What is a virus and its characteristics?

Learning Objectives

1. State 2 living and 2 nonliving characteristics of viruses.
2. List 3 criteria used to define a virus.
3. Discuss why bacteria can be cultivated on synthetic media such as nutrient broth whereas viruses cannot.
4. Define bacteriophage.

Viruses are infectious agents with both living and nonliving characteristics. They can infect animals, plants, and even other microorganisms. Viruses that infect only bacteria are called bacteriophages and those that infect only fungi are termed mycophages . There are even some viruses called virophages that infect other viruses.

Recently, viruses have been declared as living entities based on the large number of protein folds encoded by viral genomes that are shared with the genomes of cells. This indicates that viruses likely arose from multiple ancient cells.

The vast majority of viruses contain only one type of nucleic acid: DNA or RNA, but not both. Virus are totally dependent on a host cell for replication (i.e., they are strict intracellular parasites.) Furthermore, viral components must assemble into complete viruses (virions) to go from one host cell to another. Since viruses lack metabolic machinery of their own and are totally dependent on their host cell for replication, they cannot be grown in synthetic culture media. Animal viruses are normally grown in animals, embryonated eggs, or in cell cultures where in animal host cells are grown in a synthetic medium and the viruses are then grown in these cells.

Summary

1. Viruses are infectious agents with both living and nonliving characteristics.
2. Living characteristics of viruses include the ability to reproduce – but only in living host cells – and the ability to mutate.
3. Nonliving characteristics include the fact that they are not cells, have no cytoplasm or cellular organelles, and carry out no metabolism on their own and therefore must replicate using the host cell's metabolic machinery.
4. Viruses can infect animals, plants, and even other microorganisms.
5. Since viruses lack metabolic machinery of their own and are totally dependent on their host cell for replication, they cannot be grown in synthetic culture media.

This virus looks alive, but is it?

Notice the big virus. He (or she) looks very angry. But actually viruses cannot be a "he" or a "she" - or big either. In fact, viruses are the smallest things. Much smaller than most prokaryotes. We also cannot say that viruses are the smallest living things or organisms, as viruses do not meet the definition of living or of an organism.

Characteristics of Viruses

Which of the three domains of life do viruses belong to? None. Why? Viruses are usually considered to be nonliving. Viruses do not meet most of the criteria of life. They are not even made of cells.

A virus is a sub-microscopic particle that can infect living cells. Viruses are much smaller than prokaryotes, ranging in size from about 20–300 nanometers (nm), though some can be larger. Prokaryotes are typically 0.5–5.0 micrometers (µm) in length. For example, if a virus was about the size of three soccer balls lying side-by-side, then a prokaryote would be about the size of soccer field.

An individual virus is called a virion. It is a tiny particle much smaller than a prokaryotic cell. Because viruses do not consist of cells, they also lack cell membranes, cytoplasm, ribosomes, and other cell organelles. Without these structures, they are unable to make proteins or even reproduce on their own. Instead, they must depend on a host cell to synthesize their proteins and to make copies of themselves. Although viruses are not classified as living things, they share two important traits with living things. They have genetic material, and they can evolve. This is why the classification of viruses has been controversial. It calls into question just what it means to be alive. What do you think? How would you classify viruses?

The study of viruses is known as virology and people who study viruses are known asvirologists. Viruses infect and live inside the cells of living organisms. When viruses infect the cells of their host, they may cause disease. For example, viruses cause AIDS (Acquired immune deficiency syndrome), influenza (flu), chicken pox, and the common cold. Therapy is sometimes difficult for viral diseases. Antibiotics have no effect on viruses and only a few antiviral drugs are available for some diseases. One of the best ways to prevent viral diseases is with a vaccine, which produces immunity. But vaccines are available for only a few diseases.

Mimivirus, shown in the Figure below, is the largest known virus, with a diameter of 400 nm. Protein filaments measuring 100 nm stick out from the surface of the virus, which increases the diameter of the virus to about 600 nm. This is bigger than a small bacterial cell. The virus appears hexagonal under an electron microscope; the viral shape is icosahedral (having 20 faces or sides).

The largest known virus, called mimivirus, is so large that scientists first mistook it for a bacterium. It was first discovered in amoeba, in 1992, and was identified as a virus in 2003. Scientists believe that mimivirus may cause certain types of pneumonia in humans. The core contains DNA, with the majority of the DNA in genes, and only 10% DNA of unknown function ("junk" DNA).

Replication

Viruses can replicate only by infecting a host cell. They cannot reproduce on their own. Viruses are not cells; they are a strand of genetic material within a protective protein coat called a capsid. They infect a wide variety of organisms, including both eukaryotes and prokaryotes. Once inside the cell, they use the cell’s ATP, ribosomes, enzymes, and other cellular parts to replicate.

Habitats

Viruses can be found almost anywhere there is life, including living within prokaryotes. A phage is a virus that infects prokaryotes. Phages are estimated to be the most widely distributed and diverse entities in the biosphere, even more numerous than prokaryotic organisms. Phages can be found everywhere their hosts are found, such as in soil, in the intestine of animals, or seawater. Up to 109 virions have been found in a milliliter of seawater, and up to 70 percent of marine bacteria may be infected by phages. They are also found in drinking water and in some foods, including fermented vegetables and meats, where they control the growth of bacteria.

Summary

• Viruses are tiny particles, smaller than prokaryotic cells.
• Viruses are not cells and cannot replicate without help, but they have nucleic acids and can evolve.

Review

1. What is a virus?
2. What are the two main components of a virus?
3. How do viruses differ from living things? How are they similar to living things?
4. Briefly describe how viruses depend on host cells.
5. What two characteristics of life are evident in viruses?
6. What is a phage?