Viruses
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virus and is surrounded by a helically arranged protein coat. The protein coat makes up about 95 percent of the mass of this type of virus. Rabies and mumps are caused by helical viruses.

The virus that causes acquired immune deficiency syndrome, or AIDS, is even more complex. The AIDS virus is shown on the right. This virus, called the HIV, has two single strands of RNA in its core. These strands are surrounded by two layers of protein. A layer of lipids surrounds these inner protein layers. Glycoprotein molecules, proteins with sugar chains attached, are embedded in the lipid layer and form the capsid of the virus.

Classification

Because viruses are not considered living things, they are not classified by the system of nomenclature discussed in Chapter 18. Viruses instead are classified as DNA viruses or RNA viruses, depending on the type of nucleic acid in the capsid. As shown in Table 19-2, viruses contain either RNA or DNA, never both.

DNA and RNA Viruses

DNA and RNA viruses differ in the manner in which they alter the machinery of a host cell. Once inside the host cell, a DNA virus may directly produce new RNA that then makes more viral proteins. Alternatively the virus DNA may join to the DNA of the host cell and then direct the synthesis of new viruses.

RNA viruses perform in another way. Some RNA viruses enter the cell and make new proteins directly. They do so by releasing the RNA, which then migrates directly to the cytoplasm, where it uses the host ribosomes to make proteins. The polio virus, which is an RNA virus, acts in this manner.