At the beginning, we discussed the cell theory and noted that viruses do not fit the definition of living things because they are acellular. Viruses may be as small as 20 nm or as large as 300 nm.3 For reference, prokaryotes are 1–10 µm, and eukaryotes are about ten times larger. Unlike eukaryotic cells, viruses lack organelles and a nucleus. Viruses are composed of genetic material, a protein coat, and sometimes an envelope containing lipids. The genetic information may be circular or linear, single- or double-stranded, and composed of either DNA or RNA. The protein coat is known as a capsid. If an envelope is present, it will surround the capsid and is composed of phospholipids and virus-specific proteins. Enveloped viruses are easier to kill since the envelope is very sensitive to heat, detergents, and desiccation.4 On the other hand, viruses that do not have an envelope are more resistant to sterilization and are likely to persist on surfaces for an extended period of time. Because viruses cannot reproduce independently, they are considered obligate intracellular parasites. Viruses must express and replicate genetic information within a host cell because they lack ribosomes to carry out protein synthesis. After hijacking a cell’s machinery, a virus will replicate and produce viral progeny, called virions, which can be released to infect additional cells.4
Bacteriophages are viruses that specifically target bacteria. They do not actually enter bacteria; rather, they simply inject their genetic material, leaving the remaining structures outside the infected cell. In addition to a capsid, bacteriophages contain a tail sheath and tail fibers. The tail sheath can act like a syringe, injecting genetic material into a bacterium. The tail fibers help the bacteriophage to recognize and connect to the correct host cell.4
1) Villarreal, L. P. (2008, August 8). Are Viruses Alive. Scientific American.
2) Campbell, N. (2003). Biology:Concepts & Connections. San Francisco: Pearson Education.
3) Sinkovics, J., J, H., & A, H. (1998). he Origin and evolution of viruses (a review). Acta Microbiologica et Immunologica Hungarica, 349 – 390.
4) Crick FH, W. J. (1956). Structure of small viruses. Nature, 473- 475.
5) Boevink P, O. K. (2005). Virus-host interactions during movement processes. Plant Physiology, 1815–21.
6) NC State University. “Prokaryotes: Single-celled Organisms