Introduction to Organisms
Where Do Viruses Fit?
By themselves, viruses do not have all the characteristics of living organisms and are not considered "alive" by most definitions. Viruses are not cells, but consist of single or double stranded RNA or DNA surrounded by a protein shell called a capsid. The two major shapes of viruses are helical and polyhedral. Some viruses also have a protein/lipid outer membrane or envelope surrounding the capsid. Viruses do not grow, maintain homeostasis or metabolize on their own.
The structure and replication mode of viruses varies widely; however, all viruses can multiply only within a host cell (including bacteria).
Phages (viruses that infect bacteria) are the best understood of all viruses and research has led to the discovery that some double stranded DNA viruses are able to reproduce using two alternative processes, the lytic and the lysogenic cycles.
In the lytic cycle, the virus attaches to the host cell, injecting its DNA. The viral nucleic acid directs the host to produce new viral DNA and phage proteins. After assembly, new viral offspring particles are released when the host cell disintegrates, or "lyses."
In the lysogenic cycle, viral genomes remain dormant for long periods of time inside the host cell.
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell(4th ed.). New York: Garland Publishing, Inc.
- Transmission electron micrograph of influenza A virus. Courtesy of the CDC\Dr. Erskine Palmer.
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