Viruses

l./jFor three-quarters of a century, scientists have known that many diseases of man, animals, plants and even of microorganisms are caused by transmissible agents which cannot be seen under the light microscope, they afe so small that they can pass through filters fine enough to hold back the most minute bacteria. These mysterious invisible agents were given the generic namaof virusesiln the 1930's, two great discoveries were made which provided concrete informa­tion concerning the nature of viruses. It was found that some of them would be crystallized almost as readily as if they were ordinary chemical substances.

2. Chemically, the active virus particles were found to behave like
giant molecules. At about the same time, the electron microscope
became available and permitted pictures to be obtained of these crys­
tals as well as of particles present in fluids and other materials having
virus activity. Viruses would now be seen as concrete objects instead of
being merely imagined.

3.^The first unexpected fact revealed by electron microscopy was that the various viruses differ among themselves in shape and in size, as various types of bacteria. The virologist can differentiate between sever­al types of viruses on the basis of their size and shape as revealed by electron micrographs. For example, the vaccinia virus is rather large. In contrast, the polioviruses are much smaller and yield very characteristic Crystals. As to the tobacco mosaic virus, it can be crystallized in the form of thin needles having different lengths. $The viruses that attack bacteria, which are called bacteriophages, are more complex, at least in

Lesson 14 ♦ 163

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.hape. Many of them have a thin tail and a large round or cylindrical hcad.VEach active virus particle consists of at least two very different ivpes of structural components. One structure made up of nucleic acid, carries the genetic hereditary characteristics of the virus. Another, pro­tein in nature, is thought to protect this genetic apparatus and to facil­itate its transfer from one infected cell to another. For example, elec-tton micrographs revealed that virus of tobacco mosaic consisted of an inner constituent of nucleic acid lodged within an outer coat, cylindri­cal in shape and made up of protein. The central structure, the core, should be compared to the nucleus of ordinary cells in higher organ­isms, which also contains large amounts of nucleic acid and also carries the genetic endowment. In fact, the nucleic acid core of this virus is its most essential constituent. However, proteins and nucleic acids are not t he only structural components of active viruses. Certain viral particles have recently been shown to contain lipids as part of their essential structures. High-magnification electron micrographs will reveal further­more that some of them possess a distinct membrane. If we examined the structure of some of bacteria under highmagnification electron mi­croscope we should see that they possess a distinct membrane.

Упражнение 8. Прочтите и переведите текст А. Первый и второй абзацы переведите письменно.

Упражнение 9. Найдите в тексте А ответы на следующие вопросы.

1. What have scientists known about viruses until the electrone mi­croscope became available? 2. What two great discoveries were made in the 1930's? 3. Can the virologists differentiate between the types of viruses on the basis of their size or shape? 4. Are proteins and nucleic acids the only structural components of active viruses?

Упражнение 10. Составьте письменно план текста А.

Упражнение 11. Найдите в каждом абзаце текста А предложение, ныражающее основную мысль данного абзаца, и переведите его.

Упражнение 12. Найдите в каждом ряду слово, синонимичное по значению первому слову ряда.

1. ordinary - everyday, common, usual, often, habitual; 2. to reveal-to detect, to open, to find, to show, to demonstrate; 3. remarkable — usual, seldom, unusual, interesting; 4. material — findings, essence,data, evidence, matter

164 0* Learning to Understand a Medical Text