13.1 General Characteristics of Viruses (Table 13.1)

1. Viruses are non-living agents associated with all forms of life. Each virus particle consists of nucleic acid surrounded by a protein coat. They are approximately 100- to 1,000-fold smaller than the cells they infect. (Figures 13.2,13.3)

Virus Architecture

1. Different viruses have different shapes. Some are isometric, others are helical and still others are complex. (Figure 13.1)

2. The shape is determined by the protein coat (or capsid) that surrounds the nucleic acid. These make up the nucleocapsid. Each capsid is composed of capsomeres; attachment proteins project from the capsid. (Figures 13.2,13.1)

3. Some animal viruses have a lipid bilayer surrounding the coat. These viruses are enveloped. Viruses without this envelope are naked. Virtually all viruses that infect bacteria, bacteriophages, are complex and naked. (Figure 13.2)

The Viral Genome

1. Viruses contain either RNA or DNA, but never both. The nucleic acid may be single-stranded or double-stranded.

Replication Cycle—Overall Features

1. Viruses only multiply within living cells and use the machinery of the cells to support their own multiplication.

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