Introduction to Viruses
How Do Viruses Reproduce?
Although details of the mechanisms vary widely among different virus groups, all viruses must follow three basic steps to reproduce. First, they must gain entrance and deliver their genomes into cells. Bacteriophages attach to cells and inject their DNA into the host bacteria cell. Other viruses bind to specific receptors (protein molecules) on the surface of host cells. Binding of the virus can trigger the cellular process of endocytosis, essentially tricking the cell into letting the virus inside. Enveloped viruses (viruses surrounded by a lipid bilayer) use the glycoproteins, contained in the envelope, to bind to a cellular receptor. Next, the envelope fuses with the plasma membrane of the cell and the viral contents are released inside the cell. If the viral genome is still surrounded by a layer of protein after it enters the cell, the virus undergoes an “uncoating” step to make the genetic material accessible.
Second, the virus commandeers the machinery of the host cell so that it can make copies of its viral genome and synthesize viral proteins. In addition to the manufacturing machinery, the virus needs building blocks for its nucleic acid and proteins. It utilizes the host cell’s nucleotides to make copies of its genetic material and the cell’s amino acids to make proteins. At the most basic level, all viruses need to replicate their genome and produce capsid proteins. Many viruses encode additional proteins, that enhance their abilities to take over a cell, replicate to higher levels, or to evade host immune responses. RNA viruses often provide their own enzyme, a polymerase, to replicate their genome, because the host cell does not provide some the necessary enzymes. Virus-encoded polymerases are especially important for RNA viruses that need to make a DNA copy of their genome or for (-) strand RNA viruses to synthesize a (+) strand that can be translated. DNA viruses usually use the host cell DNA polymerase, which normally copies cellular DNA. Viruses use the transcription and translation machinery of the cell to manufacture virus proteins. Some viruses produce proteins that modify the cellular transcription and translation apparatus to ensure preferential synthesis of viral proteins over cellular ones, and some viruses even can completely shut down the synthesis of host cell proteins or destroy the host cell’s DNA.
Finally, the nucleic acid and protein components are synthesized, the virus particle is assembled with the protein capsid surrounding the genome. There are various mechanisms by which a virus can exit a cell. Some viruses lyse, or burst open cells to release the virus particles. This process immediately destroys the host cell. Other viruses will bud out of the cell through the plasma membrane and acquire an envelope. In any case, hundreds or thousands of infectious virus particles are released from an infected cell. These newly made virus particles then go on to infect new host cells and continue the cycle of virus reproduction.
It is constructive to keep in mind that unlike a cell, which duplicates its DNA and reproduces from a preexisting cell by dividing to form two daughter cells, a virus can use a single nucleic acid template to make hundreds or more copies of its genome. A useful analogy is that a virus genome can be reproduced multiple times from a single copy, much like a piece of paper can be reproduced numerous times in a copy machine.
Note: The relative sizes of the virus particles and the cell in the diagram are not to scale. The viruses actually are much smaller, relative to cells, than indicated.
- Campbell, N.E., & Reece, J.B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.
- Flint, S.J., Enquist, L.W., Krug, R.M., Racaniello, V.R., and Skalka, A.M. (2000). Principles of Virology: Molecular Biology, Pathogenesis, and Control. ASM Press.
- Herrmann, C. (2006). Virus replication.
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Video and transcript courtesy of Wah Chiu, PhD, National Center for Macromolecular Imaging at Baylor College of Medicine. Funding for the video provided by NCMI, NIH.