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Structure and Classification

A virus is a biological particle composed of genetic material and protein. A typical virus consists of either RNA or DNA encased in a protein coat called a capsid. When a virus causes a disease, the virus is said to be virulent (VIR-yuh-lunt). If the virus does not cause disease immediately, it is said to be temperate.

Viruses are constructed of compounds usually associated with cells, but they are not considered living organisms. They have some, but not all, of the characteristics of life listed in Chapter 1. They have no nucleus, cytoplasm, organelles, or cell membrane. They do not reproduce by either mitosis or meiosis, nor are they capable of carrying out cellular functions. Study Table 19-1, in which viruses are compared and contrasted with cells.

Because they are not cells, viruses can only reproduce by invading a host cell and using the enzymes and organelles of the host cell to make more viruses. They are therefore obligate intracellular parasites, which means they require a host cell to reproduce. Outside a host cell a virus is a lifeless particle with no control of its movements. It is spread on the wind, in water, in food, or via blood or other bodily secretions.

Studying Viruses

Scientists knew as early as 1800 that some factor existed that was smaller than bacteria and that could transmit disease. However, they were unaware of the nature of the factor until 1935, when Wendell Stanley (1904-1971) first isolated the virus that causes tobacco mosaic disease. The electron microscope was the first tool that permitted scientists actually to look at viruses. Recent advances such as tissue culture, serology, electrophoresis, and nucleic acid sequencing help virologists learn more about viruses.

Tissue culture is the growing of living cells in a controlled medium. This technique is the foundation for viral research. By


Section Objectives

Compare and contrast viruses and cells.

Describe the structure of three types of diseasecausing viruses.

Distinguish between DNA and RNA viruses.

Name and describe two disease-causing particles that are smaller than viruses.

State how Pasteur developed a rabies vaccine.

19-1 Table




Copyright 1995 Mr. Lewis Classroom for Physics & Biology. All rights reserved.