The Structure and Function of DNA: Viruses and Other Noncellular Infectious Agents

CHAPTER TEN THE STRUCTURE AND FUNCTION OF DNA

Viruses and Other Noncellular Infectious Agents

Viruses share some of the characteristics of living organisms, such as having genetic material in the form of nucleic acid packaged within a highly organized structure. A virus is generally not considered alive, however, because it is not cellular and cannot reproduce on its own. A virus is an infectious particle consisting of little more than "genes in a box": a bit of nucleic acid wrapped in a protein coat and, in some cases, an envelope of membrane. A virus cannot reproduce on its own, and thus it can multiply only by infecting a living cell and directing the cell's molecular machinery to make more viruses. In this section, we'll look at viruses that infect different types of host organisms, starting with bacteria.

Bacteriophages

Viruses that attack bacteria are called bacteriophages ("bacteria-eaters"), or phages for short. Figure ten point twenty-four shows a micrograph of a bacteriophage called T four infecting an Escherichia coli bacterium. The phage consists of a molecule of DNA enclosed within an elaborate structure made of proteins. The "legs" of the phage bend when they touch the cell surface. The tail is a hollow rod enclosed in a springlike sheath. As the legs bend, the spring compresses, the bottom of the rod punctures the cell membrane, and the viral DNA passes from inside the head of the virus into the cell.

Once they infect a bacterium, most phages enter a reproductive cycle called the lytic cycle. The lytic cycle gets its name from the fact that after many copies of the phage are produced within the bacterial cell, the bacterium lyses (breaks open). Some viruses can also reproduce by an alternative route -the lysogenic cycle. During a lysogenic cycle, viral DNA replication occurs without phage production or the death of the cell

In the lysogenic cycle, the viral DNA is inserted into the bacterial chromosome. Once there, the phage DNA is referred to as a prophage, and most of its genes are inactive. Survival of the prophage depends on the reproduction of the cell where it resides. The host cell replicates the prophage DNA along with its cellular DNA and then, upon dividing, passes on both the prophage and the cellular DNA to its two daughter cells. A single infected bacterium can quickly give rise to a large population of bacteria that all carry prophages. The prophages may remain in the bacterial cells indefinitely. Occasionally, however, a prophage leaves its chromosome; this event may be triggered by environmental conditions such as exposure to a mutagen. Once separate, the lambda DNA usually switches to the lytic cycle, which results in the production of many copies of the virus and lysing of the host cell.

Sometimes the few prophage genes active in a lysogenic bacterial cell can cause medical problems. For example, the bacteria that cause diphtheria, botulism, and scarlet fever would be harmless to people if it were not for the prophage genes they carry. Certain of these genes direct the bacteria to produce toxins that make people ill. El