16
Viruses and Subviruses

Viruses and subviruses are not cellular. Therefore, by definition they are not living because their primary units are not cells. Even so, they have the potential to interact with just about every type of living organism by interfering with their cellular processes, forcing cells to create more viruses and subviruses, which sometimes leads to serious consequences that can cause symptoms, sickness, disease, and possibly death.

VIRUSES

Viruses cannot reproduce on their own, but instead they need to use living cells to reproduce. Although viruses can reproduce with the help of other organisms and in the process they do pass traits onto their offspring, viruses are technically not considered to be living is because they do not conveniently fit in with a list that includes the following prerequisites to be included among the living.

Living organisms must be:

1. composed of one or more cells,
2. have the capacity to grow and change,
3. be able to respond to the environment,
4. maintain homeostasis,
5. have a metabolism and breathe,
6. reproduce, and
7. pass traits onto their offspring.

Viruses do have some of the above characteristics, but not all. If the definition of life had been written when we had a better understanding of viruses, it might have been written in such a way that would have included viruses among the living, but because they don't fit in perfectly with the above definition, viruses are not living. If a noncellular category of life is ever included among the official definition of what is comprised among the living, viruses would almost definitely be included. In such a case, viruses might be categorized as a separate kingdom.

Viruses are very, very small, and there are very many different kinds. Because they are so small, ranging from 10 to 300 nm in diameter (a nanometer is one millionth of a millimeter, which is the same as one-billionth of a meter). This means viruses vary in size 1/10^th to 1/100^th the size of a typical bacterium, which means they are too small to be seen with a traditional optical microscope. The invention of the electron microscope made it possible to see viruses. X-ray crystallography, which diffracts X-rays through crystallized virus particles, reveals the molecular structure of viruses. And most recently it became possible to stretch optical microscope technology in a way that converts an optical microscope into a microsphere nanoscope that allows researchers to see viruses.

Like some obligate intracellular bacteria, viruses are parasitic and unable to reproduce without having cells to inhabit. Outside of cells, viruses cannot reproduce, feed, or grow. Unlike living cells, viruses do not metabolize; that is, they do not generate their own energy. Instead, with genetic information contained in their nucleic acids (in their viral DNA or RNA), they overpower other cells, insert their nucleic acids (DNA or RNA) into their host's cell, and then direct the production of more viruses by utilizing the host's cellular machinery. All other organisms contain both DNA and RNA, however viruses contain only one or the other.

The DNA or RNA of a virus is enclosed in a protective protein coat or sheath (protein capsid or viral capsid). Some types of viruses have a viral capsid that in addition to being composed of protein, it also contains fatty acids. Such viruses can be destroyed with soap or detergent. For a virus to reproduce, it must come in contact with and then attach to a cell, penetrate the cell's exterior, travel to the cell's genetic material, and then change what the cell normally does, into doing what the virus requires to reproduce more viruses.

Outside of a host cell, a virus is inert, incapable of reproduction or of any metabolic functions that would identify it as living. The way a virus gains entry into a potential host cell is by “identifying” a receptor site on cell's exterior. By identifying the receptor site and attaching to the cell's protein coat, the virus then injects its DNA or RNA into the host, or it may enter the host intact. If the virus gets inside the host cell intact, once inside, the viral capsid dissolves and the viral DNA or RNA acts as a template for the manufacture of viral components. That is, the virus attaches its genetic material to the genetic material of the host and “tricks” it into producing more viruses though the same mechanisms the cell normally uses to replicate itself. In time, the virus particles are assembled within the host cell. Then, by lysing (dissolving) the cell membrane, the new viruses leave the host and infect new, uninfected cells (see Figure 16.1).

Viruses' capacity to interfere with an inject viral genetic information into a host's cells may play an important, and, possibly, even crucial role in evolution. By rearranging the DNA in chromosomes and by transferring genes from one species to another, viruses may be moving genetic material among plants and animals, sometimes imparting new characteristics that are adaptively significant. In fact, it is possible that certain viruses may have an evolutionary beneficial effect over time. As a result, it may even be that species reap a benefit from some viral infections.

Figure 16.1 Proliferative cycle of a virus; here the host cell is a bacterium and the virus is a bacteriophage.

It has been suggested that viruses are more closely related to their hosts than to one another, having perhaps originated as nucleic acids that escaped from cells and began replicating on their own, but returning to the cells for necessary chemicals and structures.

