Rous sarcoma virus is often presented in the educational context as the 'model' retrovirus, and there is an expectation that students have basic familiarity with rous sarcoma for the MCAT. Rous sarcoma is an enveloped virus with a positive sense RNA genome. (Just because the RNA of a virus is (+) sense, does not mean it will be immediately transcribed in the replication process. Rous sarcoma is a retrovirus. As with all retroviruses, it reverse transcribes its RNA genome into cDNA before integration into the host DNA, so rous sarcoma contains (+) RNA and propagates through a double-helical DNA intermediate.

Rous sarcoma is oncogenic. In the descriptive nomenclature, you might say that the RNA tumor virus propagates to DNA provirus then to RNA tumor virus. All RNA tumor viruses contain reverse transcriptase in their virions. The oncogenic actions of retrovirus transform susceptible cells by producing too much of certain key proteins (such as tyrosine kinases) or altering others.

Along with rous sarcoma, the other retrovirus which needs to be well-studied prior to the MCAT is HIV. The T4 cells of the immune system are the host cells for HIV. As with rous sarcoma, HIV is an enveloped virus. Interaction of a membrane envelope glycoprotein (gp120) and the CD4 receptor on the host allows membrane fusion and entry of HIV into the host cell.

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