UNIT 1

Viruses and subviruses

PRE-READING AND READING TASKS

1. Make sure you know the following words:

to interfere with	["Intq'fIq]	мешать, вредить
inscrutable	[In'skru:tqb(q)l]	загадочный, непостижимый
entity	['entItI]	сущность, существо, организм
to overpower	["qVvq'paVq]	подавлять
machinery	[mq'Si:n(q)rI]	структура
intact	[In'txkt]	нетронутый, неповрежденный, целый
crucial	['kru:S(q)l]	решающий, критический
to impart	[Im'pQ:t]	придавать, сообщать
to promote	[prq'mqVt]	способствовать, помогать
fracture	['frxktSq]	перелом, трещина, разрыв
awry	[q'raI]	неправильный
template	['templIt]	матрица
rabies	['reIbi:z]	бешенство
scrapie	['skreIpI]	почесуха (вет.)
strain	[streIn]	штамм

2. Read and translate the text.

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 interact with living organisms in many fundamental ways, interfering with their cellular processes, and are capable of causing a number of very real symptoms. In fact, these tiny, inscrutable, nonliving entities are responsible for over 60 million common colds each year in America alone, as well as for a number of other major scourges and epidemics, including AIDS.

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.