Viruses are very versatile and there are specific viruses that infect each species of plant, animal, bacteria, and archaea in the world. Some viral infections hardly cause any symptoms, while others cause diseases, some of which can spread and cause epidemics or even worse, pandemics when they spread around the world.

The viruses that only attack bacteria are known as bacteriophages (see Figures 16.2 and 16.3). There are viruses that only attack eukaryotic cells. And many viruses are extremely specific with regard to the type of cell they will attack. Some viruses that commonly attack humans include cold viruses, many of which are called rhinoviruses. Americans catch over 60 million common colds each year.

The influenza virus is the fastest-mutating virus known, capable of rapidly changing the outer protein coat through succeeding generations of the flu. People can therefore catch the flu more than once a year, since they have no antibodies to the new strain. RNA viruses cause measles, rubella (German measles), and mumps – all childhood diseases. Another childhood disease, called fifth disease, is cause by parvovirus.

Figure 16.2 Diagram of a virus that attacks bacteria, known as a bacteriophage (only two of the six collar whiskers and two of the six tail fibers are depicted here).

Figure 16.3 Scanning electron microscope (SEM) photomicrograph of a bacterial cell (Escherichia coli) being infected by many bacteriophages.

There are different forms of herpes virus. One can cause chicken pox (and when reactivated, it can cause shingles). Other forms cause cold sores in the mouth and on our lips, as well as sores in the genital area, and yet another, Epstein-Barr virus, causes mononucleosis. Another type of virus that no longer infects people, was smallpox, which killed hundreds of millions of people until it was eradicated from the wild in 1980.

Papillomaviruses, of which about 50 different types are known, cause plantar warts, genital wards, and certain wart-like rashes. Different types of viral hepatitis (hepatitis A, hepatitis B, and hepatitis C) are caused by viruses.

Retroviruses are a group of viruses named for their backward (retro) sequence of genetic replication as compared to other viruses. HIV/AIDS (human immunodeficiency virus, acquired immunodeficiency syndrome) is caused by a virus that has been brought under control with medications. Another well-known disease caused by a viral infection is rabies.

Influenza, which is also called the flu, is caused by different strains of the influenza virus. In 1918 a strain of this virus, called Spanish flu, spread around the world and killed 500 million people. It is hoped that a similar version of this deadly and highly contagious virus never starts to make people sick again, although there's no way of knowing.

Ebola (Ebola hemorrhagic fever) is very rare, but it is also very contagious and deadly. There is no a vaccine that can prevent people from catching this disease. Another disease caused by a virus may have heard of that are caused by virus is yellow fever, which was also brought under control with a vaccine.

The coronaviruses include a number of viruses, some of which are associated with common cold-like ailments. Usually the varieties of coronaviruses that attack humans are not particularly harmful, but on occasion, when a virus jumps from another species to humans, we humans have little or no immunity and the results can be catastrophic. Luckily, the jumping of a “novel” virus from a non-human species to humans is rare, however this occurred in 2003 when the SARS coronavirus (SARS-CoV). SARS is short for “severe acute respiratory syndrome,” which describes the respiratory illness that this coronavirus (CoV) causes in people. This virus is believed to have jumped from a species of bat to people, possibly when someone was handling the bat. Again in 2019 another coronavirus jumped, possibly originating with a bat species, and somehow it jumped to a human, and then spread around the world causing the disease called Covid-19 (Co is short for corona, vi is short for virus, d is short for disease, 19 is for 2019, when it first appeared in humans). This first appeared in Wuhan, Hubei, China, and then it rapidly spread to people all around the world, causing a pandemic (a disease that spreads to many people all around the world) that killed hundreds of thousands of people.

SUBVIRUSES

The smallest infectious agents known to researchers are termed subviral infectious agents, or subviruses. Scientists have identified three major types, the satellite viruses, viroids, and prions.

Viroids are minute rings of RNA that infect certain plants. Satellite viruses are tiny pieces of RNA that make full-size viruses work for them. These tiny nucleic acids multiply inside viruses that are inside cells.

Prions are usually much smaller than viruses, sometimes they are 100 times smaller, though some are as large as bacteria. Prions are known to cause some diseases and are implicated as the cause of others. Included among these diseases that prions are associated with are Mad Cow Disease, Creutzfeld-Jacob Disease, scrapie, and several similar degenerative brain diseases.

It has been theorized that prions may be radically different from any other known self-replicating entities. There is no evidence that prions contain any nucleic acids, DNA, and/or RNA. Instead, they appear to be little more than dots of protein. Even if they were found to contain nucleic acids, prions are so small that there is little chance they contain a nucleic acid any longer than 50 nucleotides. This is not large enough to encode a protein containing more than about 12 amino acids. Somehow prions are able to cause proteins to fold the wrong way in animals that suffer from such a prion transmission.

Despite indications to the contrary, it has even been suggested that prions may actually be conventional viruses, but this is quite unlikely. It appears equally unlikely that they will be found to represent a new category of proto-organismal material that reproduces in living cells, employing a technique that has yet to be elucidated. It has even been suggested that they may reproduce using a technique similar to that employed by viruses, without being viruses.

Some researchers have suggested that the mode of prion reproduction might involve fracture and continued growth, which would explain their small and uncertain molecular weights, their rod-like appearance, their varying lengths, and the unpredictability of which amino acid occurs terminally. The most recent work has shown that prions may be proteins produced somewhat abnormally by infected genes that somehow go awry.

Among the other subviruses are the viroids, minute rings of RNA that infect certain plants. Virusoids appear to be loops of RNA that occur inside regular viruses. Virinos, like viruses, need an outer coat of protein, which they are unable to make on their own, but which they induce host cells to manufacture. Virogenes are otherwise normal genes that generate infectious particles under certain circumstances. Satellite viruses are tiny pieces of RNA that make full-size viruses work for them. These tiny nucleic acids multiply inside viruses that are inside cells.

KEY TERMS

AIDS	prions	viral capsid
bacteriophages	retroviruses	virinos
hepatitis	rhinoviruses	virogenes
herpes virus	satellite viruses	viroids
influenza virus	subviral infectious agents	viruses
papillomaviruses	subviruses	virusoids
parvovirus

SELF-TEST

Multiple-Choice Questions

Viruses

1. Viruses contain ___________.
   1. nucleic acids
   2. a protein coat
   3. DNA or RNA
   4. viral capsid
   5. all of the above
2. Viruses do not ___________.
   1. metabolize
   2. generate their own energy
   3. replicate (or duplicate, or reproduce) without injecting cells
   4. all of the above
   5. none of the above
3. The information contained in the viral DNA or RNA is ___________.
   1. inserted into its host's cellular machinery
   2. contained in the viral nucleic acids that are inserted into their host's DNA
   3. used to direct the host to produce more viruses
   4. all of the above
   5. none of the above
4. Each of the many different types of viruses “knows” which cells to attack by identifying ___________ on the potential host's outer ___________.
   1. receptor sites, protein coat
   2. nucleic acids, viral capsid
   3. receptacles, bacteriophages
   4. all of the above
   5. none of the above
5. Viruses that attack only bacteria are known as ___________.
   1. DNA viruses
   2. RNA viruses
   3. retroviruses
   4. bacteriophages
   5. all of the above
6. Some viruses infect a host cell by ___________.
   1. attaching to the host's protein coat while injecting the viral DNA or RNA into the host
   2. entering the host intact
   3. injecting viral genetic information into a host's cells
   4. all of the above
   5. none of the above
7. It is possible that viruses may be moving genetic material from ___________.
   1. plants to animals
   2. animals to plants
   3. plants to plants
   4. animals to animals
   5. all of the above
8. Viruses may prove, in some cases, to be the simplest of ___________.
   1. all symbionts
   2. all parasites
   3. all living things
   4. all of the above
   5. none of the above

Subviruses

9. Prions have been said to be ___________.
   1. the smallest infectious agents known
   2. the largest infectious agents known
   3. 100 times smaller than viruses to almost as large as mitochondria and bacteria
   4. the cause of certain diseases
   5. a, c, and d
10. Prions ___________.
    1. have been found to contain nucleic acids
    2. have not been found to contain nucleic acids
    3. are viruses
    4. are not viruses
    5. b and d
11. Recent work has shown that prions may be ___________.
    1. proteins produced somewhat abnormally by “infected” genes that somehow go awry
    2. bacteria
    3. viruses
    4. protists
    5. mitochondria

ANSWERS

1. e
2. d
3. d
4. a
5. d
6. d
7. e
8. d
9. e
10. e
11. a

Questions to Think About

1. Describe viruses. Are they considered living? Support your answer.
2. What might account for the origin of viruses?
3. How do viruses increase in number? Describe the different mechanisms.
4. What are five diseases affecting humans that are caused by viruses?
5. Describe similarities and differences between viruses and subviruses.