Animal Virus Life Cycles: An Overview

IV. VIRUSES

G. BACTERIOPHAGE LIFE CYCLES: AN OVERVIEW

The overall purpose of this Learning Object is to briefly compare the lytic and the lysogenic live cycle of bacteriophages.

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.

Life Cycle of Bacteriophages

As mentioned in an earlier section, bacteriophages are viruses that only infect bacteria (also see Fig. 1C and Fig. 2E). There are two primary types of bacteriophages: lytic bacteriophages and temperate bacteriophages.

1. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages, and are so named because they lyse the host bacterium as a normal part of their life cycle.

2. Bacteriophages capable of a lysogenic life cycle are termed temperate phages. When a temperate phage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage.

We will now look at the lytic life cycle and lysogenic life cycle of bacteriophages.

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