Rna Tumor Viruses and Proto-Oncogenes
In 1911 Peyton Rous reported that a class of RNA viruses can cause tumors in animals. These RNA tumor viruses, called "retroviruses," carry an RNA genome that, once inside a cell, is copied into DNA, which then is inserted randomly into the genome of a host cell. Some retroviruses are slow to cause tumors. After infection and spread to a large number of cells, a DNA copy of the viral genome, by chance, integrates into a host cell's DNA next to a normal gene that plays an important role in cell growth. If this viral integration disrupts the expression or structure of the normal cellular gene, it induces abnormal growth signals that can lead to cancer.
Other retroviruses cause tumors to appear very quickly. In the process of copying viral RNA into DNA, RNA that is expressed from cellular genes can be mistakenly copied into the viral genome. The progeny of the virus transfer the cellular gene to many other cells. If this "captured" cellular RNA is from a gene that stimulates cell growth, it then causes abnormal growth stimulation, leading to cancer.
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