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(a) Tobacco mosaic virus (helical)

(b) Adenovirus (polyhedral)

(c) Influenza (enveloped)

figure 24-2

Viruses have a variety of sizes and shapes. (a) The tobacco mosaic virus is about 18 nm in diameter and has a helical shape. (b) The adenoviruses are about 80-110 nm in diameter and have the shape of an icosahedron. (c) The spherical influenza viruses are between 50-120 nm in diameter.

Calculating Nanometers

Materials meterstick with millimeter marks, paper, scissors, tape, pencil Procedure Cut the paper into strips. Tape the strips together to form one strip that is 2 m long, and label 1 m, 20 cm, 2 cm, and 2 mm.


1. Write an equation at the 1 m mark and at the end of the strip that shows the relationship between the length of the paper in meters and nanometers.

2. Write equations beside the 2 cm and the 20 cm marks to show the relationship of centimeters to nanometers.

3. Write an equation by the 2 mm mark to show its relationship to nanometers.

(b) Adenovirus (polyhedral)

(c) Influenza (enveloped)

Viral Size and Structure

Viruses are some of the smallest particles that are able to cause disease. But they vary in size and shape, as shown in Figure 24-2. The shape of a virus is the result of its genome and the protein coat that covers the genome. A protein coat, or capsid (KAP-sid), is the only layer surrounding some viruses. The capsid of some viruses, such as TMV, forms a helix, shown in Figure 24-2a. The rabies and measles viruses are also helical viruses. As shown in Figure 24-2b, the adenovirus capsid has the shape of an icosahedron (lE-koh-suh-HEE-druhn), a shape with 20 triangular faces and 12 corners. Other viruses with this shape include those that cause herpes simplex, chickenpox, and polio. The influenza virus, shown in Figure 24-2c, is spherical in shape.

Some viruses have a bilipid membrane called an envelope that surrounds the capsid. The envelope is formed from either the nuclear membrane or the cell membrane of the host cell as the viral capsid buds from the host cell. Proteins in the envelope, such as those of the influenza virus shown in Figure 24-2c, help new viruses recognize host cells. Enveloped viruses include the chickenpox virus (varicola virus) and human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS).

Classification of Viruses

Viruses can be classified by whether they have RNA or DNA as their genome and whether their genome is single stranded or double stranded and linear or circular. Viruses are also classified based on the nature of their capsid and on the presence or absence of an envelope. Table 24-1 describes some viruses that affect human health. For example, the virus that causes severe acute respiratory syndrome (SARS) is a coronavirus. Corona is the Latin word for "crown." The SARS virus has single-stranded, linear RNA and an envelope with lollipop-shaped proteins that make the envelope look like a crown.

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