Comrehension check

VIRUSES

The fact that viruses share properties of most living organisms provides good reason to include them in their own noncellular category of life. Were this to be sanctioned by most scientists, it would require expanding the definition of life as it currently stands to include noncellular entities that contain genes. Should such a revision ever occur, viruses might form their own kingdom.

Viruses are, on the average, from 10 to 100 times smaller than the typical bacterium, making them too small to be seen by most optical microscopes. In 1931, the invention of the electron microscope broke this light barrier. And X-ray crystallography, a technique by which X-rays are diffracted through crystallized virus particles to reveal their molecular structure, enabled researchers to study these forms.

Like some obligate intracellular bacteria, viruses are parasitic and unable to reproduce without having cells to inhabit. Viruses, like living cells, contain nucleic acids, which are enclosed in a protective coat of protein, sometimes called the viral capsid, which ranges from 20 to 250 nanometers across (1 nanometer = 1 millionth of a millimeter).

Outside cells, viruses neither reproduce, feed, nor grow. And, unlike living cells, viruses do not metabolize; that is, they do not generate their own energy. Instead, with the information contained in their viral DNA or RNA, they overpower other cells, inserting their nucleic acids into their host’s cell to direct the production of more viruses by utilizing the host’s cellular machinery. While all other organisms contain both DNA and RNA, viruses contain only one or the other.

Outside of a host cell, a virus is inert, incapable of reproduction, or of any metabolic functions that would identify it as living. However, each of the many different types of viruses “identify” receptor sites on a potential host’s outer coat of protein and thereby “know” which cells to attack. A virus may infect a host cell either by attaching to the host’s protein coat while injecting the viral DNA or RNA into the host or by entering the host 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 that of the host and “tricks” it into producing more viruses through 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.

Viruses’ capacity to interfere with and 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 evolutionarily beneficial effect over time.

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.

Some viruses known as bacteriophages attack only bacteria. Other attack only eukaryotic cells. And many are extremely specific with regard to the type of cell they will attack. The major types of viruses that attack humans include cold viruses, or rhinoviruses, which cause most common colds.

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 virus. RNA viruses cause measles, rubella (German measles), and mumps – all childhood diseases. Another childhood disease, called fifth disease, is caused by parvovirus.

Different forms of herpes virus include those that cause cold sores, genital sores, chicken pox (or, if it’s reactivated, shingles), and Epstein-Barr virus, which causes mononucleosis.

Papillomaviruses, of which forty-six types are known, cause plantar warts, genital warts, and certain wartlike rashes. Hepatitis is also caused by a virus.

Retroviruses are a group of viruses named for their backward (retro) sequence of genetic replication as compared to other viruses. The AIDS viruses are in this group. Another well-known disease caused by a viral infection is rabies.

SUBVIRUSES

The smallest infectious agents known to researchers are termed subviral infectious agents, or subviruses. Scientists have identified at least six different strains: satellite viruses, virinos, viroids, virusoids, virogenes, and prions.

Members of one of the better understood strains, prions, range in size from considerably smaller than viruses, sometimes 100 times smaller, to almost as large as mitochondria and bacteria. Prions have been found to cause certain diseases and are implicated as the cause of others. Included in this list of diseases that prions seem to promote are scrapies 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.

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 protoorganismal 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.

COMREHENSION CHECK

1. Choose the right variant for the multiple-choice statements.

1. Viruses contain

a. nucleic acids b. a protein coat

c. DNA or RNA d. viral capsid

e. all of the above.

2. Viruses do not

a. Metabolize b. generate their own energy

c. replicate (or duplicate, or reproduce) without injecting cells

d. all of the above e. none of the above.

3. The information contained in the viral DNA or RNA is

a. inserted into its host’s cellular machinery

b. contained in the viral nucleic acids that are inserted into their host’s DNA

c. used to direct the host to produce more viruses

d. all of the above e. none of the above.

4. Each of the many different types of viruses “know” which cells to attack by identifying ______________ on the potential host’s outer _____________.

a. receptor sites, protein coat b. nucleic acids, viral capsid

c. receptacles, bacteriophages d. all of the above

e. none of the above.

5. Viruses that attack only bacteria are known as _______________.

a. DNA viruses b. RNA viruses

c. retroviruses d. bacteriophages

e. all of the above.

6. Some viruses infect a host cell by

a. attaching to the host’s protein coat while injecting the viral DNA or RNA into the host

b. entering the host intact

c. injecting viral genetic information into a host’s cells

d. all of the above e. none of the above.

7. It is possible that viruses may be moving genetic material from

a. plants to animals b. animals to plants

c. plants to plants d. animals to animals

e. all of the above.

8. Viruses may prove, in some cases, to be the simplest of

a. all simbionts b. all parasites

c. all living things d. all of the above

e. none of the above.

9. Prions have been said to be

a. the smallest infectious agents known

b. the largest infectious agents known

c. 100 times smaller than viruses to almost as large as mitochondria and bacteria

d. the cause of certain diseases e. a, c, and d.

10. Prions

a. have been found to contain nucleic acids

b. have not been found to contain nucleic acids

c. are viruses d. are not viruses

e. b and d.

11. Recent work has shown that prions may be

a. proteins produced somewhat abnormally by “infected” genes that somehow go awry

b. bacteria c. viruses

d. protists e. mitochondria.

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