Virus

Viruses are obligate intracellular parasites, meaning that in order to replicate, they need to be inside a host cell. Viruses lack the machinery and enzymes necessary to reproduce; the only synthetic activity they perform on their own is to synthesize their capsids.

The infection cycle of most viruses follows a basic pattern. Bacteriophages are unusual in that they can infect a bacterium in two ways (although other viruses may replicate in these two ways as well). In the lytic cycle of replication, the bacteriophage destroys the bacterium it infects. In the lysogenic cycle, however, the bacteriophage coexists with its bacterial host, and remains inside the bacterium throughout its life, reproducing only when the bacterium itself reproduces.

An example of a bacteriophage that undergoes lytic replication inside a bacterial host is the T4 bacteriophage which infects E. coli. T4 begins the infection cycle by docking with an E. coli bacterium. The tail fibers of the bacteriophage make contact with the cell wall of the bacterium, and the bacteriophage then injects its genetic material into the bacterium. Inside the bacterium, the viral genes are transcribed. One of the first products produced from the viral genes is an enzyme that destroys the bacterium's own genetic material. Now the virus can proceed in its replication unhampered by the bacterial genes. Parts of new bacteriophages are produced and assembled. The bacterium then bursts, and the new bacteriophages are freed to infect other bacteria. This entire process takes only 20-30 minutes.

In the lysogenic cycle, the bacteriophage reproduces its genetic material but does not destroy the host's genetic material. The bacteriophage called lambda, another E. coli-infecting virus, is an example of a bacteriophage that undergoes lysogenic replication within a bacterial host. After the viral DNA has been injected into the bacterial host, it assumes a circular shape. At this point, the replication cycle can either become lytic or lysogenic. In a lysogenic cycle, the circular DNA attaches to the host cell genome at a specific place. This combination host-viral genome is Figure 1. In addition to the capsid, some viruses are surrounded by an envelope, consisting of protein and lipid, which is acquired as the virus exits the host cell. Viruses that have an envelope exit host cells by exocytosis, in which they "bud out" from the host cell. As they bud out, they take pieces of the host cell's plasma membrane with them and use these pieces as the protective envelope. Illustration by Hans & Cassidy. Courtesy of Gale Group. called a prophage. Most of the viral genes within the prophage are repressed by a special repressor protein, so they do not encode the production of new bacteriophages. However, each time the bacterium divides, the viral genes are replicated along with the host genes. The bacterial progeny are thus lysogenically infected with viral genes.

Interestingly, bacteria that contain prophages can be destroyed when the viral DNA is suddenly triggered to undergo lytic replication. Radiation and chemicals are often the triggers that initiate lytic replication. Another interesting aspect of prophages is the role they play in human diseases. The bacteria that cause diphtheria and botulism both harbor viruses. The viral genes encode powerful toxins that have devastating effects on the human body. Without the infecting viruses, these bacteria may well be innocuous. It is the presence of viruses that makes these bacterial diseases so lethal.