13.16–13.19: Viruses are at the border between living and non-living.

Colored transmission electron micrograph of T2 bacteriophage viruses (red) attacking an E. coli bacterium.

A virus is not a cell, and that is why viruses do not fit into any of the three domains of life. A virus particle (called a virion) consists of genetic material inside a container made of protein. Some viruses also contain a few enzymes. That’s all there is to a virus. The protein container is called the capsid, and the genetic material can be either DNA or RNA. Some viruses wrap themselves in a bit of the plasma membrane of the host cell as they are released. A virus of this type is called an enveloped virus—the flu virus is an example. Non-enveloped viruses are enclosed only by the protein container; the virus that causes the common cold is an example of a non-enveloped virus (FIGURE 13-24).

Figure 13.24: The simple structure of viruses.

There is almost nothing inside the capsid of a virus except DNA or RNA. A virus particle does not carry out any metabolic processes, and it does not control the inward or outward movement of molecules to make conditions inside the virus particle different from conditions outside. Viruses just wait for a chance to insert their genetic material into a living cell.

We speak of “catching” a cold, but what actually happens is that a virus catches us. Viruses identify the cells that they can infect by recognizing the structure of glycoprotein molecules on the surfaces of cells. Every cell in your body has these molecules. They are embedded in the plasma membrane and extend outward from the cell. Your immune system uses these proteins to identify the cells as part of you, and it does not react to proteins that it recognizes as self rather than non-self.

Viruses have cracked the glycoprotein code, and when they find a cell with the appropriate glycoprotein on its surface, they bind to that cell’s plasma membrane and insert their genetic material into the cell (FIGURE 13-25). Inside the host cell, the viral DNA or RNA takes over the cellular machinery and uses it to produce more viruses. Viruses carry out nearly all of their activities by hijacking materials and organelles in the host cell. Viral proteins are synthesized in exactly the same way as host cell proteins—mRNA binds to ribosomes, and tRNA matches the correct amino acid to each mRNA codon. The mRNA is produced from the virus’s DNA or RNA, but all of the protein-building machinery comes from the host cell, as does the ATP required to synthesize the new viral protein.

Figure 13.25: Making more viruses. A virus duplicates its own genetic material (DNA in the example shown here) by taking over the resources and metabolic machinery of a host cell.

There are also some other types of non-living infectious agents. Prions, for example, are misfolded proteins that form plaques and interfere with normal tissue. Usually acquired through ingestion of an infected animal (or its bodily fluids), prions can cause a variety of degenerative neurological diseases in humans. One of these, Creutzfeldt-Jakob disease, is sometimes referred to as a human form of mad cow disease: it is characterized by the development of holes in brain tissue, causing the tissue to appear sponge-like. It leads to dementia, memory loss, and disruption of balance and coordination. No cure or treatment is known, and few victims survive more than a year following the appearance of symptoms.