Final answer:
Proto-oncogenes can become oncogenes due to mutations that cause overexpression or permanent activation of proteins. These genetic changes disrupt cell cycle control, leading to uncontrolled cell growth and cancer. Viruses can play a role in oncogene activation, with some causing rapid and others gradual transformation of cells.
Step-by-step explanation:
Proto-oncogenes and Cancer
Proto-oncogenes are essential genes that normally help in cell division but can be converted into oncogenes through various genetic changes. When this occurs, the balance of cell cycle regulation is disrupted, allowing uncontrolled cell growth, which can lead to cancer. Mutations can turn proto-oncogenes into oncogenes through overexpression, producing too much of a protein, or causing a structural change in a protein so it is always active. Other mutations might inactivate tumor suppressor genes, removing the checks and balances that normally keep cell division under control.
Several types of mutations working together are usually necessary for cancer to develop. A single mutation in a proto-oncogene or a tumor suppressor gene is not typically sufficient to cause cancer because of various cellular mechanisms that safeguard against uncontrolled growth. However, multiple mutations that activate oncogenes and inactivate tumor suppressor genes are needed for the transformation of a normal cell into a cancer cell. These mutations accumulate over time and can be caused by factors such as environmental influences, genetic predisposition, or random errors in DNA replication.
Viruses can also play a role in the transition of proto-oncogenes to oncogenes. In acutely-transforming viruses, a viral-oncogene (v-onc) is immediately expressed upon infection, leading to rapid transformation of the host cell. Meanwhile, slowly-transforming viruses integrate near proto-oncogenes in the host's DNA, leading to overexpression and gradual transformation into cancerous cells.