Unit Four. The Evolution and Diversity of Life
16. Prokaryotes: The First Single-Celled Creatures
16.7. The Structure of Viruses
The border between the living and the nonliving is very clear to a biologist. Living organisms are cellular and able to grow and reproduce independently, guided by information encoded within DNA. The simplest creatures living on earth today that satisfy these criteria are prokaryotes. Viruses, on the other hand, do not satisfy the criteria for “living” because they possess only a portion of the properties of organisms. Viruses are literally “parasitic” chemicals, segments of DNA (or sometimes RNA) wrapped in a protein coat. They cannot reproduce on their own, and for this reason they are not considered alive by biologists. They can, however, reproduce within cells, often with disastrous results to the host organism.
Viruses are very small. The smallest viruses are only about 17 nanometers in diameter. Viruses are so small that they are smaller than many of the molecules in a cell. Most viruses can be detected only by using the higher resolution of an electron microscope.
The true nature of viruses was discovered in 1935, when the biologist Wendell Stanley prepared an extract of a plant virus called tobacco mosaic virus (TMV) and attempted to purify it. To his great surprise, the purified TMV preparation precipitated (that is, separated from solution) in the form of crystals. This was surprising because precipitation is something that only chemicals do—the TMV virus was acting like a chemical rather than an organism. Stanley concluded that TMV is best regarded as just that—a chemical matter rather than a living organism.
Each particle of TMV virus is in fact a mixture of two chemicals: RNA and protein. The TMV virus, pictured in figure 16.9b, has the structure of a Twinkie, a tube made of an RNA core (the green springlike structure) surrounded by a coat of protein (the purple structures that encircle the RNA). Later workers were able to separate the RNA from the protein and purify and store each chemical. Then, when they reassembled the two components, the reconstructed TMV particles were fully able to infect healthy tobacco plants and so clearly were the virus itself, not merely chemicals derived from it.
Viruses occur in all organisms, from bacteria to humans, and in every case the basic structure of the virus is the same, a core of nucleic acid surrounded by protein. There is considerable difference, however, in the details. In figure 16.9 you can compare the structure of bacterial, plant, and animal viruses—they are clearly quite different from one another and there is even a wide variety of shapes and structures within each group of viruses. Bacterial viruses, called bacteriophages, can have elaborate structures, like the bacteriophage in figure 16.9a that looks more like a lunar module than a virus. Many plant viruses like TMV have a core of RNA, and some animal viruses like HIV (figure 16.9c) do too. Several different segments of DNA or RNA may be present in animal virus particles, along with many different kinds of protein. Like TMV, most viruses form a protein sheath, or capsid, around their nucleic acid core. In addition, many viruses (like HIV) form a membranelike envelope, rich in proteins, lipids, and glycoprotein molecules, around the capsid.
Figure 16.9. The structure of bacterial, plant, and animal viruses.
(a) Bacterial viruses, called bacteriophages, often have a complex structure. (b) TMV infects plants and consists of 2,130 identical protein molecules (purple) that form a cylindrical coat around the single strand of RNA (green). The RNA backbone determines the shape of the virus and is protected by the identical protein molecules packed tightly around it. (c) In the human immunodeficiency virus (HIV), the RNA core is held within a capsid that is encased by a protein envelope.
Key Learning Outcome 16.7. Viruses are genomes of DNA or RNA, encased in a protein shell, that can infect cells and replicate within them. They are chemical assemblies, not cells, and are not alive.