Size and Shapes of Viruses

IV. VIRUSES

B. SIZE AND SHAPES OF VIRUSES

The overall purpose of this Learning Object is to learn the general size and the various shapes of viruses.

LEARNING OBJECTIVES FOR THIS SECTION

Viruses are infectious agents with both living and nonliving characteristics.

1. Living characteristics of viruses

a. They reproduce at a fantastic rate, but only in living host cells.

b. They can mutate.

2. Nonliving characteristics of viruses

a. They are acellular, that is, they contain no cytoplasm or cellular organelles.

b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.

c. The vast majority of viruses possess either DNA or RNA but not both.

Size and Shapes of Viruses

1. Size (see Fig. 1A, Fig. 1B, and Fig. 1C)

Viruses are usually much smaller than bacteria and are submicroscopic (def). Most range in size from 5 to 300 nanometers (nm) (def), although some Paramyxoviruses can be up to 14,000nm long. For a comparison of the size of a virus, a bacterium, and a human cell, scroll down to how big is... on the CELL'S ALIVE web page.

2. Shapes (see Fig. 1A, Fig. 1B, and Fig. 1C)

a. Helical viruses (def) consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure (see Fig. 2A).

b. Polyhedral viruses (def) consist of nucleic acid surrounded by a polyhedral (many-sided) shell or capsid, usually in the form of an icosahedron (def); (see Fig. 2B).

Transmission electron micrograph of Adenoviruses; courtesy of CDC.

Transmission electron micrograph of poliomyelitis viruses; courtesy of CDC.

Transmission electron micrograph of poliomyelitis viruses; courtesy of Dennis Kunkel's Microscopy.

c. Enveloped viruses (def) consist of nucleic acid surrounded by either a helical or polyhedral core and covered by an envelope (see Fig. 2C and Fig. 2D).

Transmission electron micrograph of Hepatitis B viruses; courtesy of CDC.

Transmission electron micrograph of an Influenza A virus; courtesy of CDC.

Transmission electron micrograph of HIV; courtesy of CDC.

Transmission electron micrograph showing envelope and glycoprotein spikes Coronaviruses; courtesy of CDC.

Transmission electron micrograph of herpes simplex viruses; courtesy of Dennis Kunkel's Microscopy.

d. Binal (complex) viruses (def) have neither helical nor polyhedral forms, are pleomorphic (irregular shaped), or have complex structures (see Fig. 2E).

Transmission electron micrograph of the bacteriophage coliphage T4; courtesy of Dennis Kunkel's Microscopy.

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