mutations, leading to cancer and other genetic diseases.

Tumor-suppressor genes often play a role in DNA repair pathways, producing proteins that detect and fix errors in DNA. When these genes are mutated, the DNA repair process can fail, leading to an accumulation of mutations and potentially causing cancer. Understanding DNA repair mechanisms is essential for grasping the concept of mutagenesis and its implications in oncology.

Apoptosis

Apoptosis, or programmed cell death, is a form of cell death that occurs in a regulated and controlled manner, effectively allowing the body to remove damaged or unnecessary cells without causing harm to the surrounding tissue.

This process is essential for the development and maintenance of healthy tissues and plays a critical role in preventing the development of cancer. Tumor-suppressor genes are instrumental in initiating apoptosis when cells are damaged beyond repair. By understanding apoptosis, researchers can uncover novel therapies to induce cell death in cancer cells that may have evaded this vital safeguard.

Heterozygous Mutation

A heterozygous mutation refers to the presence of different alleles at a gene locus where one is mutant and the other is normal. In the context of tumor-suppressor genes, an individual with a heterozygous mutation carries one normal allele and one mutated allele.

Usually, the normal allele is capable of compensating for the mutated one, which maintains the function of the gene. However, the presence of this mutation can still mildly increase cancer risk, especially if the individual is exposed to other risk factors. This concept is critical in understanding genetic predispositions to cancers.

Recessive Gene Behavior

Recessive gene behavior refers to how certain traits are expressed only when two copies of a gene are mutated or dysfunctional. For a recessive trait to manifest, an individual must inherit one copy of the mutated gene from each parent.

With tumor-suppressor genes, when both alleles are mutated (homozygous), it can lead to a loss of function and a significantly increased risk of cancer. In contrast, if only one allele is mutated (heterozygous), the normal allele often provides sufficient function to suppress tumor development. Understanding this recessive behavior is key in genetic counseling and assessing cancer risk.