Final answer:
It is false that oncogenes are always contributed by a virus; while oncogenic viruses can cause cancer, oncogenes can also result from environmental factors or spontaneous mutations.
Step-by-step explanation:
The statement that cancer-causing genes or oncogenes are always contributed by a virus is False. While some viruses, known as oncogenic viruses, do contribute to cancer development by interfering with the normal regulation of the host cell cycle and potentially introducing oncogenes or disrupting tumor suppressor genes, not all oncogenes are virally derived. Oncogenes may also arise from mutations due to environmental factors such as tobacco smoke, radiation, and certain chemicals.
Most cancers are caused by mutations in the DNA that may occur spontaneously or due to exposure to carcinogens. These mutations may result in genes being expressed at inappropriately high levels or in altered forms, both of which can promote cancer growth. Some known oncogenic viruses include Human Papilloma Virus (HPV), Hepatitis B and C viruses (HBV, HCV), and Epstein Barr Virus (EBV), among others. Such viruses may cause cancers like cervical cancer, liver cancer, T-cell leukemia, and various types of lymphoma